U.S. patent application number 11/926417 was filed with the patent office on 2008-07-24 for endpoint detection for photomask etching.
This patent application is currently assigned to APPLIED MATERIALS, INC.. Invention is credited to Michael Grimbergen.
Application Number | 20080176149 11/926417 |
Document ID | / |
Family ID | 39641588 |
Filed Date | 2008-07-24 |
United States Patent
Application |
20080176149 |
Kind Code |
A1 |
Grimbergen; Michael |
July 24, 2008 |
ENDPOINT DETECTION FOR PHOTOMASK ETCHING
Abstract
Apparatus and method for endpoint detection are provided for
photomask etching. The apparatus provides a plasma etch chamber
with a substrate support member. The substrate support member has
at least two optical components disposed therein for use in
endpoint detection. Enhanced process monitoring for photomask
etching are achieved by the use of various optical measurement
techniques for monitoring at different locations of the
photomask.
Inventors: |
Grimbergen; Michael;
(Redwood City, CA) |
Correspondence
Address: |
PATTERSON & SHERIDAN, LLP
Suite 1500, 3040 Post Oak Boulevard
Houston
TX
77056
US
|
Assignee: |
APPLIED MATERIALS, INC.
|
Family ID: |
39641588 |
Appl. No.: |
11/926417 |
Filed: |
October 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60863490 |
Oct 30, 2006 |
|
|
|
60969328 |
Aug 31, 2007 |
|
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Current U.S.
Class: |
430/5 ;
216/60 |
Current CPC
Class: |
C23F 4/00 20130101; G03F
1/80 20130101; H01J 37/32963 20130101 |
Class at
Publication: |
430/5 ;
216/60 |
International
Class: |
G03F 1/00 20060101
G03F001/00; C23F 1/00 20060101 C23F001/00 |
Claims
1. A method for processing a substrate, comprising: detecting an
orientation of a patterned photomask reticle while disposed on a
substrate support in a plasma etch chamber; and selecting and/or
altering an etch process in response to the detected
orientation.
2. The method of claim 1, wherein detecting the orientation further
comprises: detecting the presence of a test pattern in a peripheral
region of the photomask reticle.
3. The method of claim 2, wherein selecting and/or altering the
etch process further comprises: changing the etch process in a
manner that rotates process results about 90 degrees.
4. The method of claim 2, wherein selecting and/or altering the
etch process further comprises: changing the process for the
substrate being etched.
5. The method of claim 2, wherein detecting the orientation further
comprises: determining if the orientation of the reticle is
different from an orientation of a previously etched reticle; and
compensating the etch process in response to the determination.
6. A method for processing a substrate, comprising: placing a
patterned photomask reticle on a substrate support in a plasma etch
chamber; prior to etching, detecting an orientation of the
patterned photomask reticle while on the substrate support using at
least one of a light passing through the photomask reticle or
reflected from a surface of a layer comprising the photomask
reticle; selecting an etch process in response to the detected
orientation; and etching the photomask reticle using the etch
process.
7. The method of claim 6, wherein detecting the orientation of the
patterned photomask reticle further comprises: passing optical
signals to at least one detector, the optical signals collected
through at least a first window and a second window, the first and
second windows disposed in the substrate support below the
photomask reticle and adjacent different edges of the photomask
reticle.
8. The method of claim 6, wherein detecting the orientation of the
patterned photomask reticle further comprises: passing optical
signals to at least one detector, the optical signals collected
through a first edge window, a second edge window, a third edge
window and a fourth edge window, the edge windows disposed in the
substrate support below the photomask reticle and adjacent
different edges of the photomask reticle.
9. The method of claim 7 further comprising: determining a center
etch rate from analyzing a signal collected below a center of the
photomask reticle.
10. The method of claim 7 further comprising: determining a
difference in a center to edge etch rate from analyzing a signal
collected below a center of the photomask reticle and signals
collected through at least one of the first and second windows.
11. The method of claim 7 further comprising: determining an etch
rate profile from analyzing a signal collected below a center of
the photomask reticle and signals collected through the edge
windows.
12. The method of claim 6 further comprising: determining an etch
rate from analyzing OES information collected through a sidewall of
the etch chamber.
13. The method of claim 6 further comprising: determining an etch
rate from analyzing OES information collected through a sidewall of
the etch chamber and information optical information collected
below the photomask reticle.
14. A method for processing a substrate, comprising: placing a
patterned substrate on a substrate support a plasma etch chamber;
etching a substrate; and detecting an etching endpoint using a
light having a wavelength absorbed by a layer disposed on the
substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/863,490 filed Oct. 30, 2006, U.S. Provisional
Application No. 60/969,328 filed Aug. 31, 2007. Both of which are
herein incorporated by reference there their entireties.
[0002] Additionally, the subject matter of this application is
related to the subject matter disclosed in U.S. patent application
Ser. No. 10/672,420, entitled "Interferometer Endpoint Monitoring
Device", filed on Sep. 26, 2003, by Nguyen, et al. (Attorney Docket
Number APPM/8349); U.S. patent application Ser. No. 11/844,838,
entitled "Endpoint Detection for Photomask Etching", filed on Aug.
24, 2007, by Grimbergen (Attorney Docket Number APPM/11455); U.S.
patent application Ser. No. 11/844,868, entitled "Endpoint
Detection for Photomask Etching", filed on Aug. 24, 2007, by
Grimbergen (Attorney Docket Number APPM/11455-02); U.S. patent
application Ser. No. ______, entitled "Endpoint Detection for
Photomask Etching", filed Oct. 29, 2007, by Grimbergen (Attorney
Docket No. APPM/11455-03); and U.S. patent application Ser. No. ,
entitled "Endpoint Detection for Photomask Etching", filed Oct. 29,
2007, by Grimbergen (Attorney Docket No. APPM/11455-05), all of
which are hereby incorporated hereby by reference in their
entireties.
BACKGROUND
[0003] 1. Field of the Invention
[0004] Embodiments of the present invention generally relate to the
fabrication of integrated circuits and to the fabrication of
photomasks useful in the manufacture of integrated circuits.
[0005] 2. Description of the Related Art
[0006] The fabrication of microelectronics or integrated circuit
devices typically involves a complicated process sequence requiring
hundreds of individual steps performed on semiconductor, dielectric
and conductive substrates. Examples of these process steps include
oxidation, diffusion, ion implantation, thin film deposition,
cleaning, etching and lithography. Using lithography and etching
(often referred to as pattern transfer steps), a desired pattern is
first transferred to a photosensitive material layer, e.g., a
photoresist, and then to the underlying material layer during
subsequent etching. In the lithographic step, a blanket photoresist
layer is exposed to a radiation source through a reticle or
photomask containing a pattern so that an image of the pattern is
formed in the photoresist. By developing the photoresist in a
suitable chemical solution, portions of the photoresist are
removed, thus resulting in a patterned photoresist layer. With this
photoresist pattern acting as a mask, the underlying material layer
is exposed to a reactive environment, e.g., using wet or dry
etching, which results in the pattern being transferred to the
underlying material layer.
[0007] The pattern on a photomask, which is typically formed in a
metal-containing layer supported on a glass or quartz substrate, is
also generated by etching through a photoresist pattern. In this
case, however, the photoresist pattern is created by a direct write
technique, e.g., with an electron beam or other suitable radiation
beam, as opposed to exposing the photoresist through a reticle.
With the patterned photoresist as a mask, the pattern can be
transferred to the underlying metal-containing layer using plasma
etching. An example of a commercially available photomask etch
equipment suitable for use in advanced device fabrication is the
Tetra.TM. Photomask Etch System, available from Applied Materials,
Inc., of Santa Clara, Calif. The terms "mask", "photomask" or
"reticle" will be used interchangeably to denote generally a
substrate containing a pattern.
[0008] During processing, endpoint data from the etching of the
photomasks may be used to determine whether the process is
operating according to required specifications, and whether the
desired results such as etch uniformity are achieved. Since each
photomask generally has its own set of features or patterns,
different photomasks being etched using the same process recipe may
yield different endpoint data, thereby making it difficult to
determine if the desired etch results are obtained for a specific
photomask.
[0009] With ever-decreasing device dimensions, the design and
fabrication of photomasks for advanced technology becomes
increasingly complex, and control of critical dimensions and
process uniformity becomes increasingly more important. Therefore,
there is an ongoing need for improved process control in photomask
fabrication, such as improved apparatus and method for generating
endpoint data that would be consistent for each photomask.
SUMMARY
[0010] Embodiments of the invention generally provide a method and
apparatus for etching a substrate. The invention is particularly
suitable for etching photomasks, among other substrates used vacuum
processing.
[0011] In one embodiment, a method for etching a substrate is
provided that includes (a) providing an etch chamber having a
substrate support member, the substrate support member comprising
at least a first window in a center region and a second window in a
peripheral region, (b) providing a substrate on the substrate
support member, (c) introducing a process gas into the etch
chamber, (d) generating a plasma from the process gas for etching
the substrate, (e) detecting a first optical signal through the
first window and a second optical signal through the second window
using an endpoint detection system, and terminating the plasma
based on information obtained from at least one of the detected
first and second optical signals.
[0012] In another embodiment, a method for etching a substrate
includes (a) providing an etch chamber having a substrate support
member, the substrate support member comprising a first window and
a second window, (b) providing a substrate on the substrate support
member, (c) generating a plasma from a process gas for etching the
substrate, (d) providing an endpoint detection system comprising a
photodetector, (e) monitoring at least one optical signal through
at least one of the first window and the second window using the
photodetector, and (f terminating the plasma based on information
obtained from the at least one optical signal.
[0013] In another embodiment of the invention, an apparatus for
substrate etching is provided that includes a plasma etching
chamber, a substrate support member inside the chamber, the
substrate support member having a first window disposed in a center
region and a second window disposed in a peripheral region, and an
endpoint detection system operatively coupled to the chamber
through the first and second windows.
[0014] In another embodiment, an apparatus for substrate etching
may include a plasma etching chamber comprising a substrate support
member, an endpoint detection system configured for operating in at
least one of a reflection mode and a transmission mode, wherein the
endpoint detection system comprises a first optical component
disposed in a center region of the substrate support member and a
second optical component disposed in a peripheral region of the
substrate support member.
[0015] Another embodiment provides an apparatus for substrate
etching that includes a plasma etching chamber, a substrate support
member inside the chamber, the substrate support member having a
first window and a second window disposed therein, the first window
being in a center region of the support member, and an endpoint
detection system operatively coupled to the chamber through one of
the first and second windows.
[0016] In another embodiment, a method for etching a substrate may
include providing an etch chamber having a substrate support
member, the substrate support member comprising a first window in a
center region and a second window in a peripheral region, providing
a substrate on the substrate support member, introducing a process
gas into the etch chamber, generating a plasma from the process gas
for etching the substrate, detecting a first optical signal through
the first window and a second optical signal through the second
window using an endpoint detection system, and terminating the
plasma based on information obtained from at least one of the
detected first and second optical signals.
[0017] In another embodiment, a method for processing a substrate
includes etching a patterned substrate disposed on a substrate
support in a plasma etch chamber, detecting a first signal
reflected from the substrate during etching, the first signal
collected through a first window of the substrate support,
detecting a second signal transmitted through the substrate during
etching, the second signal collected through a second window of the
substrate support, that second window spatially separated from the
first window, and determining an endpoint of the etch process using
the first and second signals.
[0018] In another embodiment, a method for processing a substrate
includes placing a patterned substrate on a substrate support a
plasma etch chamber, etching a substrate, and detecting an etching
endpoint using a light having a wavelength absorbed by a layer
disposed on the substrate.
[0019] In another embodiment, a method for processing a substrate
includes detecting an orientation of a patterned photomask reticle
while disposed on a substrate support in a plasma etch chamber and
selecting and/or altering an etch process in response to the
detected orientation.
[0020] In another embodiment, a method for processing a substrate
includes etching a substrate through a patterned masking layer in a
plasma etch chamber, the substrate having a non-etching side
disposed on a substrate support and an etching side facing away
from the substrate support, exposing the etching side of the
patterned substrate while etching to radiation from a radiation
source while etching the substrate, collecting a signal from the
radiation source from the non-etching side of the patterned
substrate, and controlling the etch process in response to the
collected signal.
[0021] In yet another embodiment, a method for etching a substrate
includes (a) providing an etch chamber having a substrate support
member, the substrate support member having a first window and a
second window, (b) providing a substrate on the substrate support
member, (d) generating a plasma from a process gas for etching the
substrate, (e) providing an endpoint detection system that includes
a photodetector, (f) monitoring at least one optical signal through
at least one of the first window and the second window using the
photodetector, and (g) terminating the plasma based on information
obtained from the at least one optical signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] So that the manner in which the above recited features,
advantages and objects of the invention are attained and can be
understood in detail, a more particular description of the
invention, briefly summarized above, may be had by reference to the
embodiments thereof which are illustrated in the appended
drawings.
[0023] It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are
therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
[0024] FIG. 1A illustrates a process chamber incorporating one
embodiment of the present invention;
[0025] FIG. 1B illustrates a cross-sectional view of two optical
configurations for endpoint monitoring according to embodiments of
the present invention;
[0026] FIG. 1C is a schematic top view of one embodiment for
endpoint detection;
[0027] FIG. 2 illustrates sample locations on a substrate for
endpoint detection;
[0028] FIG. 3 illustrates a top view of a 6-inch substrate with
peripheral locations for endpoint detection;
[0029] FIGS. 4A-C are illustrate schematically structures of
several types of photomasks during fabrication;
[0030] FIG. 5 is an illustration of various optical signals used
for endpoint detection;
[0031] FIG. 6 is a flow diagram of one embodiment of a process for
etching a photomask;
[0032] FIG. 7 is a schematic diagram of another embodiment of an
etch chamber having an endpoint detection system that includes an
optical fiber bundle.
[0033] FIG. 8 is a schematic diagram of one embodiment of an
exemplary detector;
[0034] FIG. 9 is a schematic diagram of another embodiment of an
etch reactor suitable for etching a photomask reticle having an
endpoint detection system;
[0035] FIG. 10 is a schematic diagram of a top view of a substrate
support illustrating the distribution of a center window and edge
windows within an area covered by a reticle during processing;
[0036] FIG. 11 is graphs of average (side OES) endpoint, left
endpoint and top endpoint for one embodiment of a photomask etching
process;
[0037] FIG. 12 is a graph of endpoint data taken using side OES,
center, left, top and upper right corner during one embodiment of a
photomask reticle etching process;
[0038] FIG. 13 is a graph of endpoint data taken through the mask
and side OES (and resultant transmission) obtained during two
periods of the same photomask reticle etching process;
[0039] FIG. 14 is a flow chart of one embodiment of a method for
monitoring an etch process using normalized transmission endpoint
information;
[0040] FIGS. 15-16 are schematic diagrams illustrating thin film
interference occurring between the top and bottom of a masking
layer for light in which the film is substantially transparent, and
the absence of interference when the light is substantially
absorbed;
[0041] FIG. 17 shows endpoint data which demonstrates the combined
transmission from the component signals shown in FIG. 18;
[0042] FIG. 18 depicts the magnitude of the transmission signals
T1, T2 and T3 shown in FIG. 16;
[0043] FIGS. 19-20 are illustrative of an etching process utilizing
an endpoint monitoring signal by using a wavelength that is
absorbed by the photoresist layer;
[0044] FIG. 21 depicts the magnitude of the transmission signals T1
and T2 shown in FIG. 19 and demonstrated in the data of FIG.
20;
[0045] FIG. 22 is another graph of endpoint data obtained during
one embodiment of a photomask reticle etching process using deep UV
signal and its derivative;
[0046] FIG. 23 depicts an endpoint signal dominated by optical
interference from a photoresist layer;
[0047] FIG. 24 depicts an endpoint signal wherein optical
interference from a photoresist layer is minimized by utilizing
deep UV wavelength monitoring signals, wherein the left graph shows
the transmission and the right graph shows its derivative;
[0048] FIG. 25 is a schematic diagram of one embodiment of a
processing chamber for etching a photomask reticle and having an
endpoint detection system that utilizes both OES and TEP endpoint
information; and
[0049] FIG. 26 shows optical reflection and transmission curves for
photoresist on a mask showing increased absorption in the deep-UV
portion of the spectrum.
[0050] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. It is contemplated that elements
and features of one embodiment may be beneficially incorporated in
other embodiments without further recitation.
[0051] It is to be noted, however, that the appended drawings
illustrate only exemplary embodiments of this invention and are
therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
DETAILED DESCRIPTION
[0052] The present invention provides a method and apparatus for
etching a photomask substrate with enhanced process monitoring, for
example, by providing for optical monitoring at different regions
of the photomask. Although the discussions and illustrative
examples focus on the etching of a photomask substrate, various
embodiments of the invention can also be adapted for process
monitoring of other suitable substrates, including transparent or
dielectric substrates.
[0053] FIG. 1A is a schematic cross sectional view of a plasma etch
chamber 10 in accordance with one embodiment of the invention.
Suitable plasma etch chambers include the Tetra.TM. II photomask
etch chamber or the Decoupled Plasma Source (DPS.TM.) chamber
available from Applied Materials, Inc., of Santa Clara, Calif.
Other process chambers may also be used in connection with
embodiments of the invention, including, for example, capacitive
coupled parallel plate chambers and magnetically enhanced ion etch
chambers, as well as inductively coupled plasma etch chambers of
different designs. The particular embodiment of the etch chamber 10
shown herein is provided for illustrative purposes and should not
be used to limit the scope of the invention. It is contemplated
that the invention may be utilized in other processing systems,
including those from other manufacturers.
[0054] The process chamber 10 generally includes a cylindrical
sidewall or chamber body 12, an energy transparent ceiling 13
mounted on the body 12, and a chamber bottom 17. The ceiling 13 may
be flat, rectangular, arcuate, conical, dome or multi-radius
shaped. At least one inductive coil 26 is disposed above at least a
portion of the ceiling 13. In the embodiment depicted in FIG. 1A,
two concentric coils 26 are shown. The chamber body 12 and the
chamber bottom 17 of the process chamber 10 can be made of a metal,
such as anodized aluminum, and the ceiling 13 can be made of an
energy transparent material such as a ceramic or other dielectric
material.
[0055] A substrate support member 16 is disposed in the process
chamber 10 to support a substrate 220 during processing. The
support member 16 may be a conventional mechanical or electrostatic
chuck with at least a portion of the support member 16 being
electrically conductive and capable of serving as a process bias
cathode. While not shown, a photomask adapter may be used to secure
the photomask on the support member 16. The photomask adapter
generally includes a lower portion milled to cover an upper portion
of the support member and a top portion having an opening that is
sized and shaped to hold a photomask. In one embodiment, the top
portion of the photomask adapter has a square opening. A suitable
photomask adapter is disclosed in U.S. Pat. No. 6,251,217, issued
on Jun. 26, 2001, which is incorporated herein by reference to the
extent not inconsistent with aspects and claims of the
invention.
[0056] Process gases are introduced into the process chamber 10
from a process gas source 48 through a gas distributor 22
peripherally disposed about the support member 16. Mass flow
controllers (not shown) for each process gas, or alternatively, for
mixtures of the process gas, are disposed between the process
chamber 10 and the process gas source 48 to regulate the respective
flow rates of the process gases.
[0057] A plasma zone 14 is defined by the process chamber 10, the
substrate support member 16 and the ceiling 13. A plasma is
generated in the plasma zone 14 from the process gases by supplying
power from a power supply 27 to the inductive coils 26 through an
RF match network 35. The support member 16 may include an electrode
disposed therein, which is powered by an electrode power supply 28
and generates a capacitive electric field in the process chamber 10
through an RF match network 25. Typically, RF power is applied to
the electrode in the support member 16 while the body 12 is
electrically grounded. The capacitive electric field, which is
transverse to the plane of the support member 16, influences the
directionality of charged species to provide more anisotropic
etching of the substrate 220.
[0058] Process gases and etchant byproducts are exhausted from the
process chamber 10 through an exhaust port 34 to an exhaust system
30. The exhaust system 30 may be disposed in the bottom 17 of the
process chamber 10 or may be disposed in the body 12 of the process
chamber 10 for removal of process gases. A throttle valve 32 is
provided in the exhaust port 34 for controlling the pressure in the
process chamber 10.
[0059] FIG. 1A further illustrates an endpoint detection system 164
operatively coupled to the process chamber 10 in accordance with
one embodiment of the invention. According to embodiments of the
invention, at least two optical access ports or viewports, are
provided in different regions of the substrate support member 16.
In one embodiment, at least one access port is provided in a
non-peripheral region. In yet another embodiment, the substrate
support member 16 is provided with at least one window in a center
region. In the example shown in FIG. 1A, the two optical access
ports comprise respectively a window 110 at a peripheral region
16P, and a window 112 at a central region 16C. The endpoint
detection system 164 is configured to detect optical signals
through one or more of these windows, which allows optical
monitoring of various locations on a photomask substrate 220 from
its backside during etching. In one embodiment, a third window (not
shown) may also be provided in the peripheral region 16P of the
substrate support member 16. Alternatively, different numbers of
windows may be provided at other locations of the substrate support
member 16.
[0060] In general, a larger window facilitates the installation of
optical components within the substrate support member 16. However,
for apparatus in which the substrate support member 16 is RF
biased, the size of the window, especially in the central region
16C of the substrate support member 16, is selected to be
sufficiently large for optical monitoring, yet small enough to
avoid potential adverse impact for the RF bias. Selecting a small
window also improves the lateral temperature uniformity of the
support member 16. The optical access port may generally comprise a
flat window made of quartz or other materials that transmit light
over a broad wavelength spectrum. A more detailed discussion of
different optical configurations will be provided in a later
section.
[0061] Referring first to FIG. 2, FIG. 2 illustrates schematically
several locations of the photomask substrate 220 that are monitored
for endpoint detection according to one embodiment of the present
invention. A central region 225 of the substrate 220 may be defined
as the area of the photomask that is patterned for lithographic
purposes, while a peripheral region is outside of the patterned
central region, and may include patterns or features utilized for
endpoint or monitoring of other process parameters. Several windows
disposed in the substrate support member 16 are shown in phantom.
For example, when the photomask substrate 220 is centrally disposed
with respect to the substrate support member 16, optical access
through window 112 allows monitoring of an area 222 around the
center of the photomask 220, while areas 224 and 226 in a
peripheral region 227 of the photomask 220 can be monitored through
windows 114 and 110, respectively. In one embodiment, monitored
areas 224 and 226 are located respectively along one side and at a
corner of the photomask 220. In another embodiment, the monitored
area 224 is located on one side of the photomask 220, e.g., at a
midpoint of the side, along a x-direction with respect to the
center of the photomask 220, while another area 224A located on an
adjacent side of the photomask 220, e.g., along a y-direction with
respect to the center of the photomask 220, is monitored through
another window 114A. Optical signals obtained through windows such
as 112, 114 and 114A can be used to obtain center to edge etch
uniformity along the x- and y-directions, or more generally, along
directions that are perpendicular to each other.
[0062] One or more windows 112A may also be provided in the
substrate support member 16 to allow for monitoring of different
areas such as 222A in the central region 225 of the photomask 220.
The additional windows 112A, 114A facilitate determination of an
edge to center etch profile. For example, information regarding
process uniformity, such as the edge to center etch profile, can be
obtained by comparing the endpoint results at different regions or
locations of the photomask 220, e.g., based on signals from areas
222, 224 and 224A. The windows may also be used for ensuring that
at least one window 112, 122A is below a feature being etched.
[0063] Referring back to FIG. 1A, the endpoint detection system 164
comprises optical setup for operating in reflection or transmission
modes, and is configured for different types of measurements such
as reflectance or transmittance, interferometry, or optical
emission spectroscopy. Depending on the application of interest,
e.g., the material layers or substrate structure being processed,
endpoints may be detected based on a change in the reflectance or
transmittance intensities, the number of interference fringes, or
changes in optical emission intensities at specific wavelengths, or
a combination thereof.
[0064] The reflection mode of operation allows reflectance (or
reflectometry) and interferometric measurement to be performed. The
endpoint system 164 generally comprises an optical source 166, a
focusing assembly 168 for focusing an incident optical beam 176
from the optical source 166 onto an area or spot 180 on the
backside of substrate 220, and a photodetector 170 for measuring
the intensity of a return optical beam 178 reflected off the area
180 of the substrate 220. The photodetector 170 may generally be a
single wavelength or multi-wavelength detector, or a spectrometer.
Based on the measured signal of the reflected optical beam 178, a
computer 172 calculates portions of the real-time waveform and
compares it with a stored characteristic waveform pattern to
extract information relating to the etch process. In this case, the
calculation may be based on slope changes or other characteristic
changes in the detected signals, either in reflection or
transmission mode, for example, when a film is etched through.
Alternatively, the calculation may be based on interferometric
signals as the depth of a trench or the thickness of a film changes
during etching. In other embodiments, more detailed calculations
may be performed based on reflection and transmission data obtained
over a wide spectrum in order to determine the depth or thickness
at any point in the etch process, or to determine the lateral
dimensions of the features being etched.
[0065] The light source 166 may be monochromatic, polychromatic,
white light, or other suitable light source. In general, the
optical signal from the reflected beam 178 may be analyzed to
extract information regarding the presence or absence of a layer
(e.g., metal-containing layer), or the thickness of certain
material layers within the area 180. The intensity of the incident
light beam 176 is selected to be sufficiently high to provide a
return beam 178 with a measurable intensity. In one embodiment, the
light source 166 provides polychromatic light, e.g., from an
Hg--Cd, Hg--Ar or Xe lamp or a light emitting diode (LED), which
generates light in a wavelength range from about below 200 nm to
about above 800 nm, or about 400 to about 800 nm, respectively. The
polychromatic light source 166 can be filtered to provide an
incident light beam 176 having selected frequencies. Color filters
can be placed in front of the light detector 170 to filter out all
wavelengths except for the desired wavelength of light, prior to
measuring the intensity of the return light beam 178 entering the
light detector 170. The light can be analyzed by a spectrometer
(array detector with a wavelength-dispersive element) to provide
data over a wide wavelength range, such as ultraviolet to visible,
from about 200 nm to 800 nm. The light source may be configured to
operate in a continuous or pulsed mode. With continuous detection,
it is preferable to have a light source with an output intensity
that is higher than that of the plasma emission. In the case of a
light source with multiple wavelength outputs, one can select a
wavelength whose intensity is higher than that of the corresponding
wavelength from the plasma. For pulsed mode operation, such
requirements of the light source intensity may be relaxed, as long
as the detector is not saturated by the intensity from the light
source and plasma.
[0066] Various light source options are available for pulsed mode
operation. For example, the light source 166 may be any suitable
source that provides a steady or continuous radiation output. A
shutter (not shown) can be provided to block and unblock the output
beam from the light source 166 so as to provide alternate beam
off/on cycles for signal detection. A signal acquired during the
beam "on" period will include contributions from the plasma
emission and the signal induced by the light source 166, while a
signal acquired during the beam "off" period will correspond to the
plasma emission. Subtracting the beam "off" signal from the beam
"on" signal can result in improved measurement because potential
interference from the plasma emission can be eliminated. Such a
data subtraction routine can be provided as part of algorithm
associated with the endpoint detection system.
[0067] Pulsed mode operation may also be achieved by configuring
the light source 166 to be switched on and off in alternate cycles,
for example, as shown in FIG. 7. In the embodiment of FIG. 7, a
reticle 700 is positioned in an etch chamber below a plasma 702. An
endpoint detection system 704 is positioned to interface with the
bottom of the reticle 700. The endpoint detection system 704
includes an optical fiber bundle 706 having one end positioned to
view the bottom of the reticle 700 through one or more windows
formed in the substrate support (not shown). The optical fiber
bundle 706 carries a signal generated from the light source 166 and
reflected off the reticle 700 to a detector 170. In general, the
shuttering or switching of the light source can be performed at
various combinations of duty cycles and signal acquisition times,
e.g., with the light source duty cycle selected to match that of
the detector duty cycle for background subtraction. The light
source intensity may also be adjustable to avoid saturating the
detector 170, such as a charge-coupled device (CCD) or other
suitable device. If the pulse duration is shorter than the detector
sampling time, the lamp may be pulsed a number of times to form a
higher total intensity by integration. In one embodiment, a 50
percent duty cycle is used. When the light source is on (or shutter
is open), light sensed by the detector includes both light from the
lamp and from the plasma. When the light source is off (or shutter
is closed), light sensed by the detector includes only light from
the plasma. Utilizing the difference in the signals, the background
contribution of light from the plasma may be subtracted the
detected signal, thereby providing a more accurate endpoint
indication.
[0068] Alternatively, unequal sampling periods may also be used for
background subtraction. For example, the sampling time for the
detector, e.g., a CCD, can be kept short during the light source
"on" period, followed by a longer sampling time during the light
source "off" period, during which the background plasma emission is
collected. This may be useful for reducing the noise in the
background plasma emission if the emission itself is used as a
secondary signal, e.g., as in transmission monitoring.
[0069] The selection of the signal acquisition time and the light
source "on" period may depend on the specific application and the
intensity of the light source. In general, using a light source
with a relatively low intensity output will require a longer signal
acquisition time. In one embodiment, the beam "on" period can range
from about 0.1 second to about 2 seconds.
[0070] The light source 166 may be a monochromatic source that
provides optical emission at a selected wavelength, for example, a
He--Ne or ND-YAG laser, or a solid state source such as a light
emitting diode (LED). Other options include various discharge lamps
such as hydrogen (H.sub.2), deuterium (D.sub.2), vapor lamps such
as those disclosed in Grimbergen, U.S. Pat. No. 6,534,756, or
hollow cathode lamps, with radiation outputs at multiple
wavelengths. In one embodiment, the light source 166 includes a
number of LEDs providing radiation outputs at different wavelength
regions. For example, the light source 166 may include at least one
of the following: a LED in the ultraviolet (UV) region, a LED in
the infrared (IR) region, and a LED with broadband (e.g., white
light) output, or any combinations thereof. Using a combination of
LEDs with different output wavelengths, e.g., 370 nm (UV), 390 nm
(UV), 400-700 nm (white), 800 nm (IR), 1300 nm (IR), 1500 nm (IR),
spectral output from the UV to the IR region can be achieved, e.g.,
from about 350 nm to about 1500 nm. In this case, the light source
166 can be provided with an output fiber bundle with fibers
coupling to respective LEDs.
[0071] Referring back to the embodiments depicted in FIG. 1A, one
or more convex focusing lenses 174 a , 174 b may be used to focus
the incident light beam 176 to the area 180 on the substrate
surface, and to focus the return light beam 178 back on the active
surface of light detector 170. The area 180 should be sufficiently
large to compensate for variations in surface topography of the
substrate 220 and device design features. This enables detection of
etch endpoints for high aspect ratio features having small
openings, such as vias or deep narrow trenches, which may be
densely present or more isolated. The area of the return light beam
should be sufficiently large to activate a large portion of the
active light-detecting surface of the light detector 170. The
incident and return light beams 176, 178 are directed through a
transparent window 110 in the process chamber 10 that allows the
light beams to pass in and out of the processing environment.
Although lenses 172 a and 174 b are shown in FIG. 1A as mounted
away from the window 110, in practice, they may also be mounted
close to the window 110, as shown in FIG. 1B. It is also understood
that the incident and return light beams 176, 178 can generally be
coupled via optical fibers to the endpoint detection system 164.
The use of fiber optics for coupling light beams to and from the
windows also allows electrical isolation to be maintained between
the substrate support member 16 and the detector electronics.
[0072] The diameter of the beam spot 180 is generally about 2 mm to
about 10 mm. However, if the beam spot 180 encompasses large
isolated areas of the substrate containing only a small number of
etched features, it may be necessary to use a larger beam spot in
order to encompass a greater number of etched features. The size of
the beam spot can therefore be optimized, depending on the design
features for a particular device. If the signal is sufficient, a
large beam spot or field of view will enable process control
without precisely matching the position of the substrate support
hole and the etched area of the substrate giving rise to the
signal.
[0073] Optionally, a light beam positioner 184 may be used to move
the incident light beam 176 across the substrate 220 to locate a
suitable portion of the substrate surface on which to position the
beam spot 180 to monitor an etching process. The light beam
positioner 184 may include one or more primary mirrors 186 that
rotate at small angles to deflect the light beam from the light
source 166 onto different positions of the substrate surface.
Additional secondary mirrors may be used (not shown) to direct the
return light beam 178 on the photodetector 170. The light beam
positioner 184 may also be used to scan the light beam in a raster
pattern across the backside of the substrate 220. In this
embodiment, the light beam positioner 184 comprises a scanning
assembly consisting of a movable stage (not shown), upon which the
light source 166, the focusing assembly 168 and the detector 170
are mounted. The movable stage can be moved through set intervals
by a drive mechanism, such as a stepper motor or galvanometer, to
scan the beam spot 180 across the substrate 220.
[0074] The photodetector 170 comprises a light-sensitive electronic
component, such as a photovoltaic cell, photodiode, or
phototransistor, which provides a signal in response to a measured
intensity of the return light beam 178. The signal can be in the
form of a change in the level of a current passing through an
electrical component or a change in a voltage applied across an
electrical component. The photodetector 170 can also comprise a
spectrometer (array detector with a wavelength-dispersive element)
to provide data over a wide wavelength range, such as ultraviolet
to visible, from about 200 nm to 800 nm. The return light beam 178
undergoes constructive and/or destructive interference which
increases or decreases the intensity of the light beam, and the
light detector 170 provides an electrical output signal in relation
to the measured intensity of the reflected light beam 178. The
electrical output signal is plotted as a function of time to
provide a spectrum having numerous waveform patterns corresponding
to the varying intensity of the reflected light beam 178.
[0075] A computer program on a computer system 172 compares the
shape of the measured waveform pattern of the reflected light beam
178 to a stored characteristic (or reference) waveform pattern and
determines the endpoint of the etching process when the measured
waveform pattern is the same as the characteristic waveform
pattern. As such, the period of the interference signal may be used
to calculate the depth and etch rate. The program may also operate
on the measured waveform to detect a characteristic waveform, such
as, an inflection point. The operations can be simple mathematic
operations, such as evaluating a moving derivative to detect an
inflection point. Although FIG. 1A shows the computer system 172
connected to the endpoint system 164, it is also used for
processing data from other endpoint detectors in the system.
[0076] FIG. 1A is meant to illustrate the relative positioning of
the optical access ports or windows 110 and 112 in the substrate
support member 16. A close-up cross-section view of two alternative
optical configurations is shown schematically in FIG. 1B. The
substrate support member 16 is provided with recessed portions 132
and 134, which are separately connected to openings or channels 136
and 138 to allow optical access to the backside of substrate 220.
The recess portions 132 and 134 are provided with O-rings and
grooves 142 and 144 for vacuum sealing to windows 124 and 126,
respectively. One configuration illustrates endpoint detection
based on reflection measurements through window 124, with incident
light in a fiber 121 being focused by lens 123 onto the substrate
220. The signal returning from the backside of substrate 220 is
then collimated by the lens 123 and coupled via a fiber 125 to the
endpoint detection system 164. Different focal lengths may be used
for lens 123, and in one embodiment, a focal length of about 15 mm
is used. In other embodiments, collimating lens 123 may be omitted,
in which case, fibers 121 and 125 can be mounted up against the
window 124. Depending on the specific measurements and optical
configurations, fibers 121 and 125 may refer to either a single
fiber or a fiber bundle (having more than one fiber). The use of
multiple fibers offers additional capabilities, including, for
example, improved signal strengths and simultaneous sampling of
different areas.
[0077] Another configuration illustrates endpoint detection based
on transmission measurements through window 126. A transmission
signal, e.g., plasma emission or external light source, passes
through window 126 and is collected by fiber 127 for detection. As
shown in FIG. 1B, the opening or channel 138 is provided with a
tapered or conical section 140 near the top surface of the
substrate support member 16. The conical section 140 has a larger
diameter (or lateral dimension) at the top compared to the interior
portion, i.e., the portion closer to the recessed portion 134. This
design has an advantage of providing a wider field of view or
sampling area at the substrate 220, without requiring the use of a
larger size window 126. In one embodiment, the conical section 140
is shaped to provide a field of view with a full angle of about
25.degree. for use with a fiber having a numerical aperture of
about 0.22. The field of view can also be changed by adjusting the
distance between the fiber 127 and the window 126. Optionally, a
diverging lens may also be used for coupling the emission to the
fiber 127.
[0078] The various optical components are mounted and secured
inside the substrate support member 16 using a variety of hardware
known to one skilled in the art, and have been omitted in FIG. 1B
for the sake of clarity. Since the substrate support member 16 is
made of a conductive material, e.g., anodized aluminum, the
mounting hardware are either non-conducting or otherwise insulated
from the substrate support member 16. The size of the openings 136,
138 and recessed portions 132, 134 may vary according to specific
design and/or process needs, for example, taking into account
factors such as the optical beam spot size, desired sample areas,
minimal impact on RF bias, and so on. For example, the recessed
portions 132, 134 may have diameters ranging from several
millimeters (mm) to several centimeters (cm), while openings 136,
138 may have diameters up to about one centimeter. In one
embodiment, an opening with a diameter of about 7 mm is used with a
beam spot size of about 2 mm. Other design alternatives may include
providing a conductive grid or conductive transparent coating on
the windows in order to minimize potential impact on the RF bias to
the substrate support member 16.
[0079] The endpoint detection system 164 can be configured to
detect patterns disposed in any region of the substrate surface.
Depending on the specific endpoint detection technique, the
patterns on the substrate may be any suitable device features on
the photomask, or they may be test patterns with specific feature
design or dimension to facilitate endpoint detection. For example,
such test patterns may be line/space patterns with a single or
varying pitch and/or linewidth.
[0080] FIG. 1C is a schematic top view showing one embodiment of
the relative positions of openings 136, 138, windows 124, 126,
substrate support member 16 and the substrate 220. The side or edge
220E of the substrate 220 extends beyond the edge 16E of the
substrate support member 16. As shown, the separation between the
peripheral region 227 and the central region 225 of the substrate
220 is indicated by a dashed line. Opening 138 is used for
monitoring endpoint in the central region 225. Although opening 136
covers an area that includes both the peripheral region 227 and the
central region 225 of the substrate 220, it can still be used for
endpoint monitoring purposes, e.g., to obtain etch uniformity
information, among others. Alternatively, if the endpoint
monitoring through opening 136 is based on a signal from a specific
test pattern provided in the peripheral region 227, such endpoint
monitoring can be effectively performed, as long as the detected
signal is substantially free from interference that might arise
from features in the central region 225 that are within the field
of view of opening 136. In general, to avoid undesirable
interference, test patterns are provided at locations sufficiently
separate from features in the central region 225 of the substrate
220. In one embodiment, one or more test patterns are provided at
distances up to about 10 mm from the edge 220E of the substrate
220, and openings are provided at corresponding locations of the
substrate support member 16 for endpoint monitoring.
[0081] FIG. 3 illustrates a top view of a 6-inch square substrate
with various locations in the peripheral region for endpoint
monitoring. In one embodiment, endpoint detection is performed
based on the monitoring of one or more test patterns 330 disposed
in the peripheral region 315 or at the corners 325 of the
substrate, and the endpoint detection system 164 may be disposed
directly below these regions of the substrate. For example, with a
6 inch by 6 inch substrate, the windows of the endpoint detection
system 164 may be disposed at least about 2.6 inches, such as
between about 2.6-2.9 inches, from a horizontal center line 310 of
the substrate 220 and at least about 2.6 inches, such as between
about 2.6-2.9 inches, from a vertical center line 320 of the
substrate 220, as illustrated in FIG. 3. The window 112 is
generally located at the intersection of lines 310, 320. Windows
112A are generally located less than 2.6 inches from the center in
the plane of the substrate support member 16 for monitoring areas
within the central region 225 of the substrate. In one embodiment,
the test pattern has a size that is about the same or larger than
the beam spot.
[0082] The light beams reflected from each substrate having the
same test patterns are configured to have the same waveform
patterns when detected by the endpoint detection system 164. In
this manner, the waveform patterns derived from the same test
patterns may be used to determine whether the chamber is operating
according to a particular process recipe, and whether the desired
etch results are obtained for different substrates.
[0083] While test patterns or various dimensions and/or designs can
readily be provided in the peripheral region, the placement of such
patterns in the central region of the photomask is much more
restrictive. Thus, the availability of features for endpoint
monitoring in the central region usually depend on the device
design and layout on the photomask. If the monitored area does not
provide sufficiently strong optical signal for monitoring, e.g.,
due to insufficient open areas, alternative optical configurations
may be used to increase the field of view or to provide multiple
sampling areas. Such alternatives may include the use of optical
components, e.g., lenses and fibers, with higher numerical
apertures (NA), including fibers with tapered ends or the use of
fiber bundles to sample different areas. The use of larger NA
optics allows the sampling area to be increased without necessarily
increasing the size of the window. The use of multiple fibers
(e.g., fiber bundle) allows optical signals to be monitored at
different areas of the substrate. Depending on the specific
features and detection techniques, signals from these different
areas, such as different locations across the center region of the
substrate, may be added together to provide an improved signal, or
the different signals may be compared with each other and the best
one selected for use in endpoint detection. In most embodiments,
the collection optics is configured to sample optical signals in a
direction substantially perpendicular to the plane of the
substrate. In another embodiment, the collection optics may also
sample signals from an oblique view angle, i.e., not perpendicular
to the substrate. This oblique viewing configuration will also
result in an increased sampling area compared to the perpendicular
configuration using the same collection optics.
[0084] In the transmission mode of operation, the endpoint
detection system 164 monitors the transmittance (e.g., total light
intensity) or optical emission signals (e.g., wavelength-resolved
emission) as a function of time. In one embodiment, the plasma in
the chamber 10 serves as the light source for the optical emission
monitoring. This configuration has the advantage of a simpler
optical setup compared to the reflection mode, because it does not
require an external light source and only one optical fiber is
needed.
[0085] The plasma emission typically includes light at discrete
wavelengths that are characteristic of various species present in
the plasma. For example, emission can be monitored at one or more
wavelengths that correspond to one or more etchant/reactant or etch
product species. At the etch endpoint, e.g., when a certain
material layer is completely etched and an underlying layer is
exposed, the monitored emission intensity changes according to
whether there is an increase or decrease of the emitting species
being monitored. In general, the optical emission detection
apparatus 150 of the endpoint detection system 164 comprises light
collection assembly 152, a wavelength dispersive element 156 and a
photodetector 158. In one embodiment, the light collection assembly
152 includes an optical fiber 153, and optionally, a lens 154 for
coupling the optical signal to the fiber 153. The wavelength
dispersive element 156 may be a spectrometer for separating the
optical signal 178 into its component wavelengths. In other
embodiments, the light collection optical assembly 152 may include
various bulk optical components such as lenses and mirrors, and the
wavelength dispersive element 156 may be a variety of filters to
pass a selective range of wavelengths. Depending on the specific
arrangements, the photodetector 158 may be configured to detect
optical signals at a specific wavelength, or it may detect the
signals at different wavelengths simultaneously. Suitable
photodetectors may include a photodiode, photomultiplier tube or a
charged-coupled device, among others.
[0086] Although the embodiment in FIG. 1A shows different optical
signals from windows 110 and 112, e.g., reflection and transmission
signals, coupled to different optical components of the endpoint
detection system 164, the two signals monitored through windows 110
and 112 may also be the same type of optical signals, e.g., both
being reflectance signals or transmittance signals, and so on. In
addition, the two optical signals from windows 110 and 112 may be
coupled to the same photodetector. For example, if an imaging
photodetector is used, a plasma emission signal from one window may
be imaged onto a first set of detector elements or pixels of the
detector, and the other emission signal from the second window may
be imaged onto a second set of detector elements or pixels of the
same detector.
[0087] Furthermore, even though FIG. 1A shows only windows 110 and
112 as being disposed in the substrate support member 16, while
other optical components are shown as external to the substrate
support member 16, such depiction is partly illustrative, and
partly for the sake of clarity in the figure. It is understood that
one or more optical components, e.g., windows, optical fibers,
lenses, photodetectors, among others, of the endpoint detection
system 164 may also be disposed or embedded in the substrate
support member 16, or be integrated with the optical access window
110 or 112. Other combinations of different optical measurements
and configurations of signal detection can also be advantageously
used for endpoint monitoring at two or more locations of the
substrate.
[0088] In another embodiment, the use of an external light source
190, in conjunction with or in place of the plasma source, can
expand the capabilities or provide advantages for transmittance
measurements. For transmission mode, the external light source 190
will be coupled into the chamber 10 through a window 192 provided
on the ceiling 13. The use of the external light source 190 for
transmittance measurements has an advantage over the plasma source
because it can provide a more stable signal than plasma emission,
which may be subjected to fluctuations arising from the etch
process. The external light source 190 may be configured to allow
monitoring at selected wavelengths that are free from potential
interferences from the plasma species. Similar to light source 166,
the external light source 190 can also be operated in a pulsed mode
to allow for various signal processing options for enhancing
endpoint detection capabilities, e.g., by subtracting out possible
fluctuations from plasma emission, and so on. Details for pulsed
source operation with light source 190 are similar to those
previously described for source 166. Other embodiments may involve
the use of a pulsed source for both reflection and transmission
measurements. In another embodiment, the external light source 190
may be provided through an optical access window (not shown) in the
substrate support member 16, and the transmission signal monitored
through the window 192.
[0089] As an example of reflectance monitoring, output from the
light source 190 is coupled via a fiber 194 to pass through the
window 192 onto the substrate 220 such as a photomask. Reflected
light (e.g., off a feature on the photomask) is collected by a
collimating lens 196 and coupled into another fiber 197 leading to
a broadband spectrometer detector 198. The spectrometer 198
separates the light into its wavelength components, e.g., about 200
nm to 800 mm, to record a first spectrum.
[0090] A second spectrum is collected with the pulsed source off.
This provides a background spectrum which can then be subtracted
from the first spectrum. The difference spectrum, which includes
contribution from the reflected light only, and will not be
affected by plasma light. This sequence of collecting two spectra
is repeated for each data point during the etch process. As a
result, any changes in the plasma will not affect the measured
reflectance, as might happen if the plasma emission is relatively
intense.
[0091] Since the substrate (photomask) is a dielectric, e.g.,
transparent, the reflection measurement setup with background
subtraction can be performed from either side of the substrate 220.
That is, the fiber bundle and collimating optics can be placed on
the ceiling 13 for collecting a signal from the substrate 220
through a ceiling window 192, or they can be placed below the
substrate 220 for monitoring from the backside of the
substrate.
[0092] The latter configuration of endpoint monitoring from below
(i.e., through the substrate) offers at least two advantages.
First, in the case of an absorbing layer being etched, such as Cr,
the optical signal from the backside of the substrate will be less
affected by changes in the thickness of the photoresist masking
layer when viewed from below the substrate compared to viewing from
above the substrate top surface. Second, for certain applications,
a small optical sampling area is desired. For example, with quartz
etch, interferometry is most accurate when measured within a
designated test area with a uniform pattern. Thus, the use of
backside monitoring in which the collimating optics are close to
the substrate enables a smaller optical beam to be used than one
that would originate from the ceiling of the chamber.
[0093] This subtraction technique can also be applied to
transmission measurements, in which the light source and the
detector are on opposite sides of the substrate being processed.
This might entail a window in the ceiling and a window in the
substrate holder, and separate optics for collection.
[0094] FIGS. 4A-C illustrate various structures during the
fabrication of a photomask substrate that may be monitored by
different endpoint detection techniques. FIG. 4A shows a binary
photomask structure 410 with a patterned photoresist 416 for
etching a metal-containing layer 414, e.g., a chrome layer
comprising chromium oxide and chromium, which is disposed over a
glass or quartz layer 412. The endpoint for etching the chrome
layer 414 can be monitored either in reflection or transmission
mode, and reflectance, transmittance and/or optical emission
measurements can be performed.
[0095] For example, an incident optical beam 402 from the endpoint
detection system 164 may be directed, through one of the windows in
the substrate support member, onto one area of the photomask
substrate 410. A return beam 404, arising from the interaction
between the incident beam 402 and the photomask structure 410,
e.g., reflecting off the back surface of chrome layer 414 (or
interface between the chrome layer and the quartz layer), is
detected by the photodetector 170 of the endpoint detection system
164. At the etch endpoint for the chrome layer 414, the reflectance
signal decreases because the chrome layer in the open areas 415
(where there is no photoresist) of the photomask is removed,
resulting in a loss of the reflected beam from these areas, as
shown by the dashed arrow 405. Furthermore, diffraction analysis of
the reflection spectrum may be performed to estimate the etch
profile of a chrome feature, and to terminate the etch process when
the foot of the chrome feature is cleared. Such analysis will allow
the control of the etch profile of the feature.
[0096] In the transmission mode, the optical emission signal, e.g.,
from the plasma, passing through the open areas 415 is monitored.
In one embodiment, the total intensity of the emission, i.e., the
transmittance, may be measured. In another embodiment, the emission
may be coupled to a wavelength dispersive element and signals
monitored at one or more selected wavelengths. Towards the end of
the chrome etch when the remaining chrome thickness is relatively
small, the chrome thickness can also be estimated from the
transmission signal.
[0097] FIG. 4B shows another photomask structure 420 during the
fabrication of an attenuated phase shift mask. The structure 420
has a phase shifting material layer 428, e.g., molybdenum silicide
(MoSi), formed over a quartz layer 422. A chrome layer 424 is
deposited on top of the MoSi layer 428, followed by a photoresist
layer 426. The photoresist layer 426 is patterned and used as an
etch mask for the chrome layer 424. The molybdenum silicide (MoSi)
layer can then be etched with either the patterned photoresist
layer 426 acting as a mask, or with the patterned chrome layer 424
as a hardmask (after stripping of the photoresist layer 426).
Similar to chrome etching, the endpoint for MoSi etching can be
monitored in either reflection or transmission mode, and
reflectance, transmittance, or optical emission measurement can be
performed. Since MoSi is partially transmitting, interferometric
measurements can also be used for endpoint monitoring.
[0098] FIG. 4C shows another mask structure 430 for fabrication a
quartz phase shift mask, with a patterned chrome layer 434 serving
as a hard mask for etching the underlying quartz layer 432. The
original, or pre-etch, top surface 436 of the quartz substrate 432
is shown as a dashed line in FIG. 4C. In this case, the quartz
layer 432 has to be etched down to a certain predetermined depth d,
below the original surface 436. By operating the endpoint detection
system 164 in reflection mode, the return beam 178 at a particular
wavelength can be monitored as a function of time to provide
interferometric data, e.g., the appearance of fringes arising from
optical interference between different portions of the reflected
beam 178 that travel through different thicknesses of a material
layer. For example, one portion 402A of an incident optical beam is
reflected off an open area of the photomask 430, while another
portion 402B of the incident optical beam is reflected off a masked
area of the photomask 430, e.g., an area with a chrome
layer/feature 434. Interferences between the two reflected portions
405A and 405B produce interference fringes (i.e., intensity
modulations) that are indicative of the difference in quartz layer
thickness traversed by these portions 405A, 405B. By monitoring the
interference fringes in the reflected beam, the etch depth d.sub.1
can be obtained. In one embodiment, interferometric endpoint
monitoring is performed in a pulsed mode, as previously described
in connection with light source 166 in FIG. 1A. In general, any
narrow band source may be suitable for interferometric monitoring.
Thus, it is also possible to use the plasma as a light source for
interferometric monitoring, as long as the plasma emission has a
sufficiently narrow bandwidth for this purpose.
[0099] FIG. 5 shows three optical signals monitored simultaneously
as a function of time during the etching of a Cr mask using the
endpoint detection system. The chrome layer is etched using a
plasma containing chlorine and oxygen gases. The top trace 510 is
obtained by monitoring an emission signal originating from Cr, for
example, either by directly monitoring an atomic line from Cr e.g.,
at a wavelength of 520 nm, or by monitoring the Cr emission line
and a chlorine line (e.g., 258 nm) and taking a ratio of the Cr:Cl
emission signals. Typically, the signal to noise can be improved by
taking a ratio of emission signals of etch products to reactants
(or vice versa). As the chrome etch approaches endpoint, the
concentration of chromium-containing species (etch products) in the
plasma decreases, resulting in a corresponding change in the Cr
emission signal (or Cr:Cl emission ratio), as shown at point 512 of
the top trace 510. In general, the optical emission signal can be
monitored through one or more windows in chamber 10, e.g., those
provided in the substrate support member 16 or in the ceiling, by
looking directly at the plasma. In addition, a side window 193 may
be provided in the chamber wall for detecting the plasma emission,
for example, by coupling the emission to an optical emission
detector system 195, as shown in FIG. 1A. The emission monitoring
through the sidewall window 193 may be performed in conjunction
with endpoint monitoring through one or more other windows.
[0100] The middle trace 520 is obtained by monitoring a reflection
signal originating from light reflecting off the bottom surface of
the chrome layer, similar to that shown in FIG. 4A. At the chrome
etch approaches endpoint, the chrome layer in the open areas 415 of
the mask becomes thinner as the chrome is etched away, which
results in a decrease in the monitored reflectance signal
intensity, as shown in the portion 514.
[0101] The bottom trace 530 is obtained by monitoring the
transmittance. As shown in portion 516, the transmittance signal
intensity increases towards endpoint when the chrome layer in the
open areas 415 of the mask is removed, allowing the emission to be
transmitted through the quartz layer in these areas.
[0102] The use of these optical measurement techniques, coupled
with monitoring at two or more locations of the substrate, allows
improved process control by providing enhanced endpoint detection.
In one embodiment, the endpoint detection system is configured to
operate in both the reflection and transmission modes. For example,
referring back to FIG. 1A, a transmission signal (e.g.,
transmittance or plasma emission) is detected through window 112
for monitoring an area in the central region of the substrate, and
a reflection signal (e.g., reflectance or interferometric) is
detected through window 110 for monitoring an area in the
peripheral region of the substrate. Monitoring the central region
of the substrate in the transmission mode is advantageous because
the alignment requirement between the etched features and the
access window is less stringent than the reflection mode, and
furthermore, a larger area can be monitored.
[0103] Thus, one embodiment of the present invention provides a
method that can be implemented using the apparatus of this
invention. In one embodiment, the apparatus of this invention
includes a computer readable medium containing instructions, that
when executed by the controller, such as the computer 172 or other
processor suitable for controlling an etch reactor as commonly
known in the art, cause an etch chamber to perform a method such as
that shown in FIG. 6. It is contemplated that the computer readable
medium may be stored in the memory of the computer 172, which also
includes support circuits and processor. The method 600 starts at a
step 602 where an etch chamber is provided with a substrate support
member having a first window and a second window disposed
respectively in a center region and a peripheral region of the
support member. A photomask is provided on the support member in
step 604, and a process gas is introduced into the chamber in step
606. Halogen-containing gases are typically used for etching
different materials found on a photomask structure. For example, a
process gas containing chlorine may be used for etching a chrome
layer, while a fluorine-containing gas such as trifluoromethane
(CHF.sub.3) or tetrafluoromethane (CF.sub.4) may be used for
etching quartz. In step 608, a plasma is generated from the process
gas, and in step 610, a first and second optical signals are
detected through the first and second windows, respectively. In
step 612, the plasma in the chamber is terminated based on
information obtained from at least one of the two detected optical
signals. Furthermore, based on the etch profile results such as
center to edge uniformity, process parameters such as etchant gas
composition, flow rate, coil bias, and so on, can be adjusted for
optimization of the process.
[0104] By applying one or more optical measurement techniques for
simultaneous monitoring at different locations of the substrate,
embodiments of the present invention provide an improved apparatus
and method with enhanced process monitoring and control
capabilities. These improvements also allow reliable endpoint
detection for photomask etching applications with low open areas.
For example, optical emission endpoint detection has been
demonstrated for etching photomasks with open areas down to about 3
percent for chrome and about 1 percent for molybdenum silicide, and
reflectometry has been demonstrated for low open area chrome and
quartz etching for phase shift mask applications. Aside from
providing information for center to edge etch uniformity, etch rate
variations arising from areas with different pattern densities can
also be obtained by monitoring multiple optical signals using the
endpoint detection system of the present invention. For example,
test patterns with different feature size or pattern densities can
be provided in different areas of the peripheral region of a
photomask and the monitored optical signals can be used for
assessing or determining the proper etch endpoint for pattern
densities of interest.
[0105] It is also contemplated that a single window may be utilized
in the substrate support to provide substrate monitoring.
Particularly, features described above may be utilized with a
single window to enhance substrate monitoring over conventional
systems having a single window endpoint detector.
[0106] In another embodiment of the invention, an etch process
monitoring system is provided with a direct way to view plasma
(e.g., side window) and direct way to view etching through the
workpiece at one or more locations in the cathode (e.g., cathode
windows under the photomask or wafer). Different combinations of
these signals for process control can be used. In one embodiment,
the etch process monitoring system includes cathodes with 3 or 4
windows, with 2 or 3 windows in the peripheral region of the
etching area, a CCD endpoint system configured to simultaneously
collect 2 channels of information one side OES (optical emission
spectroscopy), and one "bottom" (through the photomask). The side
oes signal is indicative of the state of etching averaged over a
large area of the mask, as the entire upper surface of the mask is
exposed to the plasma. The "bottom" fiber-optic cable can be placed
under any of the 3 or 4 windows in the cathode. Typically this
installation is static and the fiber optic cable is fixed in either
the center or one of the edge locations. The bottom fiber cable is
kept electrically insulating to not transmit bias RF energy to the
endpoint system.
[0107] The side OES signal arises from a large area of the mask,
while the bottom signal arises from a localized area under the mask
determined by the optical configuration. Typically this region is
of the order of 2-5 mm, but could be changed by altering the
design.
[0108] The bottom signal can be reflection (by using a light
source, called interferometric endpoint "IEP") or transmission
(plasma as the light source, called transmission endpoint "TEP").
Although a light source and associated fiber cable may be used,
alternatively transmission mode (TEP) may be used which utilizes
light provided by the plasma signals detected through the
photomask.
[0109] Examples and benefits include: [0110] 1. Confirmation of
reaching both OES endpoint and bottom endpoint for greater
reliability, especially for low-open area etch applications. For
example, OES and bottom endpoint may be used to detect process
drift and/or inaccuracies in one of the endpoint methods. [0111] 2.
Process uniformity evaluation and monitoring to center-fast or
center-slow etch conditions by comparing endpoint times for the TEP
center and OES (average). [0112] 3. Similarly comparing an edge or
corner endpoint time to OES to determine left-right or top-bottom
etch rate pattern differences. [0113] 4. Normalizing the TEP signal
through the mask by dividing the TEP signal by the OES signal.
[0114] 4a. This normalization provides for a true transmission
measurement, largely independent of plasma brightness and
fluctuations. [0115] 4b. The normalization also allows for a
comparison between the measured spectral transmission of the mask
and a real-time model for the transmission, thereby allowing
determination of the etching layer thickness (e.g., Cr layer)
during etch. [0116] 4c. The normalization also allows for a
comparison between the measured spectral transmission of the mask
and a real-time model for the transmission, thereby allowing
determination of the masking layer thickness (e.g., photoresist)
during etch. [0117] 4d. Determination of etch selectivity by
dividing Cr etch rate (4b) by the PR etch rate (4a). [0118] 4e. For
other applications such as MoSi etch, the MoSi etch rate can also
determined in a similar fashion.
[0119] Note that all the transmission and/or reflection embodiments
described in above can be utilized herein with reference to
comparing to a direct view OES signal. It should also be noted that
embodiments described herein may be useful for endpoint monitoring
in photomask deposition applications MEMS through-wafer etching,
infrared monitoring/process control of either deposition or etching
of silicon wafers and infrared band-edge wafer temperature
measurements.
[0120] In another embodiment, improved etch process control is
facilitated by monitoring real-time transmission of the film on the
photomask being etched. The absorbing layer (e.g., Cr) has a small
but measurable transmittance at the start of etching (typically 1%
to 15%, depending on the film type), that increases in a
predictable way as the film gets thinner during etching until it is
gone completely (100% transmission) at the etch endpoint. A single
optical fiber bundle is placed beneath a window under the photomask
to collect the increasing plasma light. Typically, the viewing
region is of the order of 2-5 mm, which could be changed by
altering the design. This configuration may be referred to as
"transmission endpoint" (TEP).
[0121] Benefits of TEP include endpoint based on actual optical
clearing of absorbing film being etched. Provided the location of
the window is under an area of film being etched, endpoint may have
better reliability than OES, especially for low-open area etch
applications. The endpoint system can utilize plasma as a light
source, obviating the need for an external light source. With
plasma light source, a wide field of view can be used, thereby
minimizing the size of the opening in the cathode. The embodiment
can be as simple as placing an optical fiber near the cathode
window, without any additional optics.
[0122] TEP may be advantageously used for chromium and other etch
applications. Such applications may include an optic fiber
positioned under the photomask to detect an increase of plasma
light passing through the chromium layer as it is etched. A light
source may also be used to monitor reflection, although the single
change is somewhat smaller than the TEP signal, and as such, TEP
provides better resolution during chromium applications.
[0123] TEP may also be utilized in quartz etch applications. In
such applications, optical interferometry may be used. Transmission
interferometry can be used to monitor the etch rate and endpoint.
The plasma is used as a light source so no lamp is required. The
endpoint transmission may be normalized, as discussed above, by
dividing the TEP signal by the OES signal obtained through a side
window formed in the chamber to reduce signal enhances caused by
changes in the plasma. In applications wherein the plasma is
sufficiently stable as to provide a steady light source, no
background subtraction is required. Reflection interferometry may
also be utilized for **quartz etching which requires a light
source. The light source may be a steady lamp, for example, in the
UV region or brighter than the plasma background. Examples of such
suitable light sources include deuterium lamp, a high intensity
discharge lamp (HID), an arc lamp and a solid state UV LED lamp.
The light source may also be switched on and off so that the
contribution to the signal from the plasma background may be
subtracted from the signal, thereby providing a more accurate
signal indicative of the endpoint.
[0124] Additionally, still larger areas of the photomask can be
sampled. For lamp configurations, collimation optics may be used.
For plasma as the light source, a simple field-of-view cone in the
cathode may be used, or diverging lenses added. For either lamp
configurations, a scanning detector may also be used. If the
optical window in the cathode is large, a metallic grid may be
placed over the window or a transparent conductive film (e.g., ITO
or ZnO) can be used to maintain the RF bias needed for processing
the substrate.
[0125] In one embodiment, an exemplary detector is illustrated in
FIG. 8. The detector can be a single wavelength detector, such as a
photodiode PMT with filter or monochromator. Each window in the
substrate support may be coupled to a separate detector by a fiber
optic cable. The detector may also be a multi-wavelength detector
such as a spectrometer. The spectrometer may be imaging so that
individual portions of the fiber bundle can be treated as separate
spectrometers. More than one spectrometer may be used to
accommodate simultaneous multiple data collections from different
locations.
[0126] In an exemplary embodiment depicted in FIG. 8, a photo
detector 800 is shown interfaced with a plurality of windows 804
positioned in the substrate support below a reticle (not shown) by
a fiber optic bundle 802. The signals (reflective and/or
transmissive) from each window 804 enter the photo detector 800
through a port 806. The signals in the photo detector 800 are
interacted with a wavelength-dispersive element 810, such as a
grating or prism, prior to interacting with a spectrometer 808. The
signals from each fiber bundle 802 may be provided to a single
spectrometer 808, or the signal from each window 804 may be
analyzed separately, by sequentially providing the signals to a
single spectrometer, or by providing each signals to a separate
spectrometer.
[0127] A specific implementation may includes cathodes with 3 or 4
windows, with 2 or 3 windows in the peripheral region of the
etching area, one in the center, a CCD endpoint system configured
to simultaneously collect 2 channels of information one side OES
(optical emission spectroscopy), and one "bottom" (through the
mask). The "bottom" fiber-optic cable can be placed under any of
the 3 or 4 windows in the cathode. This installation is static and
the fiber optic cable is fixed in either the center or one of the
edge locations. The bottom fiber cable is kept electrically
insulating to not transmit bias RF energy to the endpoint system.
These embodiments may be useful for photomask deposition
applications, other substrate (e.g., wafer) etch applications, MEMS
through-wafer etching, infrared monitoring/process control of
either deposition or etching of silicon wafers and infrared
band-edge wafer temperature measurements.
[0128] One example of such a configuration is illustrated in FIGS.
9-10. FIG. 9 is a schematic diagram of an etch reactor 900 suitable
for etching a photomask reticle 902. The etch reactor 900 is
coupled to an endpoint detection system 904 which monitors etching
of the reticle 902 through windows disposed through the substrate
support 906. The substrate support 906 of the etch reactor 900 is
coupled to an RF generator 910 through an RF probe 908. The RF
probe 908 is coupled to a controller 912 configured with process
state monitoring software that actively controls the etch process
performed in the etch reactor 900.
[0129] The substrate support 906 includes a plurality of windows
through which signals indicative of etch rate and/or endpoint are
provided to the endpoint detection system 904. FIG. 10 depicts a
top view of the substrate support 906 illustrating the distribution
of a center window 1002 and edge windows 1004 formed in the top of
the substrate support 906 within the area covered by the reticle
902 during processing. Corner windows and/or windows in other
locations are contemplated. In the embodiment depicted in FIG. 10,
the edge 1004 windows are positioned below the peripheral area of
the reticle 902 as described above.
[0130] Referring back to FIG. 9, optical fibers 912 are positioned
below each window so that endpoint signals (transmissive and/or
reflective) may be provided to the detection system 902. An optical
fiber 914 is positioned to view the plasma through a window formed
through the side of the etch chamber 900 to provide OES information
to the detection system 904. In the embodiment depicted in FIG. 9
the fiber 914 is coupled to a first detector 916, such as a
spectrometer, while the fibers 914 are coupled to at least one
second detector 918. The second detector 918 may be configured as
described with reference to FIG. 8 or other suitable manner. A lamp
920 may optionally be provided to provide reflective signals. The
signals may be analyzed by a dedicated endpoint processor 922, such
as a PLC or other processor. The endpoint controller 922 is in
communication with the controller 912 configured with the process
state monitoring software to provide real time etching and/or
endpoint information. Optionally, at least one of the controllers
932 or processor 922 is coupled to a front end server 924 and/or
host controller 926 to allow integrated metrology information
sharing between the production and other tools within the facility.
The detector 920 may be configured to have up to three inputs.
Additionally, the detectors may be synchronized to all data to be
viewed as taken from a single detector.
[0131] FIG. 11 depicts graphs of average (side OES) endpoint, left
endpoint and top endpoint. Trace 1102 depicts the average endpoint,
while traces 1104 and 1106 depict the endpoint signals respectively
obtained at the left and top windows. The traces 1104 and 1106
illustrate a slight lag in the endpoint of the top location, while
the trace 1102 illustrates the average endpoint taken using side
OES.
[0132] FIG. 12 is illustrative of the ability of two spectrometers
to provide information suitable for monitoring process uniformity.
FIG. 12 depicts a first trace 1202 representing the average
endpoint taken using side OES. A second trace 1204 represents the
time to etch in the center of the photomask reticle. The third and
fourth traces 1206, 1208 represent the time to etch in the left
edge and top edge of the photomask reticle. A fifth trace 1210
represents the time to etch in the corner of the photomask reticle
and is shown with the edge traces 1206, 1208. As shown, the
endpoint signals may be utilized to determine which area is etching
faster and/or clears faster than another area. Such information is
useful for adjusting the etch process recipe for the next
substrate, or proving such information for adjusting processes
performed on the substrate from which the endpoint data was
obtained to better control and/or correct the process results.
[0133] FIG. 13 depicts endpoint signal data obtained from two
periods for the same etching cycles. Graph 1300A depicts a trace
1302 of an OES signal obtained through the photomask and a trace
1304 of an OES chamber signal obtained through the window disposed
in the side of the chamber. By normalizing the data, e.g., dividing
the OES through mask signal by the OES chamber signal, a trace 1306
of the normalized endpoint signal is generated. The data obtained
in graphs 1300A and 1310A are obtained after thirty seconds of
etching. The data shown in graphs 1300B and 1310B include data
taken after 380 seconds of etching. Again, graph 1300B includes a
trace 1322 of an OES signal taken through the mask and a trace 1324
of an OES chamber signal. The normalized signal is shown in FIG.
1310B by trace 1326. FIG. 14 depicts a flow chart of a method 1400
for monitoring an etch process using normalized transmission, such
as described with reference to FIG. 13. The method 1400 for
monitoring an etch process may use a normalized transmission
spectrum to measure the chromium thickness and/or resist thickness
from an optical thin film model. The process 400 provides
information relating to two separate areas, chromium etch rate and
photoresist etch rate. The chromium layer generally absorbs the
light when having greater than a predetermined thickness. The
transmission of light through the chromium layer increases rapidly
as the thickness becomes less than about 20 nanometers. The change
in transmission is nearly flat after the chromium clears. The
photoresist is largely transparent and shows thin film
interference. The method 1400 begins at block 1402. The process
begins at block 1402 wherein light transmitted through a workpiece
being etched (e.g., a photomask reticle) is measured by a detector.
At block 1404, the transmitted signal is divided by the emission
signal taken through the side window to calculate a normalized
transmission. At block 1406, the normalized transmission is
compared with a thin film optical model to calculate real time
thickness. The information obtained at block 1406 may be utilized
to analyze at least one of the chromium and/or photoresist
thickness and/or etch rate. At block 1408, the transmission data
obtained through the photoresist/chromium stack is analyzed. At
block 1410, the photoresist thickness in etch rate is determined
using the data analyzed at block 1408. Alternatively, or in
addition to the photoresist analysis performed at blocks 1408,
1410, the chromium layer may be analyzed at blocks 1412 and 1414.
At block 1412, the transmission data obtained through the chromium
layer is analyzed. At block 1414, chromium thickness and/or etch
rate is determined through the data analyzed at block 1412. This
method can also be applied to etching other materials such as MoSi
to determine a MoSi thickness and/or etch rate.
[0134] In addition to the embodiments described above, a method of
making endpoint detection more reliable is also provided. In one
embodiment, the reliability of endpoint detection may be improved
by eliminating thin film interference from the photoresist masking
layer. For example, optical monitoring of etching may be confounded
by the signal rising from etching of the mask rather than of the
layer being etched. Referring to FIG. 15, thin film interference
occurs between the top and bottom of the masking layer,
specifically by components R2 and R3 for reflection and T2 and T3
for transmission modes. The interference may be substantially
eliminated by using a wavelength regime in which the masking layer
is absorbing, such as a deep UV wavelength. Optical monitoring of
etching is then determined by the layer being etched and from its
exposure fraction. Thin film interference between the top and the
bottom of the masking layer is substantially eliminated by using
wavelength from a light source that is absorbed by the photoresist,
as shown by the absence of the components of R3 and T3.
[0135] Alternatively, all optical transmission and reflection
signals clearly show endpoint during etching of an unpatterned
workpiece (e.g., a photomask or wafer), a patterned etch mask can
create difficulties in determining endpoint. The optical signal can
be confounded by the presence of thin film optical interference
caused by concurrent thinning of the masking layer while the
etching layer is etched. Transmission is more immune to this
problem than reflection, especially if the etching layers are
substantially opaque, as in the case for thick chromium layers,
e.g., chromium layers having a thickness greater than 100 nm. As
technology moves to thinner layers, specifically layers of chromium
having a thickness less than 50 nm, inherent absorption is reduced
and the advantage is accordingly reduced as well. For the case of
phase shifting photomasks with an additional absorbing layer such
as MoSi, the advantage is further reduced. However, by choosing a
deep UV wavelength to monitor reflection and transmission, the
confounding interference from the masking layer is virtually
eliminated. This occurs when the optical length is short enough
that the absorbance of the masking layer is significant, thereby
spoiling the interference. This will occur for light at wavelengths
less than 240 nm for DUV resist, such as FEP 171.
[0136] FIGS. 16, 17 and 18 are illustrative of the optical
interference of the photoresist. FIG. 16 is a schematic showing the
optical interference components T2 and T3. FIG. 18 depicts the
magnitude of the transmission signals T1, T2 and T3. FIG. 18
illustrates the measured transmission signal depicted in the graph
of FIG. 17 as being the superposition of the transmission signal T1
and the vector added signals T2 and T3.
[0137] FIGS. 19, 20 and 21 are illustrative of an etching process
utilizing an endpoint monitoring signal by using a wavelength that
is absorbed by the photoresist layer. As depicted in FIG. 19, the
incident light absorbed by the photoresist masking layer will not
generate an interference between the top and the bottom of the
masking layer, specifically components R3 and T3 are eliminated.
Thus, the resultant signal monitored by the detector, as shown in
the graph of FIG. 20, is easier to analyze. As the chromium layer
being etched becomes less than 20 nm, the signal rises faster until
the chromium layer clears and the transmission signal becomes flat,
and as such, the resultant signal obtained by the detector is
indicative of thickness and/or presence of photomask masking layer
and/or chromium layer as seen in FIG. 21. Thus, the endpoint of the
chromium etch can be identified when the slope of the signal
decreases. The endpoint can be called using a sequence of
derivative calculation and smoothing of the data to determine when
the slope decreases. Such endpoint determination is illustrative in
the endpoint traces provided in FIG. 22.
[0138] The use of deep UV wavelength monitoring signals is
particularly useful in etch applications having low open area. For
example, FIG. 23 depicts an endpoint signal dominated by optical
interference from the photoresist layer. The endpoint of the
chromium etch is not readily visible from the signal depicted in
FIG. 23. By utilizing deep UV wavelength monitoring signals, the
endpoint is readily ascertainable as the slope of the trace visibly
decreases at the 260 second mark, as shown by trace 2402, as shown
in FIG. 24. By using a derivative of the deep UV endpoint signal,
the endpoint is also ascertainable at the 260 second mark, as
indicated after the peek signal of trace 2404, as shown in FIG.
24.
[0139] The use of fiber optics can also improve deep UV endpoint
applications. Conventional UV transmitting fiber optics generally
attenuate the signal at wavelengths below 235 nm. Non-solarizing
fibers may be utilized to improve transmission below 235 nm
wavelengths. Thus, the signal to noise ratio would increase,
thereby extending the endpoint capability to smaller, open
areas.
[0140] FIG. 25 illustrates a processing chamber 2500 having a
photomask reticle 2502 supported on a pedestal 2504. The pedestal
has one or more windows 2506 through which the bottom of the
photomask reticle 2502 may be viewed by an endpoint detection
system 2508. The endpoint detection system 2508 includes a side
fiber optic bundle 2510 which views a plasma 2512 disposed in the
chamber 2500 through a window 2514 disposed in the side of the
chamber. The side fiber optical bundle 2510 provides OES
information of the plasma 2512 to a detector 2516, such as a
spectrometer. A second fiber optic bundle 2518 views the bottom of
the substrate through the window 2506 in the pedestal 2504. In the
fiber optic bundle 2518 are made from non-solarizing deep UV fused
silica for enhanced transmission of signals at wavelengths below
235 nm. Optionally, the endpoint detection system 2508 may include
a lamp 2520 to provide light through the fiber bundle 2518 to
obtain information in a reflection mode.
[0141] FIG. 26 depicts a graph illustrative of the benefits of the
choice of wavelengths for photoresist absorption. The graph
illustrates that for wavelengths below 240 nm, the interference
fringes disappear because the higher absorption constant eliminates
multi-path reflection. Thus, the selection of the proper wavelength
for use as an endpoint detection monitoring a vehicle for enhancing
in-situ reflection transmission for endpoint and process
monitoring. Reduced photomask interference facilitates accurate
endpoint determination for low, open area applications, such as
contact patterns. This method also facilitates acquiring
measurement information of the actual process at specific areas,
including discrete local areas, and is not limited to endpoint
detection determined over a large area. This enables the process to
be adjusted to tune the etch rate at specific locations. The use of
deep UV monitoring signals benefits both reflection and
transmission modes of endpoint detection. As such, these techniques
can be extended for use on conventional wafer etching (top
reflection), as well as photomask (top reflection, bottom
reflection and bottom transmission) etching.
[0142] In another embodiment, endpoint hardware (cathode with
multiple windows underneath the etching substrate (e.g.,
photomask)), multiple substrate detection locations can be used in
conjunction with the substrate pattern to determine the substrate
orientation. Once the substrate orientation is known, the existing
process uniformity signature may be modified to improve the final
etch performance. For example, if the substrate has been inserted
in the chamber with a vertical orientation, and the etch pattern
has a top-down component, the process can be dynamically changed to
have more of a side-side component. This will result in improved
etch uniformity. The process change can be performed by modifying
part of the recipe while running. The process change could, for
example, entail use of the dynamic phase adjustment or change in
another processing variable and/or process knob. In another
example, if there is a change in orientation between substrates,
then the process may be adjusted to accommodate the change and
provide between substrate uniformity.
[0143] An example processing sequence (for a photomask substrate)
may include: A) providing a mask pattern having two openings on the
left and right edges; B) providing an endpoint system set up to
simultaneously collect data from window under the left edge and
window under the top edge of the substrate; C) when the mask is
inserted into the tool, the signals from the top and right
locations are analyzed to determine which one is under an opening
that is being etched; D) the recipe can be modified to accommodate
the mask orientation for improved etch performance. Exemplary
benefits of having additional substrate sensors coordinated with
the mask pattern include, an endpoint can be performed despite etch
orientation of the mask; signals from the two perimeter locations
can be summed to form a robust endpoint signal; signals can be
analyzed to determine which one matches the mask pattern and hence
mask orientation; mask orientation can be used to modify the
current recipe to produce an improved process result (e.g., better
etch uniformity).
[0144] In one embodiment, an implementation may include A) new
optical fiber bundle split 3-ways, which allows monitoring two
locations in the cathode (of the existing 4 windows), as well as
OES (optical emission spectroscopy) from a side window; B) The CCD
endpoint system firmware enables simultaneous collection of 3
channels of information. This is accomplished by changing the
mapping of the CCD pixels in the imaging spectrometer; D) three
data streams are analyzed to decide which signal to use for
endpoint (or to sum or otherwise combine the signals to form a
robust endpoint); D) The new algorithm may feed back the mask
orientation to the etch system, and the etch system (e.g. process
recipe) may make a process change during the remainder of the etch
to improve the final result.
[0145] This could be useful for wafer process applications if the
wafer orientation is variable and process results can be improved
by a recipe change based on the determined orientation.
[0146] In another embodiment, etch process uniformity may be
determined by 1) comparing endpoint times from different spatial
locations on the substrate being etched, and/or comparing etch
rates from different spatial locations, and 2) adjusting the
process accordingly.
[0147] The endpoint hardware (three-way optical fiber cable
combined with the cathode with multiple windows underneath the
etching substrate (e.g., photomask)), multiple detection locations
can be used in conjunction with the substrate pattern to determine
the process uniformity. If the monitoring shows some process
non-uniformity, the process can be modified to improve the final
etch performance.
[0148] For example, if the monitored etch pattern has a top-down
component, the process or hardware can be changed to reduce the
top-down component. This can be done as part of a chamber
setup/startup process, or possibly in real time with the process
adjusted during the remainder of the etch.
[0149] The number of locations monitored could be increased by
adding a plurality of windows and a plurality of detectors. In
practice, the existing CCD imaging spectrometer is limited to
detecting three (maximum 7, with added noise) independent signals.
If a second spectrometer is added ("Dual Spectrometer Endpoint"),
then a total of six signals, one OES from the side of the chamber,
and five under the mask, can be detected. A layout of five
locations is useful to get basic top-down, side-side, and
center-fast or center-slow process information. This information is
also useful for wafer process applications if the wafer orientation
is variable and process results can be improved by a recipe change
based on the determined orientation of the substrate (e.g.,
photomask) relative to the substrate support.
[0150] Optical monitoring of etching may often be confused by
signal arising from the mask layer etching, rather than that of the
etch layer itself. By using a wavelength regime in which the
masking layer is absorbing, such as deep UV for the case of
photoresist masking layers, optical monitoring of etching is then
determined by the layer being etched, and from its exposed area
fraction. Optical interference effects from the thinning resist are
then no longer present.
[0151] This embodiment may be described in two parts. The first
part is the selection of an absorbing wavelength for the masking
layer (<240 nm for the case of photoresist). The second part is
the hardware improvement of increasing deep UV transmission in the
optical system to provide the ability to choose deep UV wavelengths
with good signal-to-noise performance. One hardware improvement is
the use of a non-solarizing deep UV optical fiber with better
transmission below 240 nm wavelength. Additionally or
alternatively, the detector may be placed closer to the chamber to
remove the requirement of the transmission-limiting fiber, or use
free-space optics or hollow fiber-optics.
[0152] This method can be used to monitor reflection as well as
transmission, so its use can also apply to absorbing substrates,
transparent etching layers on absorbing substrates. The
transmission and reflection modes can be used for wafers as well as
photomask reticle.
[0153] Additional features of the invention are described in the
attached appendix following the drawings.
[0154] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow. What is
claimed is:
* * * * *