Apparatus and method for photoluminescent analysis

An optical system is disclosed which significantly enhances the throughput of a grating spectrometer intended to determine impurity concentrations on the surface of semiconductor materials (usually single crystal silicon) used for integrated circuits. The system, which uses a laser beam as the photo-excitation means impinging on a Dewar-contained sample, includes a pre-sample series of lenses which so shapes the laser beam that its shape at the point of impingement on the sample is proportionally similar to the shape of the monochromator slit in the spectrometer. The same lens which provides final focusing of the laser beam on the sample also collects the sample-emitted radiation, which is thereafter focused by suitable optics on the monochromator slit, where it preferably substantially matches the shape of the slit, but slightly overfills the slit.

BACKGROUND OF THE INVENTION 
This invention relates to the field of photoluminescent (PL) analysis, in 
which a light source is used to excite a sample, and the photons emitted 
by the sample are passed through a monochromator to a detector which 
provides a measurement of intensity. 
The present invention is particularly valuable as a means of determining 
impurity concentrations in single crystal silicon (Si), but it also has 
other potential uses, such as dopant measurement in gallium arsenide. 
Generally, the impurities would be those unintentionally incorporated; but 
in some cases intentionally doped impurities would be measured. Such 
impurity determinations are a very important means of determining the 
characteristics of electronic devices in integrated circuit chips. 
The use of PL analysis for this purpose is discussed in an article by 
Tajima in Applied Physics Letters (American Institute of Physics), Volume 
32, No. 11, June 1, 1978 (Page 719), and in an article by Tajima and 
Nomura in Japanese Journal of Applied Physics, Volume 20, No. 10, October, 
1981, (Page L-697). As pointed out in these articles, the PL technique 
"can be successfully applied to the characterization of silicon crystals 
as a powerful means for analysis of shallow impurities. The PL method 
makes it possible to detect non-destructively a small amount of impurities 
in a small region of a specimen". The cited articles point out that the 
concentration of an impurity is proportional to the ratio of the 
intensities of the impurity and intrinsic signals. 
The use of PL analysis of silicon chips was discussed by L. W. Shive, of 
the Monsanto Company, in October 1981, at a meeting of ASTM F1.06, The 
Electrical and Optical Measurements Committee. It was point out that, in 
his experiments, PL analysis was "designed to analyze single crystalline 
silicon only"; and that the method "assays silicon for Group II B and V B 
impurities, that is boron, phosphorus, arsenic, aluminum, and antimony". 
As summarized by Shive, PL analysis basically involves three steps: (1) 
"low temperature photoexcitation of the silicon sample", (2) "light 
emission from the sample--luminescence", and (3) "detection of the emitted 
light. The luminescence is a result of carrier recombination which takes 
place within the silicon sample". 
In discussing the apparatus used for experimental purposes, Shive stated: 
"A laser is used to photoexcite the sample--which is immersed in liquid 
helium. The light emitted by the sample is resolved by a monochromator, 
detected by a photomultiplier tube, amplified, and recorded. The sample 
luminesces continuously and a spectrum of intensities as a function of 
wavelength is recorded." 
The very promising concepts discussed above are subject to the problem of 
getting as much light as possible from the sample through the 
monochromator and onto the photomultiplier tube. To do this, one must 
collect as much of the light being emitted by the sample as possible, and 
fill the monochromator's entrance aperture and acceptance cone with this 
light. The problem at first seems straight forward. One collects the light 
from the sample with a colliminating lens of low f number, then focuses 
the light onto the monochromator's entrance slit with a lens which matches 
the monochromator's acceptance f number. Since monochromators typically 
have f numbers between f/3 and f/5, the focusing lens can be a single 
element. With this pair of lenses, the solid angle requirements are met. 
In the PL analysis systems heretofore used, only about 2% of the available 
sample-emitted light collected by the adjacent lens passes the entrance 
slit in the monochromator. The round spot illuminated by the laser (source 
of incident light at the sample), when transferred to the monochromator 
entrance slit, is still round. The slit, however, is long and narrow, 
e.g., approximately 0.012 mm.times.6 mm, causing a severe mismatch. 
Attempts to distort the spot image, to make it better match the slit, run 
afoul of the solid angle consideration which dictated the initial pair of 
lenses. Essentially, anything gained by changing the spot shape is lost by 
angular mismatch. One possible solution of the problem is the use of fiber 
optic image transformers. However, their low packing density (30%) and 
high cost make them unattractive. 
SUMMARY OF THE INVENTION 
The present invention provides, essentially, at least a five-to-one 
improvement in the percentage of sample-emitted, lens-collected light 
which passes the monochromator slit. In other words, from the 2% figure 
cited above, this invention has succeeded in raising to at least 10% the 
portion of such light which passes through the monochromator slit. (The 
invention is also applicable to other types of grating spectrometers.) 
This significant result is accomplished by incorporating, between the 
energy supplying radiation source (generally a laser) and the sample which 
is photoexcited by the laser beam, an optical combination which shapes the 
laser excitation beam in such a way that, at the point where that beam is 
incident on the sample, the shape of the beam essentially matches, or is 
similar to, the shape of the monochromator slit. The beam shape may either 
be proportional, or substantially identical, to the shape of the slit. 
The light emitted by the sample is preferably collected by a low f number 
lens located close to the sample, and then focused on the slit by a 
focusing lens which matches the monochromator's acceptance f number. 
An advantageous feature of the optical system disclosed in this application 
is the dual use of the lens which is closest to the sample as a shaping 
collimating lens in directing the laser excitation light to the sample, 
and as a collecting/collimating lens in collecting the widely dispersed 
light emitted by the sample. This is accomplished by a "spherical" lens, 
or its equivalent, i.e., a lens which can be schematically represented as 
having a flat surface facing toward the sample, and a convex, 
spherically-shaped surface facing away from the sample. This lens 
constitutes, therefore, both an element in the pre-sample optical system 
and an element in the post-sample optical system. As a practical lens at 
the sample, a lens system is preferably used which has a plurality of lens 
elements designed to provide aberration-free optical performance. 
Another advantage of the present PL system is the location of the Dewar 
aperture, through which excitation light enters, and sample-emitted light 
exits, at the bottom of the Dewar. This feature contributes greatly to the 
lens-to-sample closeness which is needed for optimum performance.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 is intended only to illustrate the theoretical basis of the PL 
system for measuring dopants in semiconductor materials. PL spectroscopy 
offers a method for measuring the group V donors phosphorus, arsenic, and 
antimony; and the group III acceptors boron and aluminum. The underlying 
physical process can be explained with the help of FIGS. 1a-1c. Since the 
impurities of interest all have low ionization potentials (approximately 
1/10 eV), they are all fully ionized at room temperature. This is depicted 
in FIG. 1a. Cooling the sample to 4.5.degree. K. results in the situation 
of FIG. 1b, which shows the net dopants neutralized. At this temperature, 
illuminating the sample with intense visible light will cause the 
situation shown in FIG. 1c. The visible light generates a high 
concentration of hole-electron pairs, which neutralizes the remaining 
impurities. FIG. 1c also shows some of the recombination processes which 
give rise to the luminescent photons. It is these photons which are used 
to measure the dopant concentration. Following the Tajima articles 
identified above, the dopant concentration can be given as a function of 
the ratio of the intensities of the dopant and intrinsic lines. Using the 
ratio of intensities makes the method immune to changes in sample 
preparation. 
FIG. 2, which is taken from the material of Shive identified above, is the 
spectrum of a silicon sample doped with both boron and phosphorus. The 
line marked "I" is an intrinsic line, whereas "B" indicates boron and "P" 
phosphorus. In this spectrum, the concentration of boron is equal to 0.27 
PPBA and the concentration of phosphorus is equal to 0.78 PPBA. The 
numbers next to each peak represent its measured height. 
FIG. 3 shows the general layout of the PL system. A sample 12 is located in 
a Dewar flask 14, which may be maintained at the desired cryogenic 
temperature (e.g. 4.5.degree. K.) by means of liquid helium. The 
excitation light may be provided by a laser 16, emitting a beam 18 which 
passes through a chopper 20, and is then shaped and impinged on the sample 
12 by a pre-sample optical system indicated generally at 22, the details 
of which will be explained below. 
The relatively dispersed PL light emitted by the sample 12 is collected and 
then focused at the entrance slit 24 of a monochromator 26 by a 
post-sample optical system indicated generally at 28, the details of which 
will be explained below. 
A stepper motor 30 is provided to drive the grating of the monochromator, 
and the exit slit 32 of the latter directs the diffracted light to a 
photomultiplier tube 34. Detector tube 34 converts any light passed by the 
monochromator 26 into an electrical signal, which may be input to a 
preamplifier 36, which in turn sends the amplified signal to a synchronous 
demodulator 38. The output of demodulator 38 is input to a switched 
integrator 40 for minimum noise bandwidth. The output of integrator 40 is 
digitized by an analog-to-digital converter 42, and buffered onto a bus 
44. 
The control function is provided by a computer 46, which has control of the 
stepper motor 30, the integrator 40, and the A/D converter 42. Since the 
computer has access to all functions, the system has maximum flexibility 
because it is fully under software control. 
As stated in discussing the background of this invention, prior systems of 
this type have only captured about 2% of the available sample-emitted 
light collected by the adjacent lens. This invention provides a very 
significant increase in the light available at the detector, primarily by 
means of the pre-sample optical system 22. This improved throughput, which 
has increased the 2% figure to at least 10%, can be used either to 
increase the operating speed, or to improve the accuracy of the analysis. 
Generally, speed increase will be emphasized. 
FIGS. 4 and 5 show schematically the details of the pre-sample optical 
system, FIG. 4 being a side view, and FIG. 5 a top view. The concentrated 
beam 18 of excitation light emitted by the laser is reshaped by a 
plurality of lenses, which have the effect of spreading the beam 18 in one 
dimension, while shrinking it in the other dimension, until it eventually 
forms a narrow line matching the dimensions of the monochromator entrance 
slit 24, e.g., approximately 0.012 mm wide by 6 mm long. If desired, the 
design can be arranged to provide a shape at the sample 12 proportional to 
that of slit 24, rather than an identical shape. Furthermore, a pre-sample 
optical system 22 which causes the shape of the excitation light at the 
sample to tend to conform to the shape of the slit can significantly 
improve performance of the PL system, even though total conformity is not 
obtained. 
Theoretically, the pre-sample optical system could be provided by only two 
lens elements, the first a cylindrical lens to "fan out" the laser beam 
18, and the second a spherical lens to collimate the beam in its larger 
dimension and narrow it in its smaller dimension, thereby approximating 
the extreme ratio of the slit dimensions. 
However, for a variety of design considerations (including the need to 
change beam direction to obtain compactness of the system), in a prototype 
system the pre-sample optical system comprises three "cylindrical" lenses 
50, 52, and 54 and a "spherical" lens 56. The first lens 50 reached by the 
beam 18 has a cylindrically-shaped side 58 facing the laser beam and a 
flat side 60 facing toward the mucn larger second lens 52, which has its 
flat side 62 facing lens 50. The relatively small lens 50 directs a beam 
18a toward the larger lens 52, which beam focuses at 51 and then fans out 
in one dimension only (the vertical dimension in FIG. 4). The 
cylindrically-shaped side 64 of lens 52 directs toward the facing 
cylindrically-shaped side 66 of lens 54 a wide flat beam 18b, which is 
seen as essentially a planar rectangle in FIGS. 4 and 5. From the flat 
side 68 of lens 54 the excitation beam 18c focuses at 69 and fans out to 
impinge on the spherical surface 70 of lens 56. The location of focal 
point 69 is dictated in part by the need (which will be explained in 
detail below) to have the beam pass through a small central hole in a 
reflecting mirror that is part of the post-sample optical system. 
The spherical lens 56, which should be located as close as possible to the 
sample 12, has the dual effect on entering beam 18c collimating it in one 
dimension and shaping it in the other. The collimating effect is shown in 
FIG. 4, and the shaping effect in FIG. 5. Thus, the excitation beam 18d, 
which exits from the flat side 72 of lens 56, and impinges on the surface 
of sample 12 at 74, will have at 74 the desired shape designed to equal, 
or be proportional to the shape of the slit 24. The pre-sample optical 
system just described reshapes the laser beam, but the area of the beam at 
the sample remains substantially the same as its initial area, so that the 
energy density is essentially unchanged. 
The light emitted by the sample, in accordance with the PL process 
discussed above, will be widely dispersed. By using lens 56 as the first 
lens in the post-sample optical system, and by designing it and locating 
it so as to collect as much as possible of the sample-emitted light, the 
efficiency on the PL system is greatly enhanced. The results of 
experimentation have demonstrated that it is difficult, if not impossible, 
to obtain a single-element lens 56 which is sufficiently aberration-free 
to provide the desired optical efficiency. Consequently, a 
multiple-element lens system is actually used as the lens 56. This lens, 
therefore, preferably should be a commercially available camera lens 
having a 50 mm focal length, an f number in the range of f/1.2 to f/1.5, 
and a resolution of at least 250-300 line pairs per millimeter. The word 
"lens", as used in the optical art, and as used in this application, 
covers either a single-element or a multiple-element lens arrangement 
which is designed and supplied as a unit, and is so constructed as to have 
the desired optical characteristics. 
Limitations are created on the amount of available sample emitted light by 
the necessity of locating the sample 12 in a Dewar flask. However, the 
efficiency and convenience of the Dewar arrangement are maximized by 
locating the Dewar above the optical system, and locating the light 
aperture in the bottom of the Dewar, a feature which both simplifies the 
design of the Dewar structure and enhances the collection of the 
sample-emitted light. The Dewar light aperture, and the location of the 
sample in the Dewar, are shown in FIG. 7 which is a sectional view in a 
vertical plane. 
Both the pre-sample optical system 22 and the post-sample optical system 28 
(FIG. 3) are preferably supported on a horizontal platform. The Dewar 14, 
which is a cylindrical vessel, is preferably located directly above lens 
56, and beam 18d from lens 56 is directed upwardly. As shown in FIG. 7, 
the lens 56 is preferably mounted as a recessed inset in the bottom plate 
of the Dewar. The beam 18d enters the Dewar through an aperture 76 covered 
by a window 78 supported on the bottom plate 80 of the Dewar. A plurality 
of decreasing diameter apertures 82 and 84, covered respectively by 
windows 86 and 88, are provided in successive Dewar plates 90 and 92. The 
sample 12 is supported on the innermost horizontal Dewar plate 94, which 
has an aperture 96. Another sample 12 is shown near the left edge of plate 
94, the Dewar preferably being designed to hold a substantial number of 
samples, e.g., 20, all carried by a holder which is rotated to bring 
successive samples into the measurement position. As can be seen, the 
samples always remain normal to the optical axis; and therefore, the 
holder does not require precise rotation. 
FIG. 6a shows schematically the post-sample optical system. As previously 
stated, the system is designed to maximize the extent to which the widely 
dispersed photons emitted by the sample 12 are collected by the same lens 
56 which focuses the excitation light from laser 16 on the sample. Since 
the photons emitted by the sample are widely scattered, the collecting, or 
condensing lens 56 should have a large diameter and be as close as 
possible to the sample 12. And, as previously stated, the fact that a 
"downward looking" Dewar 14 is used, permits the lens 56 and sample 12 to 
be much closer than would be possible with a different Dewar arrangement. 
Since lens 56 is a large diameter lens located close to the sample, it 
must, as previously stated, have a low f number. 
As seen in FIG. 6a, the sample-emitted light collected by the flat side 72 
of lens 56 forms a collimated beam 18e between the spherical face 70 of 
lens 56 and the spherical face 100 of another spherical lens 102, which is 
located nearest to the monochromator slit 24. The beam 18f which emanates 
from the flat side 104 of spherical lens 102 focuses at the monochromator 
slit 24 an image having the same general shape as the slit, thereby 
providing the maximum available energy at the slit, which constitutes the 
limiting factor in the amount of energy throughput available to the 
detector 34. FIG. 6b is an enlarged view looking at the wall in which slit 
24 is formed, emphasizing its very large length-to-height ratio. 
In order not to waste any of the available slit aperture acceptance angle, 
it is considered desirable to slightly "overfill" the slit by having lens 
102 re-image the sample onto the entrance slit 24 at f/3, whereas the 
acceptance angle of the selected monochromator 26 is f/4.2. The 
monochromator 26 comprises a grating 106, which is moved by stepper motor 
30 (FIG. 3), and a plurality of fixed reflectors 108, 110, 112 and 114. 
As will be apparent from the description of FIGS. 8 and 9, the actual 
optical system of the prototype requires several additional optical 
elements and features for design compactness and convenience. But, the 
essential features have been included in the foregoing description. 
The system as a whole includes components whose characteristics may be 
varied to suit particular requirements, but a preferred combination 
appears to have the following features. The monochromator may be an ISA 
model HR-320, having f/4.2 optics. This unit is compact and has a high 
throughput. The grating preferably has a blaze angle set for a 1 micron 
wavelength. The monochromator selection is determined by the resolution 
needed as a system parameter. Since no two spectral features are closer 
together than 1.5 A.degree., the monochromator needs to have the highest 
throughput which can resolve 1 A.degree.. This dictates an instrument in 
the 1/2 meter focal length class. Because the interesting spectral 
features for Si are all between 1.070 and 1.14 microns wavelength, an S1 
photomultiplier is preferred as the detector 34. In the prototype, the 
tube used is an LN.sub.2 -cooled E.M.I. S1 end-looker, cooled to 
-100.degree. C. by locating it in a suitable Dewar. 
The laser generator 16 preferably is a medium power argon ion laser, such 
as Lexel 0.5 watt model 85. It requires only single phase 208 V ac power, 
puts out the required 200 mw at 0.51 micron, and is compact and reliable. 
The windows 78, 86 and 88 of Dewar 14 preferably are clear fused quartz, 
anti-reflection coated for 1.0 micron wavelength. The middle window 86 
intercepts the 300.degree. K. radiation from the outer window 78. Since it 
re-radiates at 77.degree. K., the inner window 88 sees only a small 
fraction of the thermal radiation it would otherwise receive. In other 
words, the holding time of the liquid helium is effectively doubled by 
inclusion of the middle window 86. In the post-sample optical system, the 
lenses 56 and 102 should be anti-reflection coated for a wavelength of 1.0 
micron. In the pre-sample optical system, the lenses 50, 52 and 54 should 
be anti-reflection coated for a wavelength of 0.5 micron. 
FIGS. 8 and 9 show the general structure of the prototype system. In the 
plan view (FIG. 8), the laser generator 16 is shown at the top of the 
figure, mounted on a platform 120, such as an NRC table. The laser 
radiation is reflected by a flat mirror 122, set at a 45.degree. angle, 
and carried by a mirror mount 124 which permits initial position 
adjustment by tilting on two axes (vertical and horizontal). The mirror 
mount 124, and all the other optical element carriers may be supported by 
suitable brackets secured to table 120. A holder 126 may carry both the 
chopper 20 and the first, very small cylindrical lens 50 (neither of which 
are seen in FIG. 8). Lens 50 should be so mounted as to have displacement 
(translation) adjustment in two axes. The next cylindrical lens 52, which 
is shown carried by a holder 128, requires only vertical displacement 
adjustment. The combination of lenses 50 and 52 functions as an 
"up-collimator", the lens size ratio being about 10 to 1. A second flat 
mirror 130, which may be identical in structure and adjustability to 
mirror 122, is set at a 45.degree. angle to reflect radiation from lens 52 
toward lens 54, the third cylindrical lens. This lens also requires 
initial position adjustability by displacement along a vertical line. 
Since the Dewar is located above the optical system, radiation from lens 54 
is deflected upwardly by a flat mirror 132 (see FIG. 9), which is set at 
an angle of 45.degree. with respect to the vertical, thus changing the 
beam direction from horizontal to vertically upward. The vertical beam 
passes through an aperture 134 in another flat mirror 136, which is part 
of the post-sample optical system. The location of the aperture 134 with 
respect to the pre-sample optical system should be at the focal point 69 
(see FIG. 4) between the cylindrical lens 54 and the spherical lens 56 
which is mounted in the bottom of Dewar 14. The mirror reflector 132 
should have two-axis tilt adjustability. Lens 56 which, as previously 
stated, is preferably a high quality camera lens containing multiple lens 
elements, requires only focus adjustment. 
The mirror 136, which receives post-sample light emissions from lens 56, 
may be mounted on the same supporting member 138 as mirror 132, but it 
requires a separate adjustable mount having two-axis tilt adjustability. 
The post-sample radiation is reflected by mirror 136, which is set at an 
angle of 45.degree. to the vertical, thereby changing the beam direction 
from vertically downward to horizontal. This horizontal beam passes 
through the final focusing spherical lens 102, which may actually be a 
single element plano-convex lens, and which should be so mounted as to 
have translation adjustability along three axes. This adjustability may be 
obtained conveniently by using three translation stages mounted on a 
single supporting member 140. 
Radiation from spherical lens 102 is focused on the slit 24 of 
monochromator 26. And, for the reasons explained above, such radiation has 
been shaped to be similar to the shape of the slit, thereby substantially 
increasing the radiation throughput which enters the monochromator and 
reaches the photo-detector 34. 
From the foregoing description, it will be apparent that the apparatus 
disclosed in this application will provide the significant functional 
benefits summarized in the introductory portion of the specification. 
The following claims are intended not only to cover the specific 
embodiments disclosed, but also to cover the inventive concepts explained 
herein with the maximum breadth and comprehensiveness permitted by the 
prior art.