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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to solid state device manufacturing
and, more particularly, to an interferometric optical emission detection
system used for trace constituent contamination monitoring and endpoint
detection.
2. Description of the Prior Art
Sodium (Na) and other alkali metals are unwanted impurities which adversely
affect the operation of transistors and other solid state devices. Sodium
is a mobile ion and will cause the gate of a transistor to turn on and off
at different voltages. Variable operation of the gate will, in turn, cause
inconsistent operation of other devices such as memory chips and the like.
Sodium contamination of transistors can result from human contact with
chemicals and tools used in device manufacturing (i.e., sodium ions can be
transported from a person's body onto a processing tool simply by the
person touching the tool), impurities present in processing reagents such
as photoresist, from the manufacture and storage of these processing
reagents, by the sodium which is naturally present in the atmosphere
corrupting the processing plant environment (i.e., fog, mist, and
rainwater seepage can carry sodium ions from the atmosphere into a
processing plant), or by other means. Field Effect Transistors (FETs) are
four orders of magnitude more sensitive to alkali metals than bipolars;
therefore, sodium contamination is a particular concern in FET processing.
Sodium emission, as well as other trace constituent emissions, can be
detected optically. Most of today's optical emission detecting systems
used in transistor processing can be categorized as either monochromators
with photodiode array detectors or as scanning monochromators with a
photomultiplier detector. These types of optical emission systems are
available from the following companies: Tracor-Northern, Plasma-Therm
Analytical, Xenix, and EG&G PAR. One problem with these types of optical
emission systems is that they offer low resolution detection; often
greater than 1 nm and sometimes as poor as 10-20 nm. Low resolution
detection results in insufficient wavelength dispersion and convolution of
the desired emission signal with unwanted background and other emissive
interferences. Because of this, low resolution detection also provides low
sensitivity for trace species detection against the intense background
emission of plasma processing tools. This is further compounded by low
light throughput which is typical of conventional monochromators.
As a result of the low sensitivity of conventional optical emission
systems, the usual method used for detecting sodium impurities in FETs is
by electrical measurement at the end of the chip fabrication process.
Clearly, it would be advantageous to have some means which is sensitive
enough to detect trace levels of an impurity such as sodium ion before a
large number of chips have been damaged. Ideally, potential processing
problems could be identified when unacceptable levels of sodium are
detected during the fabrication of FETs, and the processing could be
halted temporarily to clean the tools and/or check the sodium levels in
the processing reagents.
Proper etching endpoint detection is also a major concern in transistor
manufacturing. One prior art method of determining the endpoint of
material removal is simply timing the etch process according to the rate
of material removal. To use a timed etch procedure, it is necessary to
empirically determine the time at which all of the desired material has
been removed, but the underlying layer has not been etched significantly.
Timing is not an ideal procedure for determining the proper etching
endpoint because it is indirect and the consequences of improper etching
are significant. Underetching will cause a degradation of the gain of the
transistor, while overetching will result in a degraded contact between
the intrinsic and extrinsic base regions.
Optical emission detection has been proposed as a means for detecting the
endpoint at which an etching operation should be halted. A monochromator
or bandpass filter can be used to select wavelengths of light at which a
desired optical emission will occur. High spectral resolution is always
preferable to low spectral resolution for monitoring and analysis of
plasma species because, with sufficiently high resolution, it is possible
to minimize interference from other emitting plasma species. Moreover, if
light throughput is held constant, peak height will increase relative to
the background as spectral resolution is improved. Accordingly, high
spectral resolution can provide considerable advantages in sensitivity for
etch endpoint detection and in the identification of weakly emitting
plasma species such as in sodium ion contaminant analysis. Unfortunately,
the usual methods for obtaining high spectral resolution, e.g., narrowing
monochromator slits or utilizing larger monochromators, have practical
limits for use in transistor manufacturing applications. When the slits of
a monochromator are narrowed, higher resolution is achieved; however, the
light throughput is significantly reduced and, thus, the overall
sensitivity is reduced. Using larger monochromators is unacceptable
because they would require costly clean room space and are generally
impractical for manufacturing applications.
Laser-induced fluorescence (LIF) is a newer technique used in transistor
manufacturing for identifying trench etch endpoints and can be used to
identify trace constituents. For example, U.S. Pat. 4,675,072 to Bennett
et al. discloses an LIF system used to detect and control the reactive ion
etch (RIE) through of a given layer in a wafer by detecting a large change
in the concentration of a selected minor species from the wafer in the
etching plasma. Although LIF is generally used for monitoring major
species, LIF may be used to detect trace Na ion contaminants during
etching; however, Na only fluoresces at the excitation wavelength of
588.996 nm or 589.593 nm and since scattered laser light is a severe
limitation on the sensitivity of LIF, the presence of Na may not be easily
detected because of unavoidable interference from scattered laser light
off the tool walls and windows. Furthermore, methods to reduce scattered
laser light, such as the use of Brewster angle windows and light baffles,
are impractical for use on commercial etch tools. LIF may also be used to
detect trace copper atoms which appear in aluminized lines as a means to
detect an etch endpoint. However, copper atoms will exhibit fluorescence
at 324.775 nm which is a wavelength that would require frequency doubling
of the dye laser; a process generally regarded as infeasible for
manufacturing applications because more powerful lasers are required for
this task and a skilled laser operator would be required.
Several laser interferometer techniques are now in standard practice for
optically detecting line widths and etch endpoints. U.S. Pat. No.
4,454,001 to Sternheim et al. and U.S. Pat. No. 4,680,084 to Heimann et
al. are directed to etch monitoring using laser interferometric methods
whereby the thickness of the region being etched is simultaneously
monitored. U.S. Pat. No. 4,838,694 to Betz et al. discloses a laser
interferometry process which uses the reflected laser beam. U.S. Pat. No.
4,717,446 to Nagy et al. discloses a method, using a monitor wafer which
is correlated to the endpoints of the working wafer, for detecting the
endpoint of the etch of an epitaxially grown silicon whereby a laser is
used to measure the etch rate of the monitor wafer by measuring the
reflected light off the oxide layer. U.S. Pat. No. 4,602,981 to Chen et
al. is directed an impedance monitoring technique for plasma etching
wherein endpoints are detected by an impedance change of the plasma;
however, the Chen et al. reference does disclose that laser interferometry
is well established in the art and points out that the laser measures the
thickness of the film removed as the etch process proceeds. U.S. Pat. No.
4,758,304 to McNeil et al. discloses an apparatus for ion etching which
utilizes an interferometer for surface monitoring. The methods described
in Chen et al. and McNeil et al. are generally insensitive to the precise
wavelength of the detected light. Before the invention thereof by the
applicants, interferometers have not been used in solid state device
manufacturing for detecting trace emitting species in a complex background
spectra.
Furthermore, several techniques for controlling interferometers are now in
common practice. U.S. Pat. No. 4,482,248 to Papuchon et al. discloses an
interferometer used for optical filtering. U.S. Pat. No. 4,711,573 to
Wijntjes et al. discloses a dynamic mirror alignment control which
utilizes an interferogram for analyzing sample materials wherein a closed
loop servo motor is used to maintain position orientation. U.S. Pat. No.
4,448,486 to Evans discloses the use of mirrors in a Fabry-Perot
interferometer used to change the bandwidth of the Fabry-Perot
interferometer. The applicants invention describes completely new methods
for using and controlling interferometers in transistor manufacturing.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide methods of
using light wave interferometry for the detection of trace plasma species.
It is another object of this invention to overcome the low light throughput
and corresponding low resolution disadvantages of using a monochromator
for wavelength selection to monitor optical emissions of trace plasma
species.
It is yet another object of this invention to describe a new, low-cost,
durable optical emission detection system which operates using
interferometric principles.
According to the invention, light wave interferometry is used in solid
state device manufacturing processes to identify trace plasma species. By
using light wave interferometry, high resolution optical emission
detection is achieved without the use of a monochromator. The light wave
interferometry methods of the present invention result in a high light
throughput efficiency, high spectral resolution, and an associated
improvement in detection sensitivity and selectivity. The light wave
interferometry methods of the present invention are useful in etch
endpoint analysis and in the identification of important trace
contaminants such as sodium impurities encountered during FET fabrication.
One method involves the use of a Fabry-Perot interferometer and relies upon
the constructive and destructive interferences of entering light between
two highly reflective parallel plates separated by a known distance of
air. As discussed by R. P. Feynman et al. in The Feynman Lectures on
Physics, Vol. III, Addison-Wesley, Reading, Mass., pp. 4-10, 1965, an
interferometer operates on the principle of maintaining a standing wave
between two reflective plates. The standing wave criterion will be met
when, k.sub.f =j.pi./L, where k.sub.f is the wavenumber (k=2.pi./.lambda.)
of the jth mode (j=any integer) and L is the separation of plates. By
varying the separation between the two plates, which can be accomplished
with a piezoelectric drive or similar means, the transmitted light may be
tuned with very high resolution (e.g., 0.001 to 0.08 nm) over a narrow
wavelength range. Such a system can typically transmit 70% of the light at
the selected wavelength which compares very favorably with monochromator
systems that have lower resolution and transmit only 5-10% of the light.
Since multiple orders are also transmitted by the air-spaced etalon, it is
also necessary to use prefilters to block out multiple order transmission.
Air-spaced Fabry-Perot interferometers are commercially available and have
been primarily used in transistor and other solid state device
manufacturing for linewidth measurements of major emitting species. For
example, R. Walkup et al. disclosed the use of air spaced Fabry-Perot
interferometers for linewidth measurements in J. Chem. Phys., 84, 2668
(1986). In contrast to prior uses of Fabry-Perot interferometers in solid
state device manufacturing, the present invention anticipates the use of a
Fabry-Perot etalon for end point detection and for the selective detection
of trace emitting species from a complex background spectra. The present
invention's use of the Fabry-Perot interferometer takes advantage of the
high light throughput which occurs with the Fabry-Perot interferometer
which is not present in other means of optical emission detection (e.g.,
monochromators). High light throughput provides no major advantage in line
width analysis because a major plasma species, as opposed to a trace
species, is usually being monitored during line width analysis.
Another method by which light wave interferometry can be used to monitor
trace species during transistor manufacturing is to use a tiltable narrow
bandpass interference filter to scan a small range of wavelengths. Narrow
bandpass interference filters are commercially available from Oriel
Optics, Andover Corp., Spindler, and Hoya and include a series of thin
dielectric layers which are used to produce constructive or destructive
interference of the incident light. Narrow bandpass interference filters
are similar to the air spaced Fabry-Perot etalon in principle; however,
the separation between layers in a narrow bandpass interference filter is
fixed and several dielectric layers are used instead of air. While the
resolution of a narrow bandpass interference filter is typically somewhat
lower than with a Fabry-Perot interferometer, multiple order transmission
problems associated with Fabry-Perot interferometers are not present when
a narrow bandpass interference filter is used. This is because a
Fabry-Perot interferometer operates in a single reflective cavity and,
narrow bandpass interference filters operate by multiple reflections
between successive dielectric layers. In this way, sensitivity to mode
effects is minimized. Narrow bandpass interference filters have been
widely used in materials processing plasmas for endpoint detection. In
fact, some commercial reactors come equipped with specific bandpass
interference filters, such as the Zylin metal etch reactor which is
equipped with a 10 nm bandpass filter for aluminum (Al) atom detection.
Harshberger et al. have described a reactor equipped with a bandpass
interference filter in J. Electronic Materials, 7, 429 (1978).
Interference filters are typically used in the same way as low resolution
optical spectrometers for etch endpoint detection. The high light
throughput of these filters provides an advantage over a fixed wavelength
monochromator, and also has a lower capital cost and greater reliability
than a monochromator. However, the fixed wavelength selection of the
filter makes these light detection systems vulnerable to errors resulting
from unwanted detection of interferences within the transmission range of
the filter. The absolute emission intensity measured by the bandpass
filter also includes intensity from the underlying continuous emission of
the plasma. This continuous emission can be significant and can dominate
over weak, wavelength-specific emission from trace or impurity plasma
species. The continuum can result from electron-ion recombination or
changes in the electron energy distribution in the plasma. Fixed
wavelength detection using filters means that the desired emission may be
indistinguishable from other emissions in the plasma.
It is known that bandpass interferometric filters can be tuned to shorter
wavelengths by tilting the filter with respect to the incident light.
Tilting the filter effectively changes the path length of the light
between the dielectric layers without changing the layer separation or
composition (as pointed out above, the distance between layers is fixed).
Hence, the transmission peak of a given bandpass interferometric filter
can be tuned slightly to shorter wavelengths by tilting the filter to an
appropriate orientation. FIGS. 1 and 2, which are taken from the Oriel
Optics catalog of 1986, illustrate the effect of varying the angle of
incidence of a collimated beam on bandpass interferometric filter. FIG. 1
shows that if the angle of incidence is increased, the peak wavelength is
shifted to shorter wavelengths. This shift is due to the fact that the
path difference between the direct transmitted beam and the beam formed by
the multiple reflection decreases. The angle shift in peak wavelength can
be estimated for angles less than thirty degrees using equation 1:
##EQU1##
where: a is the angle of incidence,
.lambda..sub.a is the peak wavelength at angle a,
.lambda..sub.o is the peak wavelength at normal incidence, and
N.sub.e is the incidence angle shift factor or the "effective index of
refraction" of the filter.
N.sub.e is not the actual refractive index of the spacer layer, rather it
is dependent on both the high and low index materials in the filter. Oriel
Optics experimentally determined that N.sub.e in a zinc sulfide/cryolite
system is 1.45 for a cryolite spacer layer and 2.0 for a zinc sulfide
spacer layer. FIG. 2 plots the approximate wavelength shift versus the
angle of incidence, as calculated according to equation 1, for two
bandpass interferometric filters which each include a sulfide/cryolite
system and where one uses a cryolite spacer layer and the other uses a
zinc sulfide spacer layer. For small angles (e.g., under 30 degrees), the
shape of the band pass does not change appreciably except for a small
reduction in transmission. However, at larger angles band pass shape
becomes highly distorted. For 5 nm and 10 nm filters, shifting by about 10
nm without drastic change in the band pass shape can be achieved.
Narrow and very narrow bandpass interference filters are a subset of
bandpass interferometric filters and include filters having a full-width
half maximum (FHWM) ranging between 0.1 nm and 1 nm. The distinctions
between bandpass interferometric filters, narrow bandpass filters, and
very narrow bandpass interference filters are qualitative. The FHWM refers
to the error in wavelength detection measured at one half the peak height.
Narrow bandpass interference filters are available from the Andover Corp.
of Massachusetts as well as other vendors. Narrow bandpass interference
filters show a critical dependence on filter orientation to collimated
incident light. For example, if a very narrow bandpass interference filter
having an FHWM=0.2 nm varied by 5.degree. from normal incidence, an
approximately 1 nm shift in peak wavelength will occur without significant
broadening of the transmission curve. Hence, unlike 5 nm and 10 nm
bandpass interferometric filters which shift about 10 nm without drastic
change in band pass shape, very narrow bandpass interference filters have
considerably less range, but have improved spectral resolution over that
range.
The present invention uses the "problem" of angular dependent wavelength
shifts with narrow bandpass filters as a means to greatly improve optical
emission detection sensitivity. Specifically, the invention contemplates
mounting a narrow bandpass filter in a holder which can be rapidly tilted
with respect to the collimated light collected from a plasma reactor. By
tilting the bandpass filter 5.degree., an approximate range of 1 nm can be
scanned with a bandpass of better than 0.2 nm at the half height or 0.05
nm at 90% transmission maximum. FIG. 3 shows a plot of the transmission of
the filter versus wavelength where the bandpass is 0.2 nm at the half
height and 0.05 nm at 90% transmission and shows how the plot shifts as
the narrow bandpass filter is tilted. A photomultiplier tube is used to
detect the transmitted light through the tilting narrow bandpass filter.
The output from the photomultiplier tube is sent to an oscilloscope or
lock-in amplifier which is synchronized with filter movement. Therefore,
the lock-in amplifier can sensitively measure a change in the
photomultiplier signal as a function of filter orientation.
Since atomic emission lines are extremely sharp (e.g., on the order of
0.0001 nm) any atomic emission detected will show a very strong signal
dependence on filter orientation. This dependency on filter orientation
provides the advantage of effectively eliminating background emission
noise (i.e., background emission is considered to be the emission which is
relatively constant over the range of wavelengths scanned). Eliminating
background emission enables the detection of v | | |
