U.S. patent application number 10/242425 was filed with the patent office on 2003-02-20 for etching end point judging device.
Invention is credited to Ikuhara, Shoji, Kaji, Tetsunori, Nakamoto, Shigeru, Nishihata, Kouji, Takahashi, Kazue, Usui, Tatehito, Yoshioka, Ken.
Application Number | 20030036282 10/242425 |
Document ID | / |
Family ID | 26447317 |
Filed Date | 2003-02-20 |
United States Patent
Application |
20030036282 |
Kind Code |
A1 |
Usui, Tatehito ; et
al. |
February 20, 2003 |
Etching end point judging device
Abstract
An etching end point judging device which uses emission
spectroscopy for dry etching. The device includes an AND converter
for obtaining time series data of emission intensity of a specific
wavelength produced during etching, a first digital filter for
performing smoothening of the time series data, a differential
operator for obtaining a differential coefficient of the smoothened
time series data, a second digital filter for smoothening the
calculated differential coefficient of the time series data, and a
discriminator for judging the etching end point by comparing said
smoothened differential coefficient with a value set
beforehand.
Inventors: |
Usui, Tatehito;
(Niihari-gun, JP) ; Yoshioka, Ken; (Hikari-shi,
JP) ; Ikuhara, Shoji; (Hikari-shi, JP) ;
Nishihata, Kouji; (Tokuyama-shi, JP) ; Takahashi,
Kazue; (Kudamatsu-shi, JP) ; Kaji, Tetsunori;
(Tokuyama-shi, JP) ; Nakamoto, Shigeru;
(Kudamatsu-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
26447317 |
Appl. No.: |
10/242425 |
Filed: |
September 13, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10242425 |
Sep 13, 2002 |
|
|
|
09452174 |
Dec 1, 1999 |
|
|
|
Current U.S.
Class: |
438/708 ;
257/E21.507; 257/E21.528; 257/E21.579 |
Current CPC
Class: |
H01L 21/76897 20130101;
H01L 2924/0002 20130101; H01L 22/26 20130101; H01L 21/7681
20130101; H01L 21/76807 20130101; H01L 21/76808 20130101; H01L
21/76805 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
438/708 |
International
Class: |
H01L 021/302; H01L
021/461 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 1, 1998 |
JP |
10-341369 |
Apr 14, 1999 |
JP |
11-107271 |
Claims
What is claimed is:
1. An etching end point judging device using emission spectroscopy
for dry etching, wherein said device comprises A/D conversion means
for obtaining time series data of emission intensity of a specific
wavelength produced during etching, first digital filtering means
for performing smoothening of said time series data, differential
operation means for obtaining a differential coefficient of said
smoothened time series data, second digital filtering means for
smoothening the calculated differential coefficient of said time
series data, and discrimination means for judging the etching end
point by comparing said smoothened differential coefficient with a
value set beforehand.
2. An etching end point judging device according to claim 1,
wherein said device further comprises means for detecting
abnormalities in the etching process, first digital filtering
correction means that corrects--in the event of detection of any
abnormality--said smoothened time series data, said differential
coefficient time series data, and said smoothened differential
coefficient time series data, correction means for correcting said
differentiation operation, and second digital filtering correction
means.
3. An etching end point judging device that judges the end point of
etching from a time series data of a differential coefficient of
light emission intensity, wherein said device comprises display
means that displays a transition of said time series data of said
differential coefficient, and means for displaying an abnormality
in the display of said time series data display of said
differential coefficient in the event of detection of any
abnormality.
4. An etching end point judging device according to claim 1,
wherein said device further comprises a photo-electronic
photo-multiplier tube that outputs a current value that indicates
the intensity of light of a specific wavelength that is emitted
during etching carried out by a plasma discharge, an I/V converter
that converts the current value output by the photo-multiplier tube
into a voltage value, an A/D converter that digitizes the offset
and gain for processing the output voltage from the I/V converter
and the output from the output and gain of the IN converter, sense
adjustment means that obtains from the data digitized in the A/D
converter a sense voltage value for using the output of the
photo-multiplier tube as a target output, dark current calculation
means for determining the dark current of the photo-multiplier
tube, gain correction means for changing the gain that is normally
used as a fixed value in the event there is an overflow in the
obtained sense voltage value, a D/A converter that converts the
sense voltage value, offset value, and gain value that were
obtained into analog data and sets the data.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Divisional application of U.S.
application Ser. No. 09/452,174, filed Dec. 1, 1999, the subject
matter of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an etching end point judging
method and to devices suitable for detecting etching processing end
points in a plasma discharge process through use of emission
spectrometry; and, the invention also relates to an insulation film
etching method using the etching end point judging method and the
etching processing detection devices. The insulation films include
a silicon oxide film (referred to as an oxide film henceforth) and
a low-k film consisting of a material having a low dielectric
constant.
[0003] During dry etching processing of a semiconductor wafer, the
emission intensity at a specific wavelength in plasma light changes
according to the etching progress of a specific film. A
conventional semiconductor wafer etching end point detection method
is available to detect changes in the emission intensity at a
specific wavelength from plasma and detect an etching end point of
a specific film based on the detection result during dry etching
processing. In this method, detection errors caused by an
irregularity in the detected waveforms due to noise must be
prevented. In this regard, methods for improving the detection
accuracy of the emission intensity changes are disclosed in
Japanese Non-examined Patent Publication No. 61-53728 and No.
63-200533. In Japanese Non-examined Patent Publication No.
61-53728, noise is reduced by a method of moving averages, and in
Japanese Non-examined Patent Publication No. 63-200533, noise is
reduced by noise square approximation processing.
[0004] In an etching end point judging device that judges an
etching end point of a wafer which is being processed by etching
with a plasma discharge using emission spectrometry, the detection
signal becomes weaker after each wafer processing due to attachment
of deposits. As described in Japanese Non-examined Patent
Publication No. 63-254732, detection signals can be corrected by
changing the gain value and offset value of the detection signal
for stable etching end point detection. Alternatively, as described
in Japanese Non-examined Patent Publication No. 04-57092, without
the addition of a gain and offset adjustment function, the
detection signal that was fetched to the optical conversion method
can be adjusted to a set value for stable etching end point
detection.
[0005] With the recent progress toward downsizing and high
integration of semiconductors, the opening rate (area to be etched)
on a semiconductor wafer is becoming smaller and the emission
intensity at a specific wavelength that is fetched by a light
detector of an optical sensor is becoming weaker. As a result, the
level of the sampling signal from a light detector is becoming
lower, making it difficult to detect an etching end point correctly
based on a sampling signal from the light detector.
[0006] As the size of a semiconductor device becomes smaller, the
electrical capacity of the silicon oxide film used for insulation
between wires increases, and the signal loss between wires can no
longer be ignored. As a solution to this problem, a method of
reducing the electrical capacity between wires is being developed
in which a low dielectric constant material is used as the
insulation material between wires. Various materials have been
developed as candidates for low dielectric constant materials
(referred to as low-k materials henceforth). As described on page
74 in the monthly magazine Semiconductor World, 1998, No. 11, these
materials include FSG (k=3.3 to 3.6), HSQ (k=2.9 to 3.1), and
Xerogel (k=2.0 or less) as inorganic low-k films, SiLk (k=2.6), BCB
(k=2.6), FLARE (k=2.8), and PAE (k=2.8) as organic low-k films, and
organic SOG (k=2.8 to 2.9) and HSG (k=2.9).
[0007] In addition, a damascene process that enables wiring using
copper that has a lower electric resistance than conventional
wiring materials is being developed through use of a flattening
process (CMP) based on chemical and mechanical corrosion
technology.
[0008] In the damascene process, the main method is the dual
damascene method in which a wiring groove is formed by plasma
etching after forming a low-k film, which is used as an insulating
material between wires and layers, and then a contact hole is
formed between the two layers for permitting electrical connection
to the lower layer. The process of the dual damascene method
differs depending on whether a contact hole or a groove is etched
initially. Currently, various methods are being examined. In either
case, grooves and contact holes must be formed on low-k films using
plasma etching. By using a process of high precision plasma etching
with fewer stages, the yield can be enhanced and the cost can be
reduced, thereby substantially enhancing the characteristics of
plasma etching (etching process and performance).
[0009] However, in the currently manufactured damascene structure,
an etching stopper layer is formed by inserting a silicon nitride
film on the boundary between the groove and the hole provided on
the low-k film. Consequently, the need for a stopper layer
formation process and the increase of the dielectric constant of
the film due to insertion of a stopper layer become problems. There
will be no problem if the dielectric constant of the stopper layer
is low. However, to satisfy requirements regarding an etching
selection ratio and adherence with the low-k film, a silicon
nitride layer is currently being used.
[0010] Even if a stopper layer is inserted, the film thickness
cannot be increased in terms of an increase of the dielectric
constant. Therefore, accurate judgment is necessary regarding
whether etching has progressed to the stopper layer. Although the
ordinary end point judging system can detect this, more accurate
judgment is required. The better method is to employ a structure
that does not require insertion of a stopper layer, however, such
etching becomes difficult under current conditions.
[0011] In an insulating film etching device, a change with the
passage of time is detected, such as deterioration of the etching
speed, as the etching is repeated. In some cases, the etching may
stop. This problem must be solved. In addition, it is important to
monitor fluctuations in the etching speed with time for stable
operation; however, in the conventional method, the end point
judgment time is simply monitored. Moreover, when the etching
duration becomes short, such as about 10 seconds, the judgment
preparation time and the judgment interval must also be reduced in
the end point judgment method. However, in the conventional method,
a sufficient measure to address this requirement has not been
taken. For an insulation film, as the etched area is often 1% or
less, the changes in the plasma emission intensity from the residue
that is generated as a result of etching is small. Consequently, an
end point judging system that can detect very small changes is
necessary. However, a practical system at a reasonable price is not
available at present.
[0012] To resolve the problem of drift in a lithography position
during etching for forming a contact hole on an insulation film, a
self-alignment contact technology has been developed. In the end
point judgment used in this technology also, since the etched area
of the last contact section is small (1% or less), a system having
a high detection sensitivity for plasma emission intensity changes
is necessary; however, the end point judging system presently
available does not satisfy the requirements for high precision and
a reasonable cost.
SUMMARY OF THE INVENTION
[0013] The primary object of this invention is to provide an
etching end point judging method and a detection device that are
capable of stable detection of etching end points of semiconductor
wafers even for a semiconductor wafer having a low opening
rate.
[0014] The secondary object of this invention is to develop a
method of obtaining high-quality etching results by detecting
plasma etching end points of semiconductor thin films using an end
point judging system that can detect very small changes of plasma
emissions and also can measure the data in a short time during
plasma processing, in particular, plasma etching processing.
[0015] Another object of this invention is to provide an etching
end point judging method and a detection device that can eliminate
any end point judging detection errors when a pulse type noise is
induced in an emission intensity sampling signal (for instance,
instantaneous termination of discharge power or an abnormality
caused by modulation of the emission intensity due to a sudden
change of the plasma state caused by an instability).
[0016] Another object of this invention is to provide an etching
end point judging method and a detection device that can easily
display occurrences of plasma discharge abnormalities as history
data.
[0017] Another object of this invention is to provide an etching
method for enabling high-precision etching in a damascene process
and a self-alignment process using a method or system that can
measure, with a high precision, end points of an insulation film
etching process of a semiconductor device.
[0018] Another object of this invention is to prevent etching
faults in an etching device, which are caused by changes in time,
by determining the etching speed through measurement of the etching
time required to etch up to the stopper layer in the damascene
process or up to the insulation film on the gate in the
self-alignment contact process, and monitoring the fluctuations. By
judging the correct time required for reaching the stopper layer in
the damascene process, corrosion of a thin silicon nitride layer
can be retarded, thereby improving the real selection ratio.
[0019] Another object of this invention is to control deterioration
of device performance by retarding excessive advancement of etching
on the bedding layer by judging the correct etching ending time,
since etching ends in a short time, in the process for removing the
silicon nitride layer that was formed on the bedding in a damascene
process or a self-alignment contact process.
[0020] An object of this invention is to provide an etching end
point judging method for use in dry etching. The method includes a
step of reducing noise by processing input signal waveforms through
a first digital filter, a step of finding a differential
coefficient (primary or secondary) of a signal waveform through
differential processing, a step of obtaining a smoothed
differential coefficient value by reducing the noise component of
the time series differential coefficient that was obtained by the
previous step, and a step of judging an etching end point by
comparing the smoothed differential coefficient value and a preset
value through discrimination.
[0021] Another object of this invention is to provide an etching
end point judging method for judging an etching end point from time
series data of an emission intensity differential coefficient. The
method includes a step of displaying a transition of time series
data of the differential coefficient through a display method and
adding the display indicating an abnormality on the display of the
time series data of the differential coefficient when an
abnormality is detected.
[0022] Another object of this invention is to provide an etching
end point judging device that uses emission spectrometry. The
device is equipped with an AD converter for obtaining emission
intensity time series data of a specific wavelength, a first
digital filtering device for performing smoothing processing for
the time series data, a differential operation device for obtaining
a differential coefficient of the smoothed time series data, a
second digital filtering device for performing smoothing processing
for the time series data of the differential coefficient that was
calculated, and a discrimination device for judging an etching end
point by comparing the smoothing differential coefficient value and
a preset value.
[0023] Another object of this invention is to provide an insulation
film etching method for judging an etching end point using the
etching end point judging method described above in the etching of
an insulation film containing a low-k film consisting of a silicon
oxide film or a low dielectric constant material.
[0024] This invention can provide a very stable method for judging
etching processing end points, since it enables accurate
calculation of emission intensity changes. In addition, in end
point judgment that includes differential coefficient calculation
processing, noise in the sampling signals from a light detector can
be reduced effectively by setting digital filtering processing
before and after differential coefficient calculation processing,
enabling stable and accurate end point judgment.
[0025] By setting coefficient correction processing in digital
filtering processing in the former stage, differential coefficient
calculation processing, and digital filtering processing of the
latter stage, noise of sampling signals from a light detector can
be reduced effectively in the case of an abnormality occurring
during the etching processing, enabling stable and accurate end
point judgment.
[0026] By drawing in a zero or the preset display position with a
specific color arrangement when an etching processing abnormality
occurs during differential coefficient display, a high-quality
device that facilitates abnormality monitoring during etching
processing can be provided.
[0027] This invention enables execution of a correct end point
judgment, thereby providing the effect of setting a lower
over-etching than obtainable with time management etching. As a
result, excessive corrosion of the bedding layer can be controlled.
Since this invention reduces the over-etching duration, improvement
of the throughput can be expected. Since changes in the passage of
the etching time can be monitored, an etching device abnormality
can be detected at an early stage, enabling prevention of a large
number of etching faults.
[0028] This invention enables determination of a correct sense
voltage value for the target output value of a photo-multiplier by
using a sense voltage value for a target output voltage value of a
photo-multiplier through a relational expression. Therefore, even
for a semiconductor wafer having a small opening rate, the etching
signals used for stable detection of etching end points of
semiconductor wafers can be controlled to a specified value
repeatedly without any irregularity between wafers.
[0029] By using the end point judging system of this invention,
judgment can be prepared in a short time and small plasma emission
intensity changes can be detected. Therefore, this system can be
applied to end point judgment of insulation film etching of a small
etched area.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a block diagram of an etching end point judging
device that represents an example of this invention.
[0031] FIG. 2 is a flow diagram showing an example of the
processing procedure employed in the device shown in FIG. 1.
[0032] FIG. 3 is a flowchart of the offset control and gain
correction in the processing procedure shown in FIG. 2.
[0033] FIG. 4 is a graph which shows an example of an emission
fluctuation measurement result when offset control and gain control
are not performed.
[0034] FIG. 5 is a graph which shows an example of an emission
fluctuation measurement result when offset control and gain control
according to this invention are performed.
[0035] FIG. 6 is a graph which shows multiplication rate
characteristics of a photo-multiplier.
[0036] FIG. 7 is a flowchart of sense voltage and gain correction
processing.
[0037] FIG. 8 is another flowchart of sense voltage and gain
correction processing.
[0038] FIG. 9 is a diagram which shows the calculation flow of
smoothed differential coefficient time series data Di according to
this invention.
[0039] FIG. 10 is a graph which shows the original waveform and
processed waveform when the digital filter method of the device
shown in FIG. 1 is not used.
[0040] FIG. 11 is a graph which shows the original waveform and
processed waveform of the device shown in FIG. 1.
[0041] FIG. 12 is a graph which shows the original waveform and
processed waveform when noise occurs according to the processing
procedure shown in FIG. 9.
[0042] FIG. 13 is a graph which shows the processing procedure in
the second implementation example of the calculation flow of
smoothed differential coefficient time series data Di according to
this invention.
[0043] FIG. 14 is a waveform diagram which shows the original
waveform and processing waveform of the implementation example
shown in FIG. 13.
[0044] FIG. 15 is a flow diagram of a second example of the
processing procedure used in the device shown in FIG. 1.
[0045] FIGS. 16 to 16(f) show an example of a self-alignment dual
damascene process.
[0046] FIGS. 17(a) to 17(e) show an example of a damascene process
that processes a groove first.
[0047] FIGS. 18(a) to 18(e) show an example of a damascene process
that processes a hole first.
[0048] FIGS. 19(a) to 19(f) show an example of a process when a
boundary layer between a hole and a groove is not formed.
[0049] FIGS. 20(a) and 20(b) show an example of the process when a
low-k film structure is introduced.
[0050] FIG. 21 shows, as an application example of this invention
to self-alignment contact technology, a pre-etching cross-section
of the self-alignment contact.
[0051] FIG. 22, as shows an application example of this invention
to self-alignment contact technology, a post-etching cross-section
of the self-alignment contact.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] The following section describes a first example of this
invention. FIG. 1 will be used to explain etching of a
semiconductor wafer. In FIG. 1, the configuration of a
semiconductor wafer etching device 1 and etching end point detector
10 is outlined.
[0053] In the etching device 1, the etching gas introduced into
etching chamber 2 is transformed to a plasma after being decomposed
by microwave power, and a semiconductor wafer is etched by the
plasma. Etching end point detector 10 detects plasma light that is
generated in the etching chamber 2 during semiconductor wafer
etching processing and performs end point judging processing based
on the detection result. Etching end point judging detector 10 is
equipped with optical detector 11, offset circuit (addition
circuit) 15, gain circuit (multiplication circuit) 16, AD converter
17, digital filter circuit 18, differential coefficient operation
circuit 19, digital filter circuit 20, RAM 21, judgment circuit 22,
DA converter 23, and CPU 30. The programs corresponding to auto
offset/auto gain control processing, sense voltage setting
processing, sampling processing that includes smoothing processing,
and end point judging processing are stored in ROM 31. These
programs are executed by CPU 30. Reference numeral 32 denotes an
external storage unit and reference numeral 33 denotes an
input/output device.
[0054] Optical detector 11 detects the emission of a specific
wavelength from the plasma that was generated in etching chamber 2
during etching processing using a spectroscope 12 and supplies the
emission to a photo-multiplier 13 via an optical fiber. The
emission intensity of the specific wavelength that was fetched is
transformed to a current detection signal by photo-multiplier 13
according to the emission intensity and is converted to a voltage
signal by IV converter 14. The voltage signal converted by IV
converter 14 is processed by differential circuit (offset) 15 and
amplification circuit (gain) 16.
[0055] The signal output by AD converter 17 as a sampling signal is
stored in RAM 21 as time series data yi. Digital filter circuit 18
performs smoothing processing for time series data yi and stores
the data in RAM 21 as smoothed time series data Yi. Differential
coefficient operation circuit 19 calculates time series data di of
the differential coefficient value (primary differential value or
secondary differential value) from the smoothed time series data Yi
and stores the data in RAM 21. Digital filter circuit 20 performs
smoothing processing for differential coefficient value time series
data di and stores the data in RAM 21 as smoothed differential
coefficient time series data Di. Judgment circuit 22 compares the
smoothed differential coefficient value with a preset value and
detects an etching end point using the raw waveform signal or
operation waveform signal.
[0056] The detection signal of the raw waveform signal or operation
waveform signal is weakened by each wafer processing performed in
etching chamber 1 due to deposits attached thereon, changing the
end point detection condition for each wafer. By changing the sense
voltage for controlling the output voltage of photo-multiplier 13
and the gain of amplification circuit 16, the detection signals of
all wafers can be made identical, enabling etching end point
detection under the same condition.
[0057] As shown in FIG. 1, etching end point judging detection
device 10 of this invention is equipped with the functions of auto
offset/auto gain control processing, sense voltage setting
processing, sampling processing including smoothing processing, and
end point judging processing. These functions will be explained
with reference to FIG. 2.
[0058] When etching processing starts, a sampling start instruction
is issued (100). The emission intensity of the specific wavelength
that changes according to the etching progress is detected by a
light detector as a light detection signal of the voltage
corresponding to the emission intensity. The light detection signal
is converted to a digital value by an AD converter as sampling
signal li and is stored in RAM. Time series data y.sub.i (101 ) of
the following expression (1) is determined by auto offset/auto gain
control at A/D conversion.
y.sub.i=l.sub.i+d.sub.i (1)
[0059] l.sub.i is a low gain at offset zero and d.sub.i is a high
gain at offset zero.
[0060] The next step is to check whether the time is within the
sense voltage setting time of light detector 11 (102). If the time
is within the voltage setting time, the system advances to sense
voltage setting processing (103). When the time exceeds the sense
voltage setting time, the system checks to see if time series data
yi is the present value, for instance 4 V or higher (106). When
time series data yi is 4 V or higher, the system changes the sense
voltage to the value for which time series data yi was preset, such
as 0.6 V or less (107).
[0061] That is, the system reduces noise through the digital filter
of the first stage and obtains smoothed time series data (108).
Then, the system obtains differential coefficient (primary or
secondary) di of the signal waveform through differential
processing (109). The system obtains smoothed differential
coefficient time series data Di that was generated by reducing the
noise component of the time series differential coefficient
described above using the digital filter of the second stage (108).
The system calculates (D.sub.i-L)*(D.sub.i-1-L) using the end point
judging level L that was preset (111).
[0062] By judging the sign (negative or positive) of
(D.sub.i-L)*(D.sub.i-1-L), the system performs end point judging
processing of the etching process. If the sign is negative, the
system judges the result as true and terminates the sampling (113).
If the sign is positive, control is returned to the first step
101.
[0063] Sense voltage setting processing (103) calculates a sense
voltage that makes smoothed time series data yi to be preset
voltage ys, using smoothed time series data yi and the dark current
value of the detector (104). The processing also checks to see
whether smoothed time series data yi is preset voltage ys as shown
in the following expression (2) in the sense voltage that was set.
If not, the sense voltage is changed (105) and control is returned
to the first step 101.
yi-ys0.ltoreq.ys.ltoreq.yi+ys0 ys0=0.1 V (2)
[0064] This invention enables optimization of optical signal
intensity by using the auto sense feature, that is, the
characteristics of the photo-electric element. As a result, the
light signal intensity can be set to the optimum level at a high
speed. This auto sense feature provides a substantial effect for
step etching.
[0065] The detection precision of sampling signal 1.sub.i is
restricted by the gain of amplification circuit 16 and the
resolution of the AD converter 17. For instance, when an
amplification circuit of gain 1 and an AD converter of restricted
voltage .quadrature.10 V and a resolution of 12 bits are used, the
minimum resolution voltage is 4.88 mV and the fluctuation detection
precision of the light detection signal of about 2.5 V is 0.2%
(0.0488 mV/2.5 V), which is not an acceptable detection precision
level. To solve this problem, the detection precision is enhanced
by controlling the offset value of the differential circuit 4 and
the gain value of amplification circuit 5 that were described
above.
[0066] FIG. 3 is a flowchart of offset value control of the
differential circuit 15 and gain value control of the amplification
circuit 16. When a sampling start instruction 100 is issued, the
system sets the offset value of the differential circuit 15 to zero
(1010) and the gain value of the amplification circuit 16 to 1
(1011). The system fetches the sampling signal I that was converted
to digital data from a light detection signal by AD converter 17
(1013). The system stores sampling signal l.sub.i in RAM 21
(1014).
[0067] Then, CPU 30 sets the offset value of the differential
circuit 15 from DA converter 23 using sampling signal value l.sub.i
described above. CPU 30 then sets the gain value of the
amplification circuit 16 to the preset value (1016). In the next
step, the light detection signal of the light detector 11 is
converted to digital data by the AD converter through amplification
circuit 16, and sampling signal .quadrature.l.sub.i; is obtained
(1017). In the next step, CPU 30 stores the value produced by
adding sampling signals li and .quadrature.l.sub.i that have been
stored, in RAM 21 as time series data yi (1018). CPU 30 performs
arithmetic operations based on time series data yi that is stored
and performs a signal intensity comparison operation and a
differential processing operation.
[0068] Auto offset control according to this invention has the
following features.
[0069] 1) Obtains an absolute value of an input signal from the sum
of an offset value and a differential amplification value.
[0070] 2) Obtains an offset value for an absolute value of an input
signal and detects a differential amplification value from the
offset value.
[0071] 3) Detects an offset value by setting the gain of the AD
conversion to a low gain.
[0072] 4) Truncates one bit of the offset value to the differential
circuit according to the resolution of the DA converter and sets
the value.
[0073] Auto offset control according to this invention has features
in its method of maximizing the resolution of the AD converter by
using a differential amplification circuit of an AD converter.
[0074] In the first step, an approximate absolute value of the
signal voltage is obtained by processing an input signal waveform
through AD conversion at low gain operation. The next step is to
obtain an input voltage to a differential amplification circuit of
the AD converter, considering the resolution of the DA converter
(truncating 10 mV and less). Input voltage V.sub.0 to the AD
converter that was obtained in the previous step is output to a DA
converter.
[0075] The differential waveform of the input signal is processed
by AD conversion at high gain operation, and a high-precision
differential signal voltage V.sub.1 is obtained. The next step is
to compose the voltage values that were obtained in the previous 2
steps.
Voltage: V=V.sub.0+V.sub.1
[0076] Auto offset control according to this invention enables
high-precision AD conversion for changes of signal waveforms with
the passage of time. This means that signals of high input waveform
intensity can be handled by measuring signal waveforms in a maximum
conversion area measuring mode.
[0077] Bit quantified errors in the AD conversion can be reduced.
Bit quantified errors in DA conversion can also be reduced. As a
result of differential amplification, the signal level that is
output is reduced and the gain of the AD converter can be
maximized, enabling high-precision measurement. High-precision
measurement is also enabled for values of input signal
waveforms.
[0078] Offset control according to this invention can be applied to
AD conversion processing of electric signals, such as bias signals,
pressure signals, and flow signals, in addition to optical signals,
such as EPD. Auto offset control can also extend the dynamic range
of an AD converter.
[0079] FIG. 4 shows an example of emission fluctuation measurement
results of the conventional method that does not perform offset
control and gain control. FIG. 5 shows an example of emission
fluctuation measurement results when offset control and gain
control according to this invention are performed. The diagrams
indicate an improvement in the emission fluctuation detection
precision from about 0.5% to about 0.02% by application of this
invention. Consequently, the differential coefficient time series
data that is used for judging an end point can be obtained with a
high precision, enabling stable end point judgment of the etching
processing. Time series data obtained by this invention is zero
when there is no plasma emission; and, when etching processing is
performed, time series data yi has a value greater than zero.
Therefore, to perform arithmetic operations based on the time
series data yi that was obtained, no special zero division
avoidance processing is required, thereby simplifying the end point
judging processing flow and reducing software processing
errors.
[0080] Sense voltage setting processing will be explained
below.
[0081] By changing the sense voltage of the photo-multiplier 13
through DA converter 23 in FIG. 1, the output voltage of the
photo-multiplier 13 can be controlled. FIG. 6 shows the
multiplication rate characteristics of photo-multiplier 13. An
exponential relationship is established for output voltage l of
high-voltage multiplier 13 to high voltage Hv of photo-multiplier
13 and the relation is characterized in the following expression
3.
l=Hv.sup.a (For instance a=7.5) (3)
[0082] When the high voltage Hv of photo-multiplier 13 cannot be
directly obtained, the voltage can be obtained by converting sense
voltage V, that is controlled by the CPU, using the following
expression (4).
Hv=50 .quadrature.V+400 (4)
[0083] Therefore, by using the relationship of expression 4 in
expression 3, the sense voltage V can be calculated for obtaining
an expected output of photo-multiplier 13 for a certain emission
amount. This relationship can be expressed by expression 4 and
expression 5.
V.sub.1=Hv.sub.0/50*Exp(1/a .quadrature.
Log(l.sub.1/(l.sub.0-l.sub.d))-8 (Hv.sub.0=50 .quadrature.
V.sub.0+400) (5)
[0084] l.sub.1 is the target output voltage of photo-multiplier 13
and V.sub.1 is the sense voltage at that time, l.sub.0 is the
initial output voltage of the photo-multiplier, ld is the output
voltage of the dark current of the photo-multiplier, and V.sub.0 is
the sense voltage at that time.
[0085] A photo-multiplier contains a dark current, and, when the
sense voltage is low, the dark current imposes a substantial
influence on the output voltage of the photo-multiplier. For
instance, l.sub.0 is the initial output voltage of the
photo-multiplier, and, when the sense voltage at the time is very
low, a correct sense voltage can be obtained by subtracting the
dark current from output voltage l.sub.0 of the photo-multiplier as
shown in expression 5.
[0086] Dark current is measured after a wafer is delivered to a
chamber and before a plasma is generated. Alternatively, the dark
current may be measured when a wafer is not delivered to a
chamber.
[0087] By setting sense voltage V.sub.1 that was obtained by the
method indicated above, a required output voltage of
photo-multiplier 13 can be output. The gain of amplification
circuit 16 is normally a multiplication of a fixed value.
[0088] There is a limit in a sense voltage; and, when a required
output voltage l1 of the photo-multiplier is not output even if a
maximum sense voltage value is set, normally the operation waveform
signal is amplified by adjusting the gain to a fixed
multiplication. For instance, if the required output voltage of the
photo-multiplier is 2 V and the output voltage is 1 V when a
maximum sense voltage value is set, the operation waveform signals
used for detecting an etching end point can be matched by setting a
gain of normal fixed value 2 for the gain of amplification circuit
16 (gain correction).
[0089] FIG. 7 is a flowchart which shows an example of a process
for obtaining a sense voltage value and a gain value. A waveform
adjustment implementation instruction (1031) is issued to perform
sense adjustment and gain correction. The current sense voltage
value (1032) and the current raw waveform signal value (1033) are
obtained by the waveform adjustment implementation instruction
(1031). For instance, the sense voltage value is obtained using
number 3 and the sense voltage value and raw waveform signal value
that were obtained as indicated above is 3, so that the raw
waveform signal becomes a target voltage value of 2 V (1034). The
sense voltage value that was obtained by photo-multiplier 13 from
the DA converter is output (1035) and the system waits for the time
required for the adjustment effects to appear (1036). Then, the
system compares the target voltage value of 2 V and the current raw
waveform signal value and checks to see whether the error is within
the standard value (1037). If the error is within the standard
value, the system terminates the sense adjustment (1038).
[0090] When the error is outside of the standard value, the system
takes the following steps. The system checks to see whether the
sense voltage value that was output as indicated above exceeds the
maximum value (1039). When the value exceeds the maximum value, the
system compares the current raw waveform signal value and 2 V,
multiplies the ratio by the gain value that is normally set (gain
correction) (1040) and terminates the sense adjustment and gain
correction (1041). If the sense voltage value that was output as
indicated above is not the maximum value, the system increases or
reduces the sense voltage value by 0.1 V and outputs the sense
voltage value (1042). The system checks to see whether the time
spent for sense adjustment is equal to or greater than the
specified standard time. If the time is equal to or greater than
the specified standard time (1043), the system terminates the sense
adjustment (1038), and if the time is less than the specified
standard time, the system returns control to the comparison between
the target voltage value of 2 V and the current raw waveform signal
value (1037), generating a loop. This loop operates, for instance,
on a cycle of 0.1 second.
[0091] FIG. 8 is a flowchart of another example of a process for a
sense voltage and gain correction. The basic processing flow is the
same as the processing flow shown in FIG. 7. When a sense value
overflows (1039) or the time spent for sense adjustment exceeds a
specified time (1043), the system obtains a ratio of the target raw
waveform output voltage, for instance 2 V, to the current raw
waveform signal value and stores the result in memory.
[0092] The sense voltage value at that time is also stored in
memory. The sense voltage output that was obtained is output to the
sense value and a fixed value is maintained as the gain. Although
the value does not become the target value, which is 2 V, the sense
adjustment is terminated by performing a computation based on the
ratio between 2 V and the current raw waveform signal value within
the program of the microcomputer (1038).
[0093] In the etching end point judging device used in this
implementation example, since a correct sense voltage value can be
obtained for the target output voltage of photo-multiplier 13,
normally, the gain is a constant value, retarding the irregularity
of the S/N ratio of each wafer by the gain and amplification
irregularity. When a sense value overflows, a target operation
waveform can be obtained by correcting the value with the gain
value or correcting the ratio between the target output voltage and
the current output voltage within the program to perform stable
etching end point judgment.
[0094] The process flow for calculation of smoothed differential
coefficient time series data Di will be explained in conjunction
with FIG. 9. For digital filter circuit 18, a secondary Butterworth
type low pass filter is used. Smoothed time series data Yi is
obtained from a secondary Butterworth type low pass filter using
the following expression (6).
Y=b.sub.1y.sub.i+b2y.sub.i+b.sub.3y.sub.i (6)
[0095] Coefficient values "b" and "a" vary according to the
sampling frequency and the cutoff frequency. For instance, the
following values are applied when the sampling frequency is 10 Hz
and the cutoff frequency is 1 Hz.
a2=-1.143, a3=0.4128, b1=0.067455, b2=0.13491, b3=0.067455
[0096] Secondary differential coefficient time series data d.sub.i
is calculated using the following expression (7), which uses a
polynomial adaptation smoothing differential method of five values
of time series data Y.sub.i.
where, w.sub.-2=2, w.sub.-1=-1, w.sub.0=-2, w.sub.1=-1, w.sub.2=2.
(7)
[0097] Calculation of the coefficients is described in the
reference material: "Analytical Chemistry" 36 (1964) p.1627 by A.
Savitzky, M. J. E. Golay.
D.sub.i=b.sub.1d.sub.i+b.sub.2d.sub.i-1+b.sub.3d.sub.i-2 (8)
[0098] As an example for comparison, FIG. 10 shows the original
waveform during etching and secondary differential coefficient time
series data di that was obtained without using digital filter
circuit 18 and digital filter circuit 20. The sampling time series
data indicates that the etching end point is reached after 4.2
seconds from the start of the processing. Secondary diffusing time
series data di of the differential coefficient value described
above, smoothed differential coefficient data Di can be obtained
from the following expression (8) through digital filter circuit 7
(secondary Butterworth type low pass filter; however, the
coefficients may be different from coefficients `a` and `b` of
digital filter circuit 5). Differential coefficient time series
data d.sub.i does not provide correct information for the judgment
due to the noise.
[0099] FIG. 11 shows the change of waveform when digital filter
circuit 18 and digital filter circuit 20 are used. As shown in the
graph, noise in the smoothed secondary differential coefficient
time series data Di is reduced, enabling detection of a clear
etching processing end point and stable end point judgment. In this
way, noise of differential coefficient time series data can be
reduced effectively by installing digital filter circuit 18 and
digital filter circuit 20 in differential coefficient operation
circuit 19. Consequently, accurate differential coefficient time
series data used for judging an end point can be obtained for
achieving stable end point judgment of etching processing.
[0100] Other examples of this invention will be described below
with reference to FIGS. 12 to 14. The same etching end point
judging method as that applied in the previous example is used.
These examples show processing performed when an abnormality occurs
during etching and when pulse type noise is induced in the emission
intensity sampling signal. FIG. 12 shows the secondary differential
waveform that was calculated according to the processing procedure
in the previous example when pulse type noise is induced during the
time from the 2.5 second point to the 3.5 second point. As shown in
the graph, a large undershoot appears in the smoothed time series
data Yi, and, as a result, the smoothed secondary differential
waveform may become inaccurate.
[0101] In this example, abnormality processing is performed by
interrupting the calculation processing procedure for smoothed
secondary differential coefficient time series data Di as shown in
FIG. 13. When an abnormality occurs at i=m, smoothing processing is
performed and the smoothed time series data is assigned as Ym-1=ym
and Ym=ym. At the step i=m+1, Ym+1=ym+1 is assigned. Ym+3 of the
step i=m+2 is obtained by the secondary Butterworth low pass filter
ring processing of digital filter circuit 18 that was described
above. At the step i=m +3, differential value time series data dm+1
is calculated by differential coefficient operation circuit 19 and
the value is assigned in dm-1, dm, Dm-1, and Dm.
[0102] Data is smoothed by using these values, and smoothed
differential coefficient time series data Dm+1 is obtained. In
steps from i=m+4, smoothed differential coefficient time series
data is calculated according to the processing procedure that is
shown in FIG. 9. By this abnormality processing procedure, the
smoothed differential coefficient time series data produced by
eliminating the past time series data change can be obtained from
the third step following the abnormality occurrence.
[0103] FIG. 14 shows smoothed time series data Yi and smoothed
secondary differential value waveform Di when the abnormality
processing is applied. On this graph, the time when the secondary
differential passes the zero point is more accurately determined
(4.5 seconds when there is no pulse type abnormality and 4.56
seconds in this processing) than the graph shown in FIG. 12. In
this way, even if there are pulse type emission intensity
fluctuations, the influence of the fluctuations can be reduced in a
short time by performing abnormality processing. Consequently,
accurate differential time series data used for judging an end
point can be obtained, enabling stable end point judgment of
etching processing.
[0104] This invention enables reduction of noise (shot noise of a
light intercepting element, plasma light fluctuation, etc.) that is
contained in a light signal by using digital filters and
differential processing (S-G method).
[0105] According to the differential processing used by this
invention, noise in an input signal waveform is initially reduced
through the first digital filter. Then, a differential coefficient
(primary or secondary) of the waveform is obtained using
differential processing (S-G method). The noise components of the
time series differential coefficient waveform that were obtained in
the previous step are reduced by the second digital filter.
[0106] In this invention, abnormality processing is performed when
the degree of change of the raw signal level exceeds the set value
instantaneously (sampling interval).
[0107] That is, end point judging processing and display processing
of a differential value smoothed signal are interrupted and an
abnormality is displayed on a display window. If the degree of
change of the raw signal level is a set value or less, the smoothed
signal time series of the first step is reversed by two steps and
the value of the current point is assigned. The differential value
signal and the differential value smoothed signal are reversed by
as many steps as the number of degrees of the S-G method and the
value at the current point is assigned.
[0108] According to the abnormality processing of this invention, a
noise reduction level and a time response characteristic can be set
by controlling the filter characteristics of the digital
filter.
[0109] Since primary or secondary differential coefficients are
calculated using differential processing of the S-G method,
differential values of mathematically high precision can be
processed at a high speed. Noise components contained in
differential values can also be removed. (Substantial effects at
integer processing)
[0110] In addition, high-speed differential processing can be
performed after an abnormality, and abnormality history data can be
displayed easily. Post-abnormality high-speed differential
processing can also be performed.
[0111] Abnormality processing according to this invention can also
process abnormality flags from devices other than light signals.
Unlike analog filter processing, digital filter processing can
incorporate raw signals in an operation processing at any time. The
processing brings about substantial effects in step etching.
[0112] Another example of this invention relates to the display
method for displaying information at steps m and m-1 where an
abnormality has occurred in the previous example. Normally,
differential coefficient time series data is plotted on the monitor
window of the display device that can monitor a state of etching
processing any time during etching processing. Diagrams such as
those diagrams (b) in FIGS. 11 and 14 are displayed on the monitor
window.
[0113] For smoothed differential coefficient time series data items
Dm-1 and Dm at the points of abnormality occurrence m and m-1, the
corrected values are stored in RAM 9 and are used for obtaining the
smoothed differential coefficient time series data of the next
step. However, on the monitor window that displays a transition of
the etching processing, the transition data is drawn on the zero or
preset display position with a specific color arrangement. Since an
etching abnormality is stored on the monitor window, etching
abnormality history data is kept in the display device, enabling
monitoring of abnormalities in real time.
[0114] Since the etching end point judging method of this example
can be used to calculate emission intensity changes at high
precision, the etching end point judging detection method can
provide a very stable method for judging etching processing end
points.
[0115] FIG. 15 is a flowchart of end point judging control, which
is another example of this invention. This feature judges an end
point by comparing two wavelengths.
[0116] When etching processing starts, the system issues a sampling
start instruction (100). Emission of a specific wavelength that
changes according to the progress of etching is detected as a light
detection signal. An AD converter converts the signal to a digital
value as sampling signal l.sub.i and stores it in RAM. During A/D
conversion, auto offset/auto gain control is performed (101 and
101'). Then, the system checks to see whether the time is within
the sense voltage setting time of light detector 11 (102 and 102').
If the time is within the voltage setting time, control is passed
to sense voltage setting processing (103). When the time exceeds
the sense voltage setting time, the system checks to see whether
time series data items yi and yi' are 4 V or higher (106 and 106').
When the time series data items yi and yi' are 4 V or higher, the
sense voltage is changed to 0.6 V or less (107). When the time
series data items yi and yi' are less than 4 V, control is passed
to smoothing processing.
[0117] In smoothing processing, initially, the ratio of time series
data yi to yi' is calculated (120). The noise is reduced by the
digital filter of the first stage and smoothed time series data yi
is obtained (108). Differential coefficient (primary or secondary)
di of a signal waveform is obtained by differential processing (S-G
method) (109). Then, smoothed differential coefficient data Di is
obtained by reducing the noise components of the time series
differential coefficient waveform described above through the
digital filter of the second stage (108). Using present end point
judging level L, the result of (D.sub.i-L)*(D.sub.i-i-L) is
obtained.
[0118] By checking a negative or positive sign of the result of
(D.sub.i-L)*(D.sub.i-1-L), etching processing end point judging
processing is performed (112). If the sign is negative, the system
determines the result to be true and terminates the sampling (113).
If the result is positive, the system returns control to the
initial step (101).
[0119] An explanation of sense voltage setting processing (103) is
omitted because this processing is the same as for FIG. 2.
[0120] By using the end point judging processing system of this
invention that has been described, end points of an insulation film
etching process of semiconductor devices can be measured with a
high precision. Consequently, this method provides a method of
implementing etching of a damascene process and a self-alignment
process with a high precision. A semiconductor manufacturing
process using such a system will be explained below.
[0121] FIGS. 16 to 19(e) relate to typical damascene processes.
FIGS. 16 to 16(f) show a self-alignment dual damascene process;
FIGS. 17(a) to 17(e) show a process that processes a groove first;
FIGS. 18(a) to 18(e) show a process that processes a hole first;
and FIGS. 19(a) to 19(f) show a process where a boundary layer of a
hole and a groove is not formed. The process shown in FIGS. 19(a)
to 19(f), is an ideal process involving the least number of steps.
However, since a boundary surface between a hole and a groove is
not formed, there are many etching characteristic requirements,
such as uniformity of etching speed within the wafer surface and
repeatability. There are many problems that are to be solved for
employing this method for mass production.
[0122] The damascene process will be explained using the example of
self-alignment dual damascene processing shown in FIGS. 16 to
16(f). To process a hole, a hole is made on resist 201 by exposure
development. Under the resist, silicon nitride film 202, low-k film
203, nitride silicon 204, and bedding 205, which is a wiring of the
lower layer, are formed, as seen in FIG. 16. Initially, a mask
corresponding to the shape of the hole is formed on resist 201, as
seen in FIG. 16(a), and then the opening section corresponding to
the hole of resist 202 is formed on silicon nitride film 202, which
becomes a stopper layer. Resist 201 is removed and low-k film 206
and oxide film 207 are formed on silicon nitride film 202, as seen
in FIG. 16(b). This low-k film 206 becomes an inter-wire insulation
film of the upper section.
[0123] Then, as seen in FIG. 16(c), resist mask 209 for processing
a groove is formed on oxide film 207 through exposure development,
and then oxide film 207 and low-k film 206 are etched by plasma
etching, as seen in FIG. 16(d). In this case, etching stops at
silicon nitride layer 202, which is the stopper layer equivalent to
the bedding of low-k film 206. When plasma etching is carried out
using oxide film 207 as the mask of groove 208 and silicon nitride
film 202 as the mask of the hole, hole 210 is formed, as seen in
FIG. 16(f). Finally, silicon nitride film 204 is etched to effect
contact with bedding 205. Then, wiring is formed by inserting a
wiring material such as aluminum or copper in the opening section
(hole 210) and smoothing the upper section.
[0124] The problem in plasma etching of self-alignment dual
damascene is that the film thickness is reduced to several nm,
since, if silicon nitride film 202 of the stopper layer is thick,
the entire dielectric constant becomes high. Since the film is very
thin, a high selection ratio to the low-k film must be set. If the
etching speed uniformity or repeatability is low, over-etching must
be applied excessively. This is also the reason for setting a high
selection ratio.
[0125] In accordance with this invention, after the etching times
of low-k films 206 and 203 are checked by the end point judging
system and the specified over-etching is applied, etching
processing is terminated. In this case, the end points must be
judged in a short interval, preferably an interval of about 0.1 s
because the silicon nitride films 202 and 204 of the stopper layer
are very thin (several nm).
[0126] Since a correct time for etching on a low-k film so as to
reach the silicon nitride film can be judged by using the end point
judging method of this invention, unnecessary etching of silicon
nitride 202 of the stopper layer can be prevented.
[0127] In accordance with this invention, the end point judging
system judges the etching termination time, applies the specified
over-etching, and ends the etching for silicon nitride film 204
that is formed on bedding 205. The method of this invention enables
reduction of etching on bedding 205. However, to realize this, the
end point judging system must support a function for judging an end
point in a short time as described above, and, at the same time,
the preparation time from the lighting of plasma and the start of
etching to completion of preparation for end point judgment must be
short. The duration is preferably 5 s or less. If an end point can
be judged in such a short interval, the amount of over-etching by
etching end point judgment can be set and corrosion of the bedding
205 can be controlled even if etching terminates in around 10
s.
[0128] FIGS. 17 and 18 show other examples relating to a damascene
process. FIGS. 17a) to 17(e) show a process that processes a groove
first, and FIGS. 18(a) to 18(e) show a process that processes a
hole first. The difference between these two processes is only
whether a hole is processed first or a groove is processed first,
and the details of the application of this invention are the same
as described above. In either case, oxide film 302, low-k film 303,
silicon nitride film 304, low-k film 305, silicon nitride film 306,
and begging 307, which becomes wiring of the lower layer are
formed.
[0129] In FIG. 17(a), initially, resist mask 301 for processing a
groove is formed by exposure development, and then groove 308 is
formed by etching oxide film 302 and low-k film 303 by plasma
etching, as seen in FIG. 17(b). At this time, etching stops at
silicon nitride layer 304, which is a stopper layer equivalent to
the bedding of low-k film 303. Then, hole 310 is formed through
exposure development by applying resist mask 309, applying plasma
etching, as seen in FIG. 17(c), and removing resist mask 309, as
seen in FIG. 17(d). Finally, silicon nitride film 306 is etched to
effect contact with bedding 307, as seen in FIG. 17(e). Then,
wiring is formed by inserting a wiring material such as aluminum or
copper in the opening section (310) and smoothing the upper
section.
[0130] As shown in FIG. 18(a), resist mask 301 for processing a
hole is formed by exposure development, and then hole 310 is formed
by etching the oxide film and the low-k film using plasma etching,
as seen in FIG. 18(b). In this case, etching stops at silicon
nitride film 306 of the stopper layer that is equivalent to the
bedding of low-k film 305. Exposure development is applied to
resist mask 311 for processing a groove, as seen in FIG. 18(c), and
groove 308 is formed by removing the resist mask through plasma
etching, as seen in FIG. 18(d). Finally, silicon nitride film 306
is etched to effect contact with bedding 307, as seen in FIG.
18(e). Then, wiring is formed by inserting a wiring material such
as aluminum or copper in the opening section and smoothing the
upper section.
[0131] The damascene process as shown in FIGS. 17(a) to 17(e) and
18(a) to 18(e) enable startup of an end point judging system in a
short time. Consequently, by implementing a specified over-etching
by judging the etching end point up to the silicon nitride film
using this system, excessive etching on a thin film such as a
stopper layer can be controlled, resulting in high precision
etching results.
[0132] FIGS. 19(a) to 19(f) show the dual damascene process when
silicon nitride film 202, which is a stopper layer shown in FIG.
16, is not formed. The process involves etching of the following;
resist 401, in which a mask for processing a hole is formed, as
seen in FIG. 19(a), oxide film 402, low-k film 403, silicon nitride
film 404, and the layer in which bedding 405 is formed. Initially,
hole 406 that reaches silicon nitride film 404 is formed on low-k
film using plasma etching, as seen in FIG. 19(b). Then, resist is
applied and exposure development is applied to form resist 407
where a mask for processing a groove is formed, as seen in FIG.
19(c). Etching stops when a groove of the specified depth is formed
on low-k film 403. Since low-k film 403 is uniform, it is not
possible to judge, as an end point, the point where etching has
reached the silicon nitride film. Therefore, by measuring the
etching speed in advance and managing the etching time, etching is
processed to the depth of the groove. In this case, strict etching
speed uniformity within the wafer surface and repeatability are
required.
[0133] As the end point judging system of this invention, by using
a high precision system that can start measurement preparation in a
short time and can judge end points in a short interval and a
system that can judge minimal changes (fluctuation of minimal
etching characteristics) of plasma, the following method is
enabled, resulting in more precise grooving processing. That is,
the low-k film structure shown in FIG. 20(a) is introduced. The
structure is formed by oxide film 501, low-k film 502, boundary
face 503, low-k film 504, silicon nitride film 505, and bedding
506. In this case, low-k film 502 and low-k film 504 are low
dielectric materials of different film types. It is important to
form boundary face 503 between low-k film 502 and low-k film 504 by
using films of slightly different specifications when the films are
of the same type, by exposing low-k film 504 to the atmosphere by
interrupting the film forming after the film is formed, or by
applying a process whose surface state is different from the bulk.
In this structure, although boundary face 503 is formed, a low
dielectric constant can be maintained because all the film
components are made of low dielectric materials.
[0134] As the next step, the film is etched by plasma etching. The
description of the mask materials is omitted because they are the
same as used for the process of FIGS. 19(a) to 19(f). After
starting from the groove processing in FIG. 19, the etching
characteristics of the bulk and boundary surface 503 become
slightly different when the groove depth reaches boundary surface
503. The time required for etching to reach boundary face 503 can
be judged using the end point judging system of this invention.
Therefore, when etching ends at this point, a dual damascene
structure that does not insert silicon nitride in the stopper layer
with the groove depth of the boundary face 503 is completed. The
performance required for the end point judgment is not only
detection of minimal changes of plasma at high precision, since
etching on boundary surface ends in a very short time, but also
judgment of the amount of change by measuring the plasma emission
in a short time interval. The end point judging system of this
invention can satisfy these requirements. The numeral 508 denotes a
hole for contact with bedding 506.
[0135] An example of applying this invention to self-alignment
contact technology will be described below. FIG. 21 shows a
pre-etching cross section of a self-alignment contact and FIG. 22
shows a post-etching cross section. The conventional contact holes
are arranged at a slightly shorter distance than the distance
between gates in order to resolve a drift at matching of the
lithography position. As shown in FIGS. 21 and 22, since an
insulation film is formed on the top and side of a gate in the
self-alignment contact structure, the gate is protected by the
insulation film even if the contact hole overlaps on the gate.
Therefore, since a greater lithography position drift can be
provided, a shorter distance between gate polarities becomes
possible.
[0136] The structure of the self-alignment contact shown in FIG. 21
comprises resist 601, oxide film 602, such as TEOS or BPSG, oxide
film 603, such as SOG, silicon nitride film 604, bedding 605, and
gate 606. The area of gate 606 is the final contact area, and in
the film structure of this example, the bottom of the hole 607 is
formed in silicon nitride film 604. Therefore, a process for
removing silicon nitride film 607 is required after etching of the
oxide film 602 is completed. For plasma etching of a self-alignment
contact film, since the process using a CF gas has been developed
and many research examples are reported regarding the etching
characteristics, a description of the etching is omitted in this
document.
[0137] As shown in FIG. 22, the problems of etching are noticeable
corrosion on the bottom of the hole in the resist (corresponds to
607) and corrosion of the shoulder section 608 of the silicon
nitride film 604. In particular, when an oxide film is etched
repeatedly, the temperature on the wall in the etching chamber
fluctuates, the deposition characteristics of the etching gas and
etching residue on the inside wall fluctuate, or the gas discharge
behavior changes. As a result, etching characteristics are changed
and sometimes etching down to the bedding is disabled. This
phenomenon is called etch stop. When etch stop occurs, a large
number of device faults occur. Therefore, etch stop must be
definitely prevented. Detection of etch stop is also important.
[0138] When the end point judging system of this invention is used,
high time precision for detecting minimal etching characteristic
fluctuations can be achieved since the system can measure in a
short time changes of plasma emission, that is, the plasma change
measuring time interval is short. The time required for etching
from the start of etching in the state shown in FIG. 21 to reaching
the upper face of oxide film 603 (boundary face between oxide film
602 and oxide film 603), is determined through progress of etching
on oxide film 602. The etching speed is obtained using this data
and the thickness of the oxide film 602 that has been measured, and
the result is recorded and stored as etching speed data for oxide
film 602. By comparing the etching speed data for the previous
etching and this data, changes in the passage of time of the
etching device can be obtained. The yield can be improved by
displaying the data on the control panel of the etching device and
checking the stability of the device during production.
[0139] The method of this invention can also be applied for
monitoring stable operation of the device since the method enables
measurement of speed easily during etching processing. The next
step is to etch a narrow area between gates by etching oxide film
603. The etching speed obtained by the method described above can
also be used as the base data for improving the selection ratio by
retarding etching of the shoulder section 608 or determining the
over-etching time after etching reaches the bottom of the hole 607
of silicon nitride film 604. In addition, by obtaining the etching
speed of oxide film 603 between gates in the same method, the data
can be used for verifying the stability of etching characteristics
or detection of etch stop. Even if the film thickness is not
available in advance, the stability of etching characteristics in
the lot can be verified by checking the degree of changes of
etching time for each wafer. As described above, the data can also
be constantly monitored by displaying it on the controller of the
device and can be used for determining timing of changing the
process condition or timing of full sweep.
[0140] Another example of this invention shows utilization of
features of the quick measurement preparation and the short end
point judgment interval. An end point is judged with a high
precision in a short time by using the end point judging system of
this invention in the case where a contact between the lower
section (bedding 605) and the upper section is formed by removing
silicon nitride 604 by etching so as to complete etching of
self-alignment contact, as shown in FIG. 22. Since the bottom 607
of the silicon nitride film 604 is very thin, bedding 605 may be
excessively etched unless a correct etching end point is judged.
Since the etching time is short (10 and several seconds), the set
plasma measurement preparation time must be shorter than usual. By
using the end point judging system of this invention, the end point
can be judged easily.
[0141] In the example shown in FIG. 1, emission of a specific
wavelength from the plasma generated within chamber 2 is obtained
by spectroscope 12; however, the same result can be achieved by
passing a light of the area of the specific wavelength and using an
optics filter that shuts out the light of other wavelength areas or
dramatically attenuates the light, instead of using spectroscope
12.
[0142] In the example shown FIG. 1, a spectroscope and a
photo-multiplier are used. However, as described in Japanese
Non-examined Patent Publication 59-18424, by digitizing the signals
corresponding to multi-wavelengths through an AD modulator, using a
slit, grading, and line sensor, the data can be stored in a storage
unit in a specified cycle, or data corresponding to the required
wavelength can be fetched in a specified cycle. This system has an
advantage of being able to set a required wavelength
electronically.
[0143] When a slit, grading, or a line sensor is used and a long
interval is set for a scan start signal of the line sensor, the
accumulated electric charge is increased, increasing the size of
the output signal. To prevent this phenomenon, automatic gain
adjustment is enabled by monitoring the size of the signal output
from the line sensor and adjusting the interval of the scan start
signal of the line sensor so that the maximum value is set to a
specified value.
[0144] When the number of elements of the line sensor is
insufficient for the required wavelength precision, the wavelength
precision can be improved by internal insertion.
[0145] Characteristic irregularity in this system results in
irregularity of wavelengths of the light that forms spectra to the
linear sensor side. Therefore, wavelengths corresponding to the
data stored in the storage unit that was described above can be
calibrated by allowing input of light from a standard light source
for calibration that has known optical spectrum in addition to the
light from the chamber as the light to be input to a slit (for
instance, using dual fibers) and setting the standard light source
for calibration to ON periodically.
[0146] This example shows judgment of an etching end point using
plasma, however, the method is also effective for judging a
cleaning end point using plasma so that the method can be used for
judging end points of plasma cleaning after etching processing or
after plasma CVD. That is, the method can be applied for judging
end points of plasma processing and has the following features.
[0147] (1) Plasma processing end point judging device with the
following features in plasma processing end point judgment using
emission spectrometry: an AD conversion method for obtaining
emission intensity time series data of a specific wavelength, a
first digital filtering method for smoothing processing of the time
series data, a differential operation method for obtaining a
differential coefficient of the smoothed time series data, a second
digital filtering method for smoothing processing of time series
data of the calculated differential coefficient, and a
discrimination method for judging an end point of plasma processing
by comparing the smoothed differential coefficient value and the
preset value.
[0148] (2) Plasma processing end point judging device with the
following features in the plasma processing end point judging
device described above: a method for detecting a plasma processing
abnormality, a first digital filtering correction method that
corrects the smoothed time series data at detection of an
abnormality and the time series data of the differential
coefficient, and the smoothed differential coefficient time series
data, differential operation correction method, and a second
digital filtering correction method.
[0149] (3) Plasma processing end point judging device with the
following features in the method of judging plasma processing end
points from time series data of emission intensity differential
coefficient: a method for displaying a transition of times series
data of the differential coefficient described above and a method
for displaying an abnormality on the time series data display of
the differential coefficient described above at detection of an
abnormality.
[0150] The etching method of this invention can calculate emission
intensity changes with a high precision. Consequently, an etching
end point detection method that uses this feature can provide a
very stable etching processing end point judgement.
[0151] This invention enables determination of a correct sense
voltage value for a target output voltage of a photo-multiplier by
obtaining and using a sense voltage value for a target output
voltage of a photo-multiplier through a relational expression.
Therefore, even for a semiconductor wafer having a small opening
ratio, this invention can provide a method of controlling the
signals used for repeatedly detecting etching end points within a
constant value without irregularity between wafers for stable
detection of etching end points of semiconductor wafers.
[0152] By using this invention, the noise of sampling signals from
a light detector can be reduced effectively, enabling stable end
point judgment by setting digital filtering processing before and
after differential coefficient calculation processing in end point
judgment for differential coefficient calculation processing for
judging end points. By providing coefficient correction processing
in the digital filtering processing of the prior stage,
differential coefficient calculation processing, and digital
filtering processing of the post stage, noise of sampling signals
from a light detector can be reduced more effectively, enabling
stable and high-precision end point judgment. In addition, in a
differential coefficient display, by drawing on the zero or a
preset display position using a specific color arrangement at the
time of an etching processing abnormality, an effective device for
facilitating abnormality monitoring during etching can be
provided.
[0153] This invention enables accurate execution of end point
judgment so that low over etching can be set in comparison to the
etching managed by time. As a result, excessive corrosion of the
bedding layer can be controlled. In addition, an over etching time
can be reduced, resulting in improvement of the throughput. Since
changes can be monitored in the passage of time, an etching device
abnormality can be detected at an early stage, preventing a large
number of etching faults.
* * * * *