Autoradiography applied to tracer studies in semiconductor research

International Journal of Applied Radiation and Isotopes, 1967, Vol. 18, pp. 829-833. Pergamon Press Ltd. Printed in Northern Ireland

Autoradiography Applied to Tracer Studies in Semiconductor Research J A M E S F. O S B O R N E Texas Instruments Incorporated Central Analytical Chemistry Facility Dallas, Texas

(Acceptedfor publication 31 May 1967) The single crystal materials and processes used in semiconductor research present problems unique to them as opposed to metals and their alloys. Autoradiography has at times been the only technique by which these problems could be investigated. The types of films, their speed and resolution and the nonconventional use of dental X-ray film used in these studies are described. The techniques and some specific applications ofautoradiography to semiconductor research problems are presented. L ' A U T O R A D I O G R A P H I E A P P L I Q U E E A U X ETUDES A I N D I C A T E U R S DANS LA R E C H E R C H E DES S E M I C O N D U C T E U R S Les mat~riaux ~ cristal unique et les proc6d~s usit~s dans la recherche des semiconducteurs prdsente des probl~mes particuliers ~t euxm~me par contre aux m6taux et ~t leurs alliages. L'autoradiographie a 6t6 parfois le seul moyen par lequel on a pu regarder ces probl~mes. O n d6crit les esp~ces de pellicule, leur rapiditd et leur rdsolution et l'emploi non-conventionnel de la pellicule ~ rayons X dentaire utilis6e dans ces &udes. O n prdsente les techniques et quelques applications spficifiques de l'autoradiographie aux probl6mes de la recherche des semiconducteurs. PA~HOABTOPPA(I)HH, IIPHMEHHEMAfl IIPH H 3 Y q E H H H M E q E H b I X ATOMOB B HCCJIE~OBAHHH HOJIVHPOBO)~HHHOB O~HoKpneTaJI~nqecgHe MaTep~a~u ~ npo~eccu, ynoTpe6~AeMue B nccne~oBaHnnnoaynpoaO~HHROB,npe~cTaBa~mT npo6~eMu, CBOIICTBeHHUeTOnbKOriM, ~ npoTnBOnOnomm~eMeTannaM n nx cnnaaaM. BpeMeHaMHpa~noaBTorpagpn~ 6u~a e;~nHCTBenno~TexHngol~, npH nOMO]~n soTopol~ aTH npo6aeMu Mor~n 6Hrb ncc~e~oBaHu. OnHcuBamTca Tnn~ ~OTOn~eHOK, HX CRopocr~ n paapemammaa cnoco6aocT~, paB~o ~ag i~ ~eoS~uHoe np~MeHe~ae 8y6HOii p e n r r e n o n ~ e ~ n npn O~OMnayueHnn. IIpe3cTaBnen~ Texno~ornuec~ne npneM~ H ~e~ovop~e cne~n~qecRne n p ~ e a e n ~ pa~noaBTorpa~a B npo6neMax nccne~oBaHna nonynpoBo~HHHOB.

A U T O R A D I O G R A P H I E UND I H R E ANWENDUNG BEI I N D I K A T O R E N U N T E R S U C H U N G E N IN DER HALBLEITER F O R S C H U N G Die Einzelkrlstall-Werkstoffe und Verfahren in der Halbleiterforschung stellen einzigartige Fragen, die yon Metallen und ihren Legierungen grundverschieden sind. Manchmal ist die Autoradiographie das einzige Verfahren, das zum Studinm dieser Probleme benutzt werden kann. Es werden die Filmarten, ihre Geschwindigkeit und Aufl6sung, und die ungew6hnliche Verwendung yon zahn~irzflichem R6ntgenfilm beschrieben. Die Verfahren und einige spezifische Anwendungen yon Autoradiographie auf Halbleiterprobleme werden erl~iutert. 829

830

James F. Osborne

INTRODUCTION

NUCLEAR AND X - R A Y E M U L S I O N S

AUTORADIOGRAPHY prior to twenty years ago T h e r e are a n u m b e r of nuclear and X - r a y was used only in specialized applications, emulsions covering a wide range of sensitivity mainly in biological work. T h e first application available from various manufacturers, e.g. of autoradiography was probably by Becquerel Eastman K o d a k and Ilford. All of these can be when in 1896 his photographic plates were used for autoradiography if sufficient activity is exposed to the gamma-radiation of uranium ore. present in the sample and the degree of contrast Later STEP and BECKE~I) used autoradiography and resolution is not critical. to demonstrate the segregation of radioactivity In semiconductor research, macro autoradiogin ore specimens. T h e discovery of artificial raphy showing the general distribution of trace radioactivity in 1934 by F. Joliot and Irene impurities in the semiconductor material is Curie greatly expanded the range of applica- usually sufficient to obtain the desired informability of autoradiography. I n 1938, GROVEN, tion. A n u m b e r of autoradiographic and X - r a y GOVAETS and GUEBEN(2) prepared autoradio- emulsions were investigated to determine which graphs using artificial radioactivity, utilizing were best for macro autoradiography of semithe beta particles emitted by radiophosphorus conductor materials, and these are listed in Table I. and neutron-activated iridium metal. Autoradiography, though most often used in A "test sample" was prepared as a common biology and medicine TM, has proved to be usefu! source of exposure for all emulsions tested. This in metallurgical and chemical work ¢~) in sample was prepared b y sputtering a 2000 A industry and, particularly, in the iron and SiO 2 film on the polished face of a GaAs slice. Using the K M E R Photo Resist TechniquO 6~ aluminum industries. T h e semiconductor industry is a compara- and a L-66 emitter window mask, a n u m b e r of tively young industry and virtually no litera- windows were cut into the oxide film, Several ture has been published on autoradiography of these windows cut into the oxide film are and its application to semiconductor materials. shown in Fig. 1. Materials used in the semiconductor industry T h e slice with the cut windows was then are termed ultrapure. W h e n impurities are placed in a nickel-plating solution tagged with deliberately added to change the electrical Ni-63 (pure fl-emitter) and the nickel was characteristics of the semiconductor they are selectively deposited into the cut windows. present at the low part per million or less The Ni-63 test sample was used to obtain concentration. Even the electron microprobe autoradiograms with the various autoradiocannot detect impurity distribution within a graphic and X - r a y films listed in T a b l e I and material at these concentration levels. Thus some of these are shown in Fig. 2. These autoradiography has been the only way, in most autoradiograms show the relative resolution of instances, where t h e actual distribution of the the films tested to be essentially equal, with the radioactive " t a g g e d " impurity within the semi- exception of the AR-10 stripping film, which is conductor material could be measured. an extremely fine-grain film, and it also affords Autoradiography has proved useful in such more intimate contact between the sample and specific applications as diffusion, impurity segre- emulsion. Certainly for macro-autoradiography gation, surface contamination, surface damage, the X - r a y films are equally as good as the autoanisotropic segregation, grain boundaries and radiographic films and, where a large n u m b e r of dislocations in semiconductor materials. T h e autoradiograms are required, a substantial cost evaluation of various nuclear and X - r a y reduction is realized. In particular the X - r a y emulsions with specific applications to some of films DF-7 and DF-58, which are dental the semiconductor problems mentioned above films, have proven useful for autoradiography will be discussed, However, the general of semiconductor materials because of the principles of autoradiography will not be dis- convenient size (3 × 4 cm) and individuallycussed as these are adequately covered by w r a p p e d film packets which facilitates handNORRIS{5) and others. ling.

Autoradiography applied to tracerstudies in semiconductorresearch

831

TABLE 1. Emulsions for autoradiography Manufacturer

Film type

Form as usually supplied

Eastman Kodak

NTB NTB2 Type A No Screen Periapical RadiaTized DF-7 Periapical UltraSpeed DF-58 Type M

On 0.06 in. glass base On 0.06 in. glass base On 0.06 in. glass base On 0.06 in. glass base Dental X-ray film-double coated Dental X-ray film-double coated Industrial X-ray filmdouble coated Industrial X-ray filmdouble coated Stripping emulsion with 10 # gelatin base on glass plates Stripping emulsion with 10/~ gelatin base on glass plates Industrial X-ray filmdouble coated

Type R Kodak Ltd.

AR-10 AR-50

Ilford

Type G

APPLICATIONS T h e application of autoradiography to semiconductor materials present problems unique to them as opposed to metals and their alloys. I n particular, the concentration levels of impurities being looked at in these materials m a y range from p a r t per trillion, for material being used to fabricate semiconductor detectors, to the low p a r t per million in transistor material, compared to the percentage range in metals and alloys. Some of the expected or deliberately added radioactive impurities, found in semiconductor materials, and their half-lives are shown in T a b l e 2. All of these isotopes are amenable to autoradiography with the proper selection of film. Some of the specific applications and rcsultant'autoradiographs are presented to show areas in semiconductor research where autoradiography has proved useful.

1. Diffusion into grain boundaries in lead telluride A thin film of Fe-59 was plated on polycrystalline PbTe. T h e P b T e was then diffused at 600°C for 48 hr. After diffusion the surface was etched lightly to bring out the crystalline structure. T h e Fe-59 activity in the P b T e at this point was 1.6 × 105 d p m / c m ~. T h e autoradiography film chosen was N T B 2-25 /~ which is particularly sensitive to betaparticles and has good resolution. An exposure

Emulsion thickness (p) 10, 25 10, 25 25 25 14 17 32 25 5 12 13

time of four days was used and the resulting autoradiogram is shown in Fig. 3. I t is apparent from the autoradiogram that the Fe-59 has preferentially diffused down the grain boundaries.

2. Diffusion into dislocations in silicon Gold-198 base and emitter rings were alloyed into a silicon wafer at 716°C for 3 min. T h e wafer was then incrementally etch-lapped on the ring side to determine the depth of gold diffusion. At a depth of 104/~, an autoradiogram showed the gold preferentially diffusing into a line of dislocations in the silicon. T h e Au-198 activity in the silicon at this time was 1.1 × 10 e d p m / c m 2. T h e autoradiographic film chosen was K o d a k DF-7 Radia-Tized (Dental film) which is a medium speed film with fair resolution. An exposure time of 20 rain produced the autoradiog r a m shown in Fig. 4. 3. Diffusion into gallium arsenide showing impurity segregation T i n - l l 3 was plated on a tantalum strip which was then placed with a slice of p-type gallium arsenlde in a I0 ml quartz tube and sealed under high vacuum. V a p o r diffusion of the S n - l l 3 into the gallium arsenide was achieved by heating for 6 hr at 835°C.

James F. Osborne

832

TABLE 2. Radioactive impurities in semiconductors Impurity isotope

Half-life

Antimony-122 Arsenic-76 Cadmium-115M Chromium-51 Copper-64

2.8 days 26.5 hr 43 days 27-8 days 12"9 hr

Gold-198 Indium-114M Iodine-131 Iron-59 Manganese-56 Mercury-203 Phosphorus-32 Silicon-31

65 hr 50 days 8.05 days 45 days 2"58 hr 47 days 14.3 days 2.6 hr

Sodium-24 Sulfur-35 Tellurium- 127M

15 hr 86.7 days 105 days

Tin- 113 Zinc-65

118 days 245 days

Since tin-113 is a pure g a m m a , X - r a y emitter, it has been difficult to obtain good autoradiograms. For this reason, the film chosen was K o d a k DF-7 Radia-Tized (Dental film) which is particularly X - r a y sensitive. An autoradiog r a m was m a d e at a depth of 200/t into the gallium arsenide where the Sn-113 activity was 7-1 × 105 d p m / c m 2. An exposure time of 16 hr produced the autoradiogram shown in Fig. 5. T h e autoradiogram shows the tin spiking into the gallium arsenide, creating a high degree of impurity segregation.

4. Imperfection in native oxides of silicon I n the study of Ag-110 electrodeposition on silicon, it had been observed that freshly-etched silicon always had a larger a m o u n t of Ag-110 deposition than silicon which had been boiled in H N O s acid. Autoradiograms of these slices were m a d e using T y p e A autoradiographic plates. In Fig. 6(a), the autoradiogram of the freshly-etched silicon slice shows a heavy and fairly uniform electrodeposition of the Ag-110. I n Fig. 6(b) the H N O a acid boiled silicon slice which has about 100# of native oxide on the surface shows only electrodeposition of the Ag110 on the silicon where imperfections in the

Semiconductor material present in Germanium, silicon Silicon, germanium Gallium arsenide Gallium arsenide Gallium arsenide, germanium, indium arsenide, silicon Silicon Lead telluride, silicon, germanium Lead telluride Gallium arsenide, lead telluride Gallium arsenide, indium arsenide Germanium Silicon, germanium Gallium arsenide, germanium, indium arsenide Silicon Gallium arsenide Gallium arsenide, indium antimonide Gallium arsenide Gallium arsenide oxide film occur, thus explaining the lower Ag-110 concentration found in slices treated in this manner.

5. Vapor diffusion into cadmium sulfide showing surface damage In-114 was vapor diffused into a polished slice of cadmium sulfide in a closed tube at 500°C for 14 min. An activity of 1 × 104 d p m / c m ~ was present in the slice after diffusion. K o d a k DF-7 Radia-Tized film was used with an exposure time of 4 days required to produce the autoradiogram shown in Fig. 7. T h e autoradiogram shows preferential diffusion of the In-114 into surface damage caused by lapping the cadmium sulfide to obtain a polished surface.

6. Anisotropic segregation of impurities in the growth of III-V intermetaUics Te-127 was added as an n-type impurity to a GaAs melt at 1235°C from which a GaAs single crystal was pulled. T h e crystal was sliced on the 111 plane into 30-rail slices and autoradiograms m a d e on each slice to show the Te-127 distribution throughout the crystal. T h e autoradiograms were m a d e using K o d a k

1. Windows emitter-window

FIG.

cut into SiO, film using L-66 mask. Magnification 250 X .

(b)

(4 FIG. 2.

(a) Type

(4 A film,

(b) NTB-2 film (c) AR-10 stripping (f) Type R film. Magnification

(f) film, 215

(d) DF-7 X.

film,

(e) DF-58

film, 832

FIG. 3. Diffusion

of Fe-59

FIG. 4. Diffusion

into grain boundaries

of Au-198

into dislocations

of lead telluride.

in silicon.

Magnification

Magnification

3.5

3.5

X.

x.

FIG. 5. Diffusion

of Sn-113

(4

into gallium

arsenide

showing

impurity

segregation.

Magnification

3.5

X.

(b)

FIG. 6. Electrodeposition of Ag-110 on silicon showing imperfections in the native oxide. silicon slice, (b) Nitric-acid-boiled silicon slice. Magnification 3.5 x .

(a) Freshly

etched

FIG. 7. Vapor diffusion of In-l 14 into cadmium

FIG. 8. Gallium

arsenide

slice showing

anisotropic

sulfide showing

segregation

surface damage.

of Te-127

impurity.

Magnification

Magnification

3.5

3.5

x.

x.