U.S. patent application number 08/873819 was filed with the patent office on 2001-08-09 for micro-mechanical probes for charge sensing.
Invention is credited to FIREBAUGH, SAMARA L., PANGAL, KIRAN, STURM, JAMES C..
Application Number | 20010011887 08/873819 |
Document ID | / |
Family ID | 26692468 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011887 |
Kind Code |
A1 |
STURM, JAMES C. ; et
al. |
August 9, 2001 |
MICRO-MECHANICAL PROBES FOR CHARGE SENSING
Abstract
A method and apparatus for measuring a charge on surface, such
as on a semiconductor wafer, arising during plasma processing is
provided. Such a charge may be measured on an insulating film
applied to such a wafer. By the present invention, the charge on
such an insulator exposed to plasma is measured in-situ using
micro-cantilevers. The micro-cantilevers include an insulating base
positioned on the substrate and a cantilevered beam extending
therefrom to over the substrate. The beam is formed of a conductive
material. A charge on the beam causes an opposite charge to form on
the substrate. The opposite charges attract to move or deflect the
beam towards the substrate. The amount of movement or deflection
corresponds to the magnitude of the charge. This movement or
deflection of the beam can be measured to determine the charge by
bouncing a light source, such as a laser, off of the beam. In
another embodiment, the cantilever includes a flexible bridge
interconnected between the base and a rigid beam. In this
embodiment, the surface of the beam does not bend. Rather, movement
of the beam is accomplished by the bending of the flexible bridge.
This allows for easier measurements of the movement of the beam
because the surface of the beam remains planar.
Inventors: |
STURM, JAMES C.; (SKILLMAN,
NJ) ; PANGAL, KIRAN; (PRINCETON, NJ) ;
FIREBAUGH, SAMARA L.; (CAMBRIDGE, MA) |
Correspondence
Address: |
MICHAEL R. FRISCIA
WOLFF & SAMSON
5 BECKER FARM ROAD
280 CORPORATE CENTER
ROSELAND
NJ
07068
US
|
Family ID: |
26692468 |
Appl. No.: |
08/873819 |
Filed: |
June 12, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60019658 |
Jun 12, 1996 |
|
|
|
Current U.S.
Class: |
324/109 |
Current CPC
Class: |
H01J 37/32935
20130101 |
Class at
Publication: |
324/109 |
International
Class: |
G01R 029/22 |
Claims
What is claimed is:
1. An apparatus for measuring charge on a surface comprising:
insulating base means positioned on the substrate; beam means
extending from the base to over the substrate; wherein when a
charge is formed on the beam, an opposite charge is formed on the
substrate causing the beam to deflect towards the substrate; and
means for measuring the deflection of the beam means.
2. The apparatus in claim 1 wherein the base means and beam means
are microscopic in dimension.
3. The apparatus of the claim 2 wherein the beam means is comprised
of polycrystalline silicon.
4. The apparatus of claim 2 wherein the means for measuring
deflection of the beam means comprises light means for reflecting
off of the beam means to a light position detector means.
5. The apparatus of claim 4 wherein the light means comprises a
laser.
6. An apparatus for measuring charge on a substrate comprising: an
insulated base positioned on the substrate; a flexible bridge
extending from base; a conductive beam interconnected with the
flexible bridge, the beam having a surface charge mirroring the
charge on the substrate, the bridge bending to move the beam
towards the substrate based on the electrical attraction between
the charge on the beam and the charge on the substrate; and means
for measuring the movement of the beam with respect to the
substrate.
7. The apparatus in claim 6 wherein the base and beam are
microscopic in dimension.
8. The apparatus of the claim 7 wherein the beam is comprised of
polycrystalline silicon.
9. The apparatus of claim 8 wherein the means for measuring the
deflection comprises bouncing a light source off the beam to a
light position detector.
10. The apparatus of claim 9 wherein the light source comprises a
laser.
11. A method of measuring a charge on a surface comprising the
steps of: positioning an insulated base on a substrate;
interconnecting a conductive beam with the insulating base, the
beam extending over the substrate; allowing a charge to form on the
substrate in response to a charge on the beam; allowing the beam to
move towards the substrate because of an electrical attraction
between the charge on the beam and the charge on substrate; and
measuring the deflection of the beam.
12. The method of claim 11 wherein the step of measuring the
deflection of the beam comprising reflecting a light source off of
the beam to a light position detector.
13. The method of claim 11 wherein the step of measuring the
deflection of the beam comprises measuring the change in angle of a
light source reflected off the beam.
14. The method of claim 11 wherein the step of measuring the
deflection of the beam comprises measuring defraction of a light
source directed at the beam.
15. The method of claim 11 further comprising positioning a
plurality of beams to extend over the substrate.
16. The method of claim 15 wherein the step measuring the
deflection of the beams comprising inspecting the beams.
17. The method of claim 15 further comprising utilizing beams of
varying size.
18. The method of claim 17 further comprising the step of allowing
the beams to deflect to contact the substrate and stick
thereto.
19. The method of claim 18 wherein the step of measuring the
deflection comprises visually inspecting the stuck beams.
20. The method of claim 11 wherein the substrate is non-conductive
and the method includes the step of applying a conductive layer
over a portion of the substrate prior to positioning an insulated
base on the substrate.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/019,658, filed Jun. 12, 1996.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to a method and apparatus
for measuring an electrical charge on a surface, and more
particularly to a system of one or more micro-cantilevers for
measuring charges on a surface layer on a substrate during
semiconductor processing on semiconductor wafers.
[0004] 2. Related Art
[0005] Plasmas are widely used in the semiconductor industry for
processing to delineate fine line pattern and deposition at low
temperature, e.g. plasma etching. During plasma exposure the wafer
is exposed to a bombardment of ions, electron, photons, and x-rays
which can lead to a charge buildup on the semiconductor device.
Such a charge build-up, known as wafer surface charging, can
degrade or destroy the device. If such charges develop, a voltage
can develop which could cause irreversible damage to the device,
especially metal oxide semi conductor (MOS) gate dielectrics. In
recent years, it has been reported that plasma non-uniformity
across the wafer is the predominant cause of charging. This
non-uniformity can arise from non-uniformities in RF current flow,
electron current flow, and ion current flow. Such charging can also
arise from handling and/or cleaning a semiconductor device. These
problems are becoming more important as gate oxides become thinner,
and hence, more vulnerable to surface charging.
[0006] It would be beneficial to measure in situ, in a plasma
reactor, the charge on a wafer to determine the magnitude of such
charges occurring during the manufacturing process. This could lead
to improved manufacturing processes. Additionally, if one could
measure the charge on a wafer that occurred during the
manufacturing process, one may be able to use such measurement as a
quality control means, i.e. by discarding chips have too great a
charge and therefore, a high likelihood of damage.
[0007] Previous efforts in this area have not yielded a suitable
method and apparatus for measuring a charge on a wafer during the
manufacturing process. One way that has been used in the past to
obtain measurements of charge is to attach leads directly to a
specimen. Another way of determining charge is to measure
degradation of the transistor after processing. Such previous
efforts include:
[0008] Murakawa, et al., "Mechanism of Surface Charging Effects on
etching Profile Defects," Jpn. J. Appl. Phys., Vol 33 (1994)
discloses using a magnet placed under a grounded electrode to
create a magnetic field at the wafer center. A probe measures the
plasma potential along the wafer. The probe comprises a silicon
wafer with an Al pad with contacts leading to Cu wires extending
out of the chamber to rf chokes and low-pass filters and then to a
DC voltmeter. This is not an elegant solution as it requires
fishing wires in and out of the reactor chamber.
[0009] Another commercially available method of measuring charge
comprises a detection device which can be employed outside the
reactor chamber after processing of a device is complete. However,
this solution is not entirely satisfactory because it measures an
accumulated affect over the total processing time and cannot
provide specific information about charge development during
processing.
[0010] None of these previous efforts teach or suggest all of the
elements of the present invention, nor do any disclose the benefits
and utility of the invention.
OBJECTS AND SUMMARY OF THE INVENTION
[0011] It is a primary object of the present invention to provide a
method and apparatus for measuring charge on a surface.
[0012] It is another object of the present invention to provide a
method and apparatus for measuring the charged caused by plasma
processing of a semi-conductor wafer.
[0013] It is another object of the present invention to provide
microscopic cantilevers for use in measuring a charge on a
surface.
[0014] It is an additional object of the present invention to
provide a method and apparatus for measuring charge on a
semi-conductor surface in situ.
[0015] It is even an additional object of the present invention to
provide means for measuring charge on a surface by measuring
deflection of a cantilever beam.
[0016] It is still even an additional object of the present
invention to provide a means for measuring a charge on a surface
wherein a rigid beam is connected to a flexible bridge so that the
surface of the beam does not bend during deflection of the
device.
[0017] It is still even a further object of the present invention
to provide a means for measuring charge on a surface which can map
charging across an electrode.
[0018] It is still another object of the present invention to
utilize a plurality of micro-cantilevers on a wafer to measure
surface charging.
[0019] It is a further object of the present invention to provide
test wafers comprising a plurality of probes for experimental
use.
[0020] It is also an object of the present invention to provide
probes on wafers in combination with actual circuits of chips,
which probes do not affect chip performance.
[0021] The present invention provides a method and apparatus for
measuring a charge on surface, such as on a semiconductor wafer,
arising during plasma processing. Accordingly, such a charge may be
measured on an insulating film applied to such a wafer. By the
present invention, the charge on such an insulator exposed to
plasma is measured in-situ, within the plasma reactor, using
micro-cantilevers. The micro-cantilevers include an insulating base
positioned on the substrate and a cantilevered beam extending
therefrom to over the substrate. The beam is formed of a conductive
material. Processing can cause external charges to occur on the
beam. A charge on the beam causes an opposite charge to form on the
substrate. The opposite charges attract to move or deflect the beam
towards the substrate. The amount of movement or deflection
corresponds to the magnitude of the charge. This movement or
deflection of the beam can be measured to determine the charge by
bouncing a light source, such as a laser, off of the cantilevered
beam. Alternatively, measuring charge can be performed by allowing
the beams to deflect to the point of contact with substrate wherein
they stick to the substrate, and the beams can be later inspected
to determine charge. In another embodiment, the cantilever includes
a flexible bridge interconnected between the base and a rigid beam.
In this embodiment, the surface of the beam does not bend. Rather,
movement of the beam is accomplished by the bending of the flexible
bridge. This allows for easier measurements of the movement of the
beam because the surface of the beam remains planar.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Other important objects and features of the invention will
be apparent from the following Detailed Description of the
Invention taken in connection with the accompanying drawings in
which:
[0023] FIGS. 1a and 1b show schematic views of a cantilever charge
sensing structure in undeflected, and deflected states,
respectively, of one embodiment of the invention;
[0024] FIGS. 2a and 2b show top and side elevational views of a
paddle structure embodiment of the invention;
[0025] FIG. 3a through 3d show the main processing steps required
for fabricating the paddle structure of FIGS. 2a and 2b, for one
embodiment of the invention;
[0026] FIG. 4 shows a plot of tip deflection angle as a function of
voltage for external calibration of the paddles of FIGS. 2a and
2b;
[0027] FIG. 5 shows a schematic view of an experimental setup of
apparatus used to detect charge in plasma, for detecting reflected
laser beams from charged and uncharged cantilevers or paddles;
[0028] FIG. 6a shows a plot of charging voltage versus RF input
power with constant chamber pressure, and constant flow rate of
oxygen;
[0029] FIG. 6b shows a plot of charging voltage versus chamber
pressure with both RF power and flow rate of oxygen held
constant;
[0030] FIG. 6c shows a plot of charging voltage versus distance
from the center of an electrode, where each of RF power, chamber
pressure, and the flow rate of oxygen are held constant;
[0031] FIG. 6d shows a plot of charging voltage versus the flow
rate of oxygen;
[0032] FIG. 7a is a plot of quasi-static C-V curves of devices
exposed at varying rf powers for 1 min;
[0033] FIG. 7b is a plot of the corresponding interface state
density as a function of position in the band gap
[0034] FIG. 8 is a comparison of QS C-V curves of devices exposed
to plasma and DC bias stressed.
[0035] FIG. 9 is a plot of the changing voltage inferred from DC
bias stressing and matching the QS C-V curves for n&p type
substrate MOS capacitors.
[0036] FIG. 10 is a comparison of charging voltage as measured by
sensor and inferred from QS C-V degradation at different rf
power.
[0037] FIG. 11 is a perspective view of a conductive layer placed
over a portion of a non-conductive substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Referring to FIG. 1a, a microscopic cantilever
("micro-cantilever"), generally indicated at 10, comprises a beam
12 supported above a substrate 8, by means of an insulator 14
positioned at one end of the beam 12. The insulator 14 electrically
isolates the beam 12 from the substrate 8. The beam 12 is formed of
a conducting material and acts as a charge-sensing structure. If an
interaction with plasma or fluid adds electrons or ions to the
surface of the beam 12, the micro-cantilever 10 becomes charged.
The substrate 8 mirrors the charge on the micro-cantilever 10,
resulting in an electric field between the micro-cantilever 10 and
the substrate 8. This electric field creates a force on the
micro-cantilever 10 leading to deflection of the beam 10 as shown
in FIGS. 1a and 1b. The charge on the micro-cantilever 10 can be
calculated from the deflection of the beam 12. It should be pointed
out that the beam 12 could be supported at both ends thereof, and
the deflection could be measured at the center of the beam 12. It
is also within the scope of the present invention to replace the
beam 12 with panel or membrane, supported at all four corners, and
measure deflection at the center thereof.
[0039] One way to measure the deflection of the beam 12 is to
reflect a light source or beam 16, such as, for example, a laser,
off the surface of the deflected beam 12 and onto a screen. As the
beam 12 deflects, the reflection of the light source shifts, as
shown by angle a, and this sift shows up at the screen. The
magnitude of deflection, and hence, the magnitude of the charge on
the substrate can be determined from this shift. The deflection
could also be measured through defraction of a light source bared
on movement of the beam 12. Another way to measure deflection is to
deflect the cantilevered beams far enough to touch the substrate
and stick thereto. This is a well-known phenomena in the field of
micro-mechanical structures, often referred to as "stiction." By
using a plurality of cantilevers of different size and/or rigidity,
by examining the "stuck" cantilevered beams, the voltage range can
be determined. Deflection could also be measured by capacitance
sensing.
[0040] Referring now to FIGS. 2a and 2b, paddle-like structures
("paddles"), generally indicated at 110, can be used. These paddles
110 are more sensitive to electrostatic force than the
micro-cantilevers 10, and also have larger reflecting area, so that
deflection is easier to detect. The paddles 110 comprise a large
pad 112 connected to the support 114 by a flexible bridge
comprising thin arms 116. An additional advantage of this structure
is that majority of the distortion occurs in the arms 116, with the
pads 112 remaining unbowed. Various sized paddles 110 can be used
to detect a particular range of voltage. The paddle 112 could be
formed of any desired material such a Poly-Si, while the support
114 could formed of SiO.sub.2.
[0041] The paddle structures and the cantilevers are fabricated by
standard surface micro-machining technique. The fabrication steps
are shown in FIGS. 3a through 3d. N.sup.+-type silicon wafers
(0.5-2.0 .OMEGA.-cm) are used as substrate. First a layer of
silicon nitride of thickness 150 nm is deposited by chemical vapor
deposition (CVD). This serves to alleviate "stiction" problems
during processing. Stiction occurs when the cantilever deflects all
the way and sticks to the substrate. The second step involves
creating the sacrificial layer which will be isotropically etched
to realize free standing structures. In this case a 2 .mu.m thick
silicon dioxide (SiO.sub.2) layer is deposited by atmospheric
pressure CVD. A 1 .mu.m thick polycrystalline silicon layer is then
deposited by low-pressure CVD and is doped heavily with phosphorus
(.about.10.sup.20 cm.sup.-3) to have high conductivity, and
annealed to reduce the internal stress in the polysilicon
cantilevers.
[0042] The polysilicon is first dry-etched in SF.sub.6 plasma (RF
power 30 W, pressure 150 mTorr & flow rate 15 sccm) as shown in
FIG. 3b. Next photoresist (AZ1518) is spin-coated on the support
structure to prevent the silicon dioxide underneath the support
from being etched during the release step (FIG. 3c). The final
processing step to create free-standing micro mechanical structures
is sacrificial layer etching of SiO.sub.2 by buffered hydrofluoric
(HF) acid (FIG. 3d). The wafers are then rinsed in deionized water
and later in acetone to remove the photoresist. Drying the wafers
under atmospheric conditions leads to stiction. Hence, various
drying methods were attempted, like vacuum drying and drying the
wafer on a hot plate. The second method gave better results. It
involved soaking the wafer in pure isopropyl alcohol for 10 min.
until the alcohol displaces the water underneath the paddles and
then evaporating the alcohol by placing the wafer on a hot plate at
200.degree. C. The reflecting pad areas of the paddles can range
from 60 .mu.m.times.50 .mu.m to 30 .mu.m.times.50 .mu.m with arm
lengths from 40 .mu.m to 30 .mu.m, although other dimensions are
within the scope of the invention. The structures preferably have
an arm width of 10 .mu.m. The dimensions of all the paddle
structures are designed for a sacrificial layer etch of about four
hours.
[0043] Prior to exposure to plasma, the devices are externally
calibrated by observing the deflection in response to an applied
electrical voltage, using a needle probe to apply voltage to the
polysilicon on top of the support. FIG. 4 shows the tip deflection
angle of the paddle as a function of voltage. Simulations of
deflection vs. voltage can be performed by finite difference
modeling.
[0044] The schematic of the experimental set-up used to detect
charging in situ in the parallel-plate reactor is shown in FIG. 5.
A He--Ne laser 30 produces a beam 32 which is directed into the
reactor chamber 34 through a quartz window 36 after passing through
a beam-splitter 38. The laser beam covers several identical paddles
so no focusing is necessary. The reflected light 40 is projected
onto a screen 42. When charging occurs, the paddles 110 deflect
(dotted lines) leading to a shift in the reflected laser spot on
the screen 42. From the tip deflection angle, which can be easily
computed from this shift, the charging voltage in the plasma can be
determined. The advantage of this technique is that it is direct
and gives fairly accurate results (<5% error). But the
disadvantage is that it requires an optical port and one cannot
measure charging voltage at different parts of the electrode.
[0045] During plasma exposure, cantilevers that deflect far enough
suffer "pull-in" and touch the substrate. When this occurs they
often suffer from "stiction", i.e. they remain stuck after the
removal of the charge. This can be quickly detected by external
inspection under an optical microscope after the plasma exposure.
The charging voltage can be estimated by having a range of paddle
sizes with different pull-in voltages on the wafer, and observing
which ones have pulled-in. The main advantage of this method is
that no optical ports are required and one can quickly map charging
non-uniformities across the electrode.
[0046] As described herein, it has been assumed that a conducting
or semiconducting substrate is used. If the substrate is
insulating, such as glass, the sensor could be fabricated on top of
a conductive coating placed over a portion of the substrate. This
is shown in FIG. 11, wherein the micro-cantilever 210 includes a
beam 212 extending from an insulator 214. A conducting layer 209 is
positioned over the insulating substrate 208, and the beam 212
extends over conducting layer 209.
[0047] Experiment
[0048] The charging voltage in a parallel-plate
reactive-ion-etching (RIE) reactor has been measured by both
techniques. The electrode diameter is 24 cm and spacing between the
electrodes is 5 cm. The gases are injected into the chamber through
a shower head in the top electrode. All experiments were done in an
Oxygen or Argon plasma with RF frequency being 13.56 MHZ.
[0049] Results
[0050] The charging voltage near the electrode edge (9.5 cm. from
the center) is measured by the in situ technique. The charging
voltage was found to increase as RF input power increased (see FIG.
6a), and charging voltage as large as 20 V was seen. The charging
voltage decreased as the chamber pressure was increased (see FIG.
6b). These results clearly demonstrate the utility and versatility
of the technique. Note that the voltages measured are sufficient to
damage thin gate dielectrics (e.g. 50 nm oxide has a breakdown
voltage of .about.7 V). Charging voltages were also measured at the
different points on the electrode by the ex-situ method. The
charging voltage was found to be lower at the center compared to
that at the edge (see FIG. 6c). The charging voltage was also found
to vary as the flow rate of the gas varied with charging voltage
decreasing as the flow rate of oxygen increased at the electrode
edge (see FIG. 6d).
[0051] When the MOS capacitors were exposed to plasma, varying the
rf power, time of exposure and the chamber pressure, the
quasi-static (QS) capacitors vs. voltage (C-V) was measured after a
fixed interval of time (14 min.). The QS C-V curve reflected the
degradation of the gate oxide and this degradation scaled with the
sensor reading. FIG. 7a shows the QS C-V curves of various devices
exposed to Argon plasma at different rf powers for one minute, and
FIG. 7b shows the corresponding interface state density, clearly
showing that the degradation of gate oxide scales with rf
power.
[0052] By varying the DC stress, electrical voltage on the gate
during plasma exposure could be estimated by comparing QS C-V
curves. It was found that the negative bias stress on the gate gave
QS C-V curve which must closely resemble that measured after plasma
exposure (FIG. 8). Similar results were found from MOS capacitors
with p-type substrates, and the plasma-induced voltage increased
with oxide thickness as shown in FIG. 9. A comparison of the
charging voltage as measured by the sensors at different plasma rf
power and that inferred from the MOS capacitors is shown in FIG.
10. There is a direct one-to-one relation between the two
measurements.
[0053] The main advantage of the present invention is that charging
of insulator surfaces exposed to plasma can be detected in-situ
without disturbing the plasma, unlike other probes. It requires no
wires and can detect the charge ex-situ too if there are no optical
ports. The technique is direct and there is no need for further
processing to estimate charging in plasmas. The probe is location
specific so that one can map the charging voltage across the
electrode and thereby determine the degree of plasma non
uniformity.
* * * * *