SEMICONDUCTOR MATERIALS RESEARCH

REPORT

FOR ANALYTICAL

CHEMISTS

SEMICONDUCTOR MATERIALS RESEARCH by P. F. Kane, Texas Instruments, Incorporated, Dallas, Texas PHILIP F. KANE is manager of the Central Analytical Chemistry Facility of Texas Instruments Incorporated, Dallas, Texas. He received his B. Sc. in Chemistry from London University in 1948. Mr. Kane started his career as a technician in the Control Laboratory of Standard Telephones and Cables, Limited in London where he was employed from 1939 to 1949. In 1949 he joined Laporte Chemicals, Limited in Luton, England, as a research analyst and in 1951 became Chief Analyst, a post he held until he came to the U. S. in 1957. He joined the Chemagro Corporation in Kansas City and directed a group engaged in pesticide analysis until 1959 when he joined Tl. Mr. Kane's principal interests have been in the analysis of peroxy compounds, pesticides and, more recently, semiconductor materials. He is a Fellow of the Royal Institute of Chemistry, and a member of ACS, Society of Applied Spectroscopy, Society for Analytical Chemistry and the Polarographic Society. rj^HE

ELECTRICAL

PROPERTIES

of

•*- semiconductors depend to a very large extent upon crystal lattice imperfections. These imperfections may be chemical (that is, foreign elements present either substitutionally or interstitially), or they may be physical (point defects, dislocations, or other areas of deficiency in the order). Both types of imperfection may give rise to charge carriers. It is the materials research man's objective to control these imperfections so as to tailor material which can give devices with the desired characteristics. It is the analytical chemist's duty to present to him information about these imperfections on which he can base his deductions. This definition of analysis is quite broad, but in this field it is more convenient to base an analytical fa-

cility on techniques, and to allow the specialists in these techniques to provide such support as they feel necessary, rather than worry unduly about defining boundaries for analytical chemistry. On this basis, some techniques described may not fit the usual concept of analysis. Although a considerable number of materials have been examined as semiconductors, in practice only a handful are of significance. The vast majority of entertainment devices are made of germanium. Most military and space devices and the new integrated circuits are made from silicon. A few research groups, including some at Texas Instruments, are investigating III-V compounds, in particular gallium arsenide. The applications described will therefore be limited to these materials.

Bulk Material

Since the first transistor in 1947, the quality of bulk material has posed a challenge to analytical scientists—one that, so far, has only been partially met. With very few exceptions, colorimetric methods have proved to be not usable; they require involved clean-up procedures for the reagents and, of course, they are too specific. Emission spectrography has been applied to this problem but, unfortunately, the spectra for both silicon and germanium are obscured by heavy continuum from oxide bands. The use of an inert atmosphere tends to repress the volatilization and, in general, the sensitivities are limited to the 10 to 100 p.p.m. range. However, for gallium arsenide this method has been considerably more successful (11). The density is VOL. 38, NO. 3, MARCH 1966

·

29 A

REPORT FOR ANALYTICAL CHEMISTS

Table I.

Sensitivity Limits in Gallium Arsenide for Mass (M) and Emission (E) Spectrography (All values in parts per million atomic)

higher, there is little or no interfer ence from bands, and, by splitting the burn into three exposures, a better signal-to-noise ratio can be obtained. Useful sensitivities are obtainable and are shown in Table I. Several elements of interest (copper, for example), have sensi tivities in the 1 to 20 p.p.b. range. For semiconductor materials re search, the mass spectrograph rep resented a break-through in sensi tivity. I t is, for this purpose, al most the ideal analytical tool, com bining broad coverage with high sensitivity. Table I also includes the sensitivities {15) for this tech nique and, as can be seen, they are generally about 100 times better and include elements which are not detectable by emission spectrog raphy. These levels are about the same for silicon and germanium, also. Despite this high sensitivity, improvements in materials have al ready made further demands on this technique. Hopefully, improve ments in detectors and in utilization of the ions generated will help to meet this challenge. Two other methods for bulk analysis might be mentioned. T h e most sensitive technique is, of course, activation analysis. This seems t o have been little used for germanium but for silicon there have been a number of radiochemi cal separations published (β). An adaptation (6) used in this labora tory is shown in Table I I . I t covers eight of the most common impur 30 A

·

ANALYTICAL CHEMISTRY

ities, and the elements are separated such t h a t the short-lived isotopes are dealt with first. A very much faster method uses γ - r a y spectros copy and a computer stripping pro gram (7). Table I I I shows the computer program for this deter mination and Table I V shows some preliminary results obtained by this process as compared with the radio chemical separation. As can be seen, the two methods give com parable results and the stripping method is a t least as sensitive. For a few specialized applica tions, stripping polarography has been found t o be applicable (14)This technique employs a hanging mercury drop electrode, into which the impurity is placed, and an anodic dissolution using a fastsweep polarograph. This suffers from m a n y of the difficulties of colorimetric analysis, b u t the reagents are kept to a minimum and it is possible to determine as little as 50 p.p.b. of some elements. T h e applications are summarized in Table V. Distribution Studies All of the foregoing methods are applicable to homogeneous samples and are of great value in assessing the quality of a finished crystal, or indeed of material a t a n y point prior to this. However, much ex perimental work in semiconductors is concerned with heterogeneous dis tribution in crystals. B y tech

niques embodying grinding or dic ing, all the techniques so far men tioned can be used to study segre gation and diffusion. I n crystal growth, segregation can lead to a concentration gradient through the crystal of any or all of the trace elements. B y introducing elements as their radioactive isotopes, straightforward slicing and count ing techniques will allow the seg regation coefficient to be calculated. Diffusion is now generally em ployed to form junctions in semi conductors, and this phenomenon can be easily studied using an ac tive form of the diffusant. Diffu sion is usually from one face of a slice of the crystal, and lapping techniques allow as little as 1 m i cron to be removed from this slice. By successively lapping and count ing, information can be fed to the computers a n d a standard least squares t r e a t m e n t returns a dif fusion profile. Figure 1 is an ideal ized representation of the two types of diffusion, limited source (Curve A ) , and infinite source (Curve B ) . Knowing the diffusion coefficients, it is possible to adjust concentra tion and temperature in such a w a y t h a t a n y desired device structure can be obtained. Failure of a junc tion to form at the predicted depth or with the expected front m a y be due to an impurity in t h e dopant or on the surface which is vitiating the results. I n this case, the dif fusion is carried out with the nor mal inactive dopant and the crys-

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Table III. Spectra

Computer Program for Analyzing Complex y-Ray

City State.

Zip Code_

Circle No. 169 on Readers' Service Card VOL. 3 8 , N O . 3, MARCH 1 9 6 6

·

31

A


REAGENTS

RADIOCHEMICAL SEPARATION

ELEMENT

IO.O

9.7 ppb

GOLD

ppb 2I.7 ppb

GALLIUM

87, 89 ppb

112,94 ppb

GOLD

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0.60, 0.33 ppm

0.60, 0.64 ppm

ARSENIC

O.Z2, 0.055 ppm

0.44, 0.46 ppm

Table IV. Comparison of Radiochemical and γ-Ray Spectroscopic Methods for Trace Elements in Silicon

tal is then irradiated. The lapping is carried out as before but y-ray spectroscopy is applied to the sample to identify the isotopes. Overlapping curves may be identi fied which give a distorted carrier distribution. These lapping methods are suc cessful only if the distribution of dopant is uniform across the slice. To check this point, autoradiograms are made of the slice as successive layers are removed from its surface. Figure 2 shows an unsuccessful ex periment in which the gold has spiked. This phenomenon may be duo to physical imperfections in the lattice. It will obviously render in valid conclusions based on diffusion theory.

Physical properties of the crystal can also profoundly affect the characteristics of a device. The growth axis of the crystal will, to a very large extent, govern the growth

STRIPPING

P0LAR0ÔRAPHY

·


OF SEMICONDUCTORS ELEMENTS DETERMINED

SILICON GERMANIUM GALLIUM ARSENIDE INDIUM ARSENIDE INDIUM ANTIM0NIDE INDIUM

ZINC INDIUM CADMIUM

Table V. 32 A

characteristics. It will normally be the case that one preferred axis will give the easiest growth and lead to the lowest dislocation den sity. Moreover, diffusion can be affected by the crystallographic di rection, particularly the shape of the diffusion front. Consequently, equipment for orienting crystals is essential; usually this is accom plished by x-ray diffraction. Dis locations can be sources of electrons or, more usually, can act as trap ping centers and may affect the mobility of the semiconductor ma terial. Much information on dislo cations can be obtained by etch pit counts but this technique is limited, since it detects only a dislocation that intersects the surface, and can not identify the type nor distinguish it from mechanical damage. A more informative technique is that of x-ray topography, and Figure 3(a) outlines the equipment for transmission topography by the Lang (16) method. Due to the absence of destructive interference

MATERIALS ANALYZED

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I9.3ppb

GALLIUM

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GAMMA SPECTROSCOPY

TIN LEAD COPPER THALLIUM BISMUTH

Applications of Stripping Polarography


Figure 1 .

Diffusion Profiles

Figure 2.

Figure 3.

at dislocation sites, the emergent x-rays are more intense from these sites than from elsewhere in the crystal. The result is an exposure that is uniform over most of the area but with more heavily exposed lines than originated at the dislo cations. A variation of this tech nique (10) uses reflected x-rays and this apparatus is shown in Figure 3(6). It is essentially the same as that used for the Lang technique except for the position of the film. The results differ, however, in that this method examines the surface to a depth of about 20 microns, whereas the transmission method examines the full thickness of the slice (up to a millimeter for sili con). Figure 4(a) shows a reflec-

Non-Uniform Diffusion of Gold into Silicon

X-Ray Topography,

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Figure 4. Reflection Topograph of Gallium Arsenide, (a) (242) Plane Diffracting (b) (440) Plane Diffracting

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tion topograph from a gallium arsenide slice t a k e n with the (242) planes diffracting. N o t e the dislo cations. Figure 4 ( 6 ) shows the same slice with the (440) planes diffracting, and it is seen t h a t the dislocation is now much more in tense. I t can be shown t h a t this in tensity is a minimum when the Burger's vector is parallel to the diffracting planes, and a m a x i m u m when it is normal. T h e Burger's vector is simply the direction in which the lattice is displaced and can be deduced from t h e crystal orientations when the intensity of the line is a m i n i m u m and a maxi mum. T h e dislocation axis can be deduced from the plane along which the slice has been taken. If the Burger's vector and the dislocation axis are normal, the dislocation is an edge dislocation ; if they are p a r allel, the dislocation is a screw dis location. If the image intensity has no point of extinction, then this is not a dislocation but a mechani cally induced fault such as a micro crack or scratch, or a precipitate.


At laboratory supply houses everywhere

Figure 5.

Figure 6.

Surface

Adsorption of Radioactive Copper from Solution

Autoradiograms of Silicon Surfaces with Absorbed Copper, (a) Freshly Etched, and (b) Nitric Acid Treated

Films

D u r i n g the discussion of this last technique, it was noticeable t h a t t h e interest was less in bulk properties t h a n in surface properties, and this is indicative of the trend in semi conductor materials research in re cent years. T h e importance of the surface to the performance of a de vice has long been acknowledged. Adsorption of ions from etch solu tions, for example, has been found (4) one of the causes of the vexa tious phenomenon of inversion, in which a supposedly η-type area be comes p - t y p e a t the surface by de pletion of electrons. Figure 5 shows the results (17) of a typical adsorp tion experiment using radioactive copper as a tracer. A freshly etched silicon surface adsorbed cop per from solution in a linear fashion; however, t r e a t m e n t with nitric acid to form a surface oxide reduced the adsorption over most

of the range by almost a factor of 10. T h i s conclusion was confirmed by autoradiography. Figure 6 ( a ) is a p r i n t from the surface of the freshly etched m a t e r i a l ; Figure 6 ( 6 ) is t h a t from the nitric acid treated slice. T h e use of oxides to protect sur faces, particularly silicon surfaces, has been widespread both for p a s sivation in p l a n a r devices or as a protective against etch in preparing mesa devices or integrated circuits. Impurities in these oxide films can degrade the electrical characteris tics by a mechanism similar to t h a t of metals on the surface ; these films have also been studied by activation analysis. I t has been suggested t h a t alkali metals are undesirable impurities. T h e distribution of sodium in an oxide film was de termined (3) by irradiation, fol lowed by etching and counting; a fairly uniform distribution through the film was found. I t was thought

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37 A

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Figure 7. Distribution of Sodium and Phosphorus in Oxide Film by Activa tion Analysis

that the overlay of a phosphorus oxide glaze might getter the sodium and concentrate it on the outer sur face, leaving the interface free. Figure 7 shows the phosphorus and sodium distribution in an oxide film that had been overlaid and heated. The device, taken from a pilot line, was irradiated and the surface etched away in 200 A. laps. The solutions were determined for so dium by γ-ray spectroscopy and for phosphorus separated radiochemically. The contents of these ele ments in these 200 A. laps is, as can be seen, in the parts per million range giving an indication of the extreme sensitivity of this method. The ability of the phosphorus glaze to getter the sodium is also quite remarkably demonstrated. The thickness of these oxide films must be determined in order to follow experiments such as the one just outlined. Thicknesses of films of this type can be visually esti mated quite closely by the color of the interference patterns from the surface but a more accurate value can be obtained by ellipsometry. When natural unpolarized light strikes a film-covered surface, an interference phenomenon gives rise to reflected light that is elliptically polarized, and the amplitude and


REFERENCE SUBSTANCES FOR USE IN

ORGANIC MICRO-ANALYSIS

@

ORGANIC ANALYTICAL STANDARDS

Name of substance

Figure 8.

Visible Light Ellipsometer

azimuth angle of this ellipse can be determined by an ellipsometer, shown in Figure 8. In practice, elliptically polarized light is used in the incident beam and adjusted until plane polarized light results in the reflected beam. The angle be tween the polarizer and the quarter wave plate is related to Δ, the phase difference between the light in the two axes of the ellipse. The angle between the polarizer and the ana lyzer is related to the azimuth angle, ψ of the ellipse. These two parameters (taken in conjunction with the refractive index of the film,

the angle of incidence, and the wavelength of the incident light) are combined in a very complex re lationship with thickness which can, however, be dealt with by com puter techniques. Figure 9 shows two curves, each for a different re fractive index, which relate Δ and ψ for transparent films on gallium arsenide. These curves are gener ated by a computer from theory. The determined values of Δ and ψ give a point which, if the refractive index is correct, will fall on the curve and give the thickness. If the point is off the curve, the values

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Figure 9. Typical Ellipsometer Curves Relating Thickness to Polarization Parameters

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Old Reliable

Figure 1 0 . Epitaxial Gallium Arsenide Showing Hillock Formation

can be fed to the computer and both thickness and refractive index calculated. T h e relationships involved in this interference phenomenon are involved and further information should be sought elsewhere {18). In practice, however, the determination is very simple. T h e q u a r t e r wave p l a t e is set p e r m a n e n t l y a t 45° to the plane of incidence and the polarizer and analyzer are m u tually adjusted to a visual null. Epitaxial

Layers

T h e use of oxide films as passivation agents or as insulators is an indication of the growing sophistication of solid state devices. M a n y integrated circuits, and even some discrete devices, are now being fabricated not from bulk material b u t from epitaxial material. I n this technique, a film of material with the right device characteristics is grown from the vapor phase on m a terial of different characteristics. This epitaxial layer must, of course, by definition be single crystal. I t must have high perfection and be free from impurities other t h a n the selected dopant. Figure 10 shows an epitaxial film of gallium arsenide which is of poor quality, characterized by the a p pearance of hillocks. These faults probably originate a t the interface and the x-ray topographic technique m a y be used to evaluate the substrate surface as described above. Since this method can distinguish between grown imperfections and mechanically induced damage due

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Figure 11.

Figure 12.

Infrared Reflection Spectra of Successive Depths of Gallium Arsenide

Infrared Interference Pattern from Film-Covered Surface

to sawing or grinding, it can be used to follow the removal of this damage by etching. This mechan ical damage can also be detected by infrared reflection {12). Figure 11 shows a series of spectra taken as the surface of a gallium arsenide slice was progressively etched away. The peak at 36 μ is due to a lattice vibration; the remainder of the spectrum is due to free carrier ab sorption. These free carriers might be trapped at lattice faults; what ever the reason, when the damage is removed the spectrum is typical of the bulk and remains constant. It is interesting to note that the damage extends to a depth about equal to that of the grit diameter. 42 A

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The crystal perfection of the layer itself can be determined by the same methods as bulk material, either etch pit counting or x-ray topography. The purity can be de termined by mass spectrography ; the substrate material is analyzed first, followed by the composite sub strate plus epitaxial film. The dopant in the substrate can be used as a means of determining the ratio of epitaxy to substrate material; the impurities in the film then cal culated. Alternatively, if the film thickness is known and the sub strate is relatively much purer than the film (which is usually the case), a preliminary analysis of the sub strate can be avoided. Epitaxial

film thickness measurements can be carried out by the same techniques as for oxide films with one compli cating factor—they are not trans parent to visible light and measure ments must be made in the infrared. The method usually employed is equivalent to the color method for oxide films—that is, the interfer ence fringes are determined (5). Light reflected from the surface gives rise to a pattern as shown in Figure 12, where the difference in wavelength between successive peaks is related to the film thick ness. This method is quite satis factory for many applications but does have a number of drawbacks, chief of which is that it is not ap plicable below about 5 μ since the peaks become too broad and diffuse. Recently the method of ellipsometry has been extended to the infrared region (8, 9), and Figure 13 shows the instrument which has been con structed. The polarizer and ana lyzer are of polyethylene film, ac tually 30 layers of 0.5-mil thick film; the quarter wave plate is of natural quartz. The reflection monochromator passes light at 55 μ. With this instrument, thicknesses down to 1 μ or less have been meas ured with an accuracy comparable to that of the conventional method. Improved wire grid polarizers, ex pected to be available shortly, should improve the reproducibility to about 1 per cent.

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Infrared Ellipsometer

Some epitaxial films are now be ing tailored to band-gap by the use of alloy systems such as gallium arsenide phosphide and indium gallium arsenide. These may be of any combination, and analytical methods are required to determine the composition. Since the systems are essentially solid solutions, Vegard's law holds and the lattice spacing changes linearly with con centration. Figure 14(a) shows an x-ray diffraction scan for gallium phosphide on gallium arsenide {19). Since the substrate is always gal lium arsenide, this peak acts as a datum point and the difference be tween this and the epitaxial ma terial peak is measured in 2 Θ. The (333) reflection is used in order that 2 θ be high and hence give good separation of the peaks. Figure 14(b) shows a scan for an actual sample of gallium arsenide phos phide on gallium arsenide. An alternative method uses vis ible reflectivity, and Figure 15 shows reflection spectra for a series of indium gallium arsenide alloys (13). As the band-gap changes, several other transitions are also affected and the change in wave length of these peaks can be used

as a measure of the composition. In a similar application, Figure 16 relates wavelength to composition for two peaks in the gallium arse nide phosphide reflection spectrum (20). Both these methods for determin ing epitaxial alloy compositions are reproducible to about 3 per cent atomic. They are really comple mentary since x-ray diffraction ex amines the whole film thickness, and visible reflectivity just exam ines the surface. By successive etching, the second method can be used to follow changes through the film. Current Trends

Almost five years ago, Burkhalter (1), in another Report for Ana lytical Chemists, discussed the tech niques and problems in this field of semiconductor analysis. Re-read ing this paper, one is struck by the fact that at that time the emphasis was almost exclusively on bulk ma terial. Problems involving purifica tion of the basic material were over-riding; without a good intrin sic material, no devices could be made at all. Consequently, the


Figure 14. Composition of Gallium Arsenide Phosphide Epitaxial Layers, (a) Gallium Phosphide on Gallium Arsenide, and (b) Gallium Arsenide Phosphide on Gallium Arsenide

Figure 16. Relationship between Wavelength and Concentration for Gallium Arsenide Phosphide Alloys

analytical scientist in this field was concentrating on bulk analysis—that is, average values for comparatively large samples but at extremely low levels. While one would hesitate to suggest that this 46 A

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Figure 15. Reflection Spectra of Indium Gallium Arsenide Alloys

type of problem has been completely solved, it is true to say that high quality bulk material can be routinely produced in large quantities. This has allowed the materials researcher to turn to more subtle properties of crystals involving heterogeneous distribution. Crystal perfection has already received considerable attention, and surfaces and surface films are now being subjected to intense study. The emphasis now, shown to a considerable degree in this review, is to smaller and smaller sample size and to the topographical distribution of impurities and faults. Five years ago the solids mass spectrograph was the tool which, it was hoped, would make the greatest contribution to the solution of the existing problems. This hope was largely realized. Today, with the new problems posed by topography, the electron probe microanalyzer may prove equally successful. The next five years will tell. Literature Cited (1) Burkhalter, T. S., ANAL. C H E M . 33,

21A (1961). (2) Cali, J. P., "Trace Analysis of Semiconductor Materials," Pergamon Press, Oxford, 1964. (3) Carlson, H. G., Fuller, C. R., Os-

borne, J. F., Electrochem. Soc. Mtg., Buffalo, Oct. 1965. (4) DeMars, G., Semiconductor Products 2, (4), 24 (1959). (5) Groves, W. O., Semiconductor Products 5, (12), 25 (1962). (6) Heinen, K. G., Larrabee, G. B., in "Standard Methods of Chemical Analysis," F . J. Welcher, éd., Vol. I l l , in press. (7) Heinen, K. G., Larrabee, G. B., unpublished data, Texas Instruments, Inc., Dallas, 1965. (8) Hilton, A. R., Jones, C. E., 4th National Mtg., Soc. for Appl. Spec, Denver, Sept. 1965. (9) Hilton, A. R., Jones, C. E., Electrochem. Soc. Mtg., Buffalo, Oct. 1965. (10) Howard, J. K., Dobrott, R. D., Appl. Phys. Letters 7, 101 (1965). (11) Jones, C. E., Andrychuk, D., Massengale, J . F., Pittsburgh Conf. on Anal. Chem. and Appl. Spec, 1961. (12) Jones, C. E., Hilton, A. R., J. Electrochem. Soc. 112, 908 (1965). (13) Jones, C. E., 4th National Mtg., Soc. for Appl. Spec, Denver, Sept. 1965. (14) Kane, P. F., Burson, K. R., in "Standard Methods of Chemical Analysis," F . J. Welcher, éd., Vol. I l l , in press. (15) Klein, H. M., unpublished work, Texas Instruments Inc., Dallas, 1964. (16) Lang, A. R., J. Appl. Phys. 29, 597 (1958). (17) Larrabee, G. B., Electrochem. Soc. Mtg., Detroit, Oct. 1961. (18) Passaglia, E., Stromberg, R. R., Kruger, J., Eds., "Ellipsometry in the Measurement of Surfaces and Thin Films," Nat. Bur. Std. Misc. Publ. 266, Washington, 1964. (19) Williams, E . W., Cox, R. H., D o brott, R. D., Electrochem. Tech., in press. (20) Williams, E. W., Jones, C. E., Solid State Comm. 3, 195 (1965).

SEMICONDUCTOR MATERIALS RESEARCH

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