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Fabrication methods for lithium drifted surface barrier silicon detectors
NUCLEAR
INSTRUMENTS
AND
METHODS
40 (1966) 330-336;
~) N O R T H - H O L L A N D
PUBLISHING
CO.
FABRICATION M E T H O D S FOR L I T H I U M DRIFTED SURFACE BARRIER SILICON DETECTORS* H M MURRAY, J W HARPSTER ~ a n d K J CASPER Department of Phystcs, Western Reserve Umverstty, Cleveland, Ohto
Received 21 September 1965 Techniques for the fabrlcatmn of llthmm surface barrier slhcon detectors are presented All important chemical procedures are described m detail The detectors made by these techniques
exhibit resolutions of 15-30 keY on internal conversion electrons when operated at room temperature.
1. Introduction
3. Crystal specification and evaluation
A n u m b e r of papers I 7) have appeared describing methods of fabricating h t h l u m drifted sohd state silicon detectors. The detectors described tn these papers either have not been of the surface-barrier type or else requtre coohng for best operatton The methods described here are consistently dependable for the preparation of surface barrier types which exhibit good r e s o l u n o n at room temperature.
The sihcon crystals are obtained as wafers shced and lapped to the desired thickness from Geoscience Instruments Corporation.§ The silicon is specified to be p-type, b o r o n doped, float-zone refined, 50-200 ohmcm, with a d t s l o c a n o n c o u n t less than 20000/cm 2 The orientation Js usually [l l 1] although crystals cut with different orientations have been used. As a first step the d M o c a h o n pattern is determined. A single slice from each ingot ts etched in the following manner' 1. The crystal is lapped first with 20 [lm and then l0/2m a l u m i n u m oxide powder, cleaned ultrasomcally m trlchloroethylene and washed successively in alcohol and distilled water 2 A 2'1 mixture of H N O 3 and H F ts prepared in a Teflon beaker and cooled in ice water. The wafer is
2. Chemical specifications Electronic grade chemicals are used t h r o u g h o u t the fabrlcatmn process Either absolute ethanol or m e t h a n o l ~s statable where the use of alcohol Is specified. The acid concentrations are 7 0 ~ H N O 3, 48% H F and glacial acetic acid and all etches are prepared and used in a Teflon beaker Baker Atomex I m m e r s i o n Gold S o l u t m n ++ is used in the electroless gold plating. The h t h l u m is obtained in the form of rods ~ and is kept stored m mineral oil For evaporation, the desired a m o u n t of h t h l u m is cut off the end of the rod with a diagonal wire cutter and cleaned with t n c h l o r o ethylene Vinyl gloves tt are worn whenever the crystal must be handled and the etched sides of the crystal are kept scrupulously clean. Generally, stainless steel tweezers are used to hold the crystal wherever possible. In discussing the crystal after h t h m m diffusion, the term " f r o n t surface" refers to the surface on which h t h m m has been diffused and "backsurface" refers to the surface toward which the lithium is drifted
1 10 Beckman Street, N e w York, N Y
* Supported in part by a grant froth the N a t i o n a l A e r o n a u t i c s
and Space Administration + Present address Department of Electrical Engineering, Ohto State University Columbus, Ohio +Engelhard Industries, lnc, Baker Platinum DtvlsJon, 113 Astor Street, Newark, N J Lithmm Corporation of America, Bessemer City, North Carolina V+Wllson Tru-Touch, Becton, Dickinson and Co, Rutherford, New Jersey 330
FJg 1 Slhcon wafer after dislocation etch, showing the high dislocation density ring.
LITHIUM DRIFTED SURFACE BARRIER SILICON DETECTORS
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4 h at r o o m temperature, after which the etch is again quenched with distilled water. In most of the crystals, an outer ring of high density dislocations is observed. Such a ring is evident m the crystal shown In fig. 1. The size of this ring is approximately the same for all of the slices of the ingot. Therefore, before any further processing, this high dislocation ring must be removed from each crystal. If the dislocation density is high even in the center of the crystal, the ingot is rejected as unsuitable
Fig 2 Lapidary wheel used for sanding sides of wafers.
then etched for 45 sec in this solution. The etch is quenched by placing the beaker under running distilled water until all o f the etchant has been displaced by water. In all of the polish etches, no air is permitted to come into contact with the wafer while it is being activated by acids. 3. The wafer is then placed immediately into a Teflon beaker containing a 10:3:1 mixture o f H O A c , H N O 3 and H F in sufficient a m o u n t to immerse the wafer. The crystal is etched in this solution for at least
F~g 4. Slhcon crystal after diffusion m furnace of fig 3 The dark tuner circle is hthmm hydroxide coating and indicates the area of diffusion
i
Fig 3 Schematic drawing of pyrex diffusion furnace. The diffused area of the crystal is limited by the size of the aperture m the evaporation table
4. Preparation of the crystal for diffusion Each crystal is sanded down to remove the high dislocation ring using the small lapidary wheel* shown in fig. 2 and cloth backed, # 320 silicon carbide discs. The crystal and the disc are kept moistened to prevent overheating of the crystal. The sides are then smoothed by light hand sanding with silicon carbide paper. The faces are lapped and the wafer is ultrasonically cleaned in tnchloroethylene and washed successively with alcohol and distilled water 5. Diffusion of lithium Two diffusion furnaces have been used. The first, shown in fig. 3, is operated in a vacuum of about 10 -5 mm. Lithium is evaporated from the basket onto the surface o f the crystal. The furnace is then raised to a temperature of 370°C as measured with the platinum * Model N B M D , Lee Lapidaries, 6617 Loram Ave., Cleveland, Ohio
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H.M. MURRAY et al.
resistor*. After 5 min, the current is t u r n e d off and air is let into the system immediately. The crystal is then cooled by placing it on a c a r b o n block. A crystal diffused in this m a n n e r is shown in fig. 4. The d a r k lithium h y d r o x i d e circle indicates the area of diffusion. Diffusion of l i t h i u m on the sides o f the crystal is minimized, b u t the final detector size is restricted by the aperture in the m o u n t i n g plate. Tungsten w~re evaporation basket
-I Stainless steel plate
Mma~
Plat+num resmtor
~\N,chrome w,re furnace
Fig 5 Diffusion furnace The heating element 1s a coil of wire between two stainless steel plates and insulated from them with mica The second furnace, shown in fig 5, is similar to the one used by L o t h r o p and Smith 7) and is also o p e r a t e d m a vacuum. Lithium metal is e v a p o r a t e d onto the surface of the unheated crystal. The crystal is then heated to a diffusion t e m p e r a t u r e o f 370 °C, as measured by m o u n t i n g the p l a t i n u m resistor on the heating plate. The valves for the p u m p s are closed and app r o x i m a t e l y 1 liter of nitrogen gas is let into the bell jar. U n d e r these conditions, the diffusion rate is 0 1 r a m / m i n u t e for silicon, and a diffused layer o f 0 1 m m for each m m o f crystal thickness is used. After diffusion, the current for the heating coil is t u r n e d off, air let into the v a c u u m system i m m e d i a t e l y a n d the wafer cooled by placing It on a c a r b o n block. After the crystal has reached r o o m temperature, the lithium h y d r o x i d e coating is washed f r o m the surface o f the silicon with distilled water. The face is l a p p e d and the sides are sanded to remove all traces o f this coating. The crystal is then scrubbed with cotton swabs moistened with distilled water. * C.E. # 30707-7A, Engelhard Industries, Inc., Instruments and
Systems Dwmlon, 850 Passaic Ave. East Newark, New Jersey.
6. Stain etehing
Stain etching techniques 8) are used to reveal the extent o f the h t h m m diffusion. The stain etch is an accurate 1000 l mixture of H F and H N O 3 in sufficient q u a n t i t y t h a t the wafer is c o m p l e t e l y covered by the solution in a small Teflon b e a k e r The beaker containing the wafer with the h t h l u m diffused face down is placed directly under an incandescent light until h y d r o g e n bubbles are seen on the sides and surface o f the wafer. After the back surface has stained darkly, the wafer is removed f r o m the etch and washed m distilled water. The p-type material should stain d a r k l y in c o m p a r i s o n to the n-type, h t h l u m - r i c h region The usual practice has been to ignore the results o f the first etch o f a freshly p r e p a r e d solution and to l a p and sand the crystal and etch it again in the same solution Stain etch solutions can be used with satisfactory results for app r o x i m a t e l y an h o u r after mixing.
Fig 6 Slhcon crystal after diffusion in the furnace of fig 5 and after stain etching The lightly stained regions are the lithium diffused areas and the darkly stained region is the undlffused p-type material Diffusion on the sides of the crystal is qmte evident
The crystal shown in fig. 6 was diffused in the furnace of fig 5 a n d has been stain etched. Thin figure shows the i m p o r t a n c e of the stain etch for determining the q u a h t y o f the j u n c t i o n Side diffusion of lithium has occurred on this crystal and must be removed before a g o o d diode can be obtained. The crystal is sanded a n d checked with staining until the sides of the wafer show a sharp, even j u n c t i o n with no evidence of diffused lithium on the sides. Fig. 7 shows the crystal of fig. 6 after this processing With the crystal shown in fig. 4, the sides are usually sanded down to the h t h l u m h y d r o x i d e circle. A t this
Fig 7 Sihcon crystal of fig 6 after sanding and stain etching. The side diffusion of hthlum has been removed, and a sharp, even junction can be seen
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cotton swabs moistened w~th distilled water. In order to provide electrical contacts to the faces of the wafer, an electroless gold solution consisting of 8 drops of A t o m e x Immersion Gold Solution, 6 drops H F and 40 ml of distilled water is prepared in a pyrex beaker. The wafer is placed in this solution and is heated on a hot plate until the surfaces of the wafer are covered with gold. It is then washed in distilled water and the flat surfaces are masked with Apiezon Wax W*. The sides are cleaned of any excess wax by using cotton swabs moistened lightly with trichloroethylene. After rinsing the wafer successively with alcohol and distilled water, the gold is removed from the sides with aqua regia and the crystal is washed in distilled water.
8. Etching and drift
Fig 8 Back surface of slhcon crystal after drifting. The dark area ~s the uncompensated p-type material point, the stain etch is used and the crystal is sanded further, if necessary, untd a g o o d junction is observed.
7. Electroless gold contacting The crystal is now lapped and sanded to remove the stain etch. As before, after sanding it is cleaned ultrasonlcally with trlchloroethylene, rinsed successively wtth alcohol and distilled water and scrubbed with
The wafer is now etched m CP-4A consxstmg o f a 5 : 3 : 3 solution of HNO3, H F and H O A c in a Teflon beaker. The etching time is 21 m m at r o o m temperature. The solution is agitated constantly during etching and the etch is quenched in distilled water. After the wafer has been washed free of etchant, it is placed in a beaker of trichloroethylene to remove the Apiezon wax. The wafer is then washed successively in trichloroethylene and alcohol and dried with nitrogen gas. The reverse current is checked at 100 V bias and, if less than 10 /~A, the wafer is drifted. If the reverse current is greater than 10/~A, the wafer is left under a * James G Biddle Co., Plymouth Meeting, Pennsylvama.
F~g. 9. Back surface of sihcon crystal, after drifting as seen under a low-power microscope. The dots are rather blurred and indistinct.
334
n . M . MURRAY et al others, have been used with no noticeable differences in the final results. The drift m a y be s t o p p e d at some desired depth, b u t it IS usually continued until the b a c k surface is reached. Some o v e r c o m p e n s a t l o n or excesswe c o n c e n t r a t i o n o f l i t h i u m m a y occur at the back surface, but, with the stain-etch procedures described in the next section, this presents no problem.
10. Preparation for final etch
Fig 10 Back surface of silicon crystal of fig 9 after additzonal lapping and stare etching reverse bias of 400 V for a p p r o x i m a t e l y 12 h. If the reverse current at 100 V is still high after this time, the front and back surfaces are r e m a s k e d and the wafer is re-etched. If the current is not reduced to 10 p A after a few etches, the trouble is usually a t t r i b u t a b l e to lineage and the crystal is discarded.
9. Drifting Three drifting m e t h o d s 3' 4, 6) described in detail by
After drifting, the wafer is l a p p e d to remove the gold and the sides are sanded to remove the etched surface. The crystal is then stain etched and two effects are observed. The first, shown in fig. 8, is t h a t the c o m pensated region is stained a tone intermediate between that of the n- a n d p-type regions. The r e m a i n d e r of the p-type material, in this case the dark, irregular region in the center o f the back surface, can be removed by l a p p i n g and sanding the surfaces and sides. The second effect is p e r h a p s the m o s t i m p o r t a n t for the f o r m a t i o n of windowless, surface barrier detectors As shown in fig. 8, small circular, d a r k l y stained regions a b o u t 50/~m in d i a m e t e r can be observed on the surface of the crystal in the c o m p e n s a t e d material. If the back surface is inspected under a low power microscope, it can be seen that the dots are not p a r t i c u l a r l y clear and distract (fig. 9). This is the result of the pile-up o f l i t h i u m on the back surface d u r i n g the drifting process. These regions are usually strongly n-type and m u s t be r e m o v e d in o r d e r to achieve a windowless detector.
Fig 11. Surface of fig 10 as seen under a low-power m~croscope The dots and circles should be compared to fig 9 to note the improvement m sharpness
335
L I T H I U M D R I F T E D S U R F A C E BARRIER S I L I C O N DETECTORS
The back surface is therefore lapped until sharp, clearly defined dots or circles such as those shown in figs. 10 and 11 can be observed. In fig. 10 a screw dislocation, not observable before drift, has become apparent as a result of the h t h m m - i o n compensation. This crystal was discarded since such a dislocation prevents the formation of a good diode structure. The front surface of the crystal is now lapped and again plated with electroless gold. A flat head stainless steel screw is mounted on the lithium side using D u P o n t Silver P r e p a r a h o n 8030 which hardens overnight at 30 °C. The front surface and the head of the screw are then masked with Apiezon Wax W. Some care must be taken since the heat required to melt the wax may loosen the screw. However, the silver cement will harden again immediately after cooling. The excess wax is removed from the sides and other faces o f the crystal using cotton swabs moistened in trlchloroethylene The
/Evaporated Silverp r i n / gold layer point c . ~ /~"-.. / / /Etched s,hcon . r ~ c:.~.,~ //.~./~ crYsial I~ / I ~-~Du Pont ~1-t~1 I I_--~ I silver Aquadog/ [ ~ ~ ~ _ -~--;7--~ [ Ipreparahon
Gold wire(dla
175p. )
0,ezon uc,,e
I
I ,
"'oc"
Fig
13. Schematic drawing of detector showing method of
mounting. crystal is then rinsed in alcohol and distilled water and the gold is removed from the exposed portion wlth aqua regia The wafer is lightly lapped and sanded and scrubbed with cotton swabs moistened with distilled water. 11. Final etch
Fig 12 Detector mounted m plastic pipette shown just after the final pohsh etch
As shown in fig. 12, the screw is inserted m a plastic pipette m order to hold the crystal m the polish etch. The crystal is etched for 2½ m m in CP-4A at r o o m temperature and the etch ~s again quenched with distilled water. After the etch, the detector is m o u n t e d in a Lucite holder as shown in fig. 13. Gold is evaporated on the face of the detector with the sides of the detector protected by a mask. An ion p u m p is usually preferred for this v a c u u m evaporation, since backstreaming of oll onto the detector may occur with oil diffusion pumps. After evaporation, the detector is stored in darkness for two to three days. The contact to the evaporated gold layer is made with a 0.175 m m gold wire as shown in fig. 14. A tiny drop of Aquadag* is placed on the surface and the wire ~s then bent at an angle to contact the detector at this point. A small drop of General Cement Silver Print (with most of the solvent removed) covers the w~re at this contact and bonds it to the Aquadag. After the contact is dry, the reverse current is measured. In general, the best detectors will have reverse currents less than 1/tA at 100 V. If the current is higher than this, further storage in darkness under bias voltages of 200 400 V will occasionally give re* Acheson Colloids C o , Port Huron, Michigan.
336
H.M. MURRAY et al. well-formed dots are obtained on the back surface, the crystal can then be etched and finished as before. This has restored good detector properties in almost every case. The detectors fabricated by these means have resolutions between 15 and 30 keV for the 975 keV conversion line of 2°7B1 when operated at room temperature. For alpha particles, resolutions between 45 and 60 keV on the 8.78 MeV alpha-ray of mesothorium have been measured. The diameter of the detectors is greater than 1 cm and the thickness ranges between 2 and 6 mm. If these detectors are operated at low temperatures, care must be taken not to expose the detector to mr while cool, since moisture will form on the surface of the detector and may loosen the gold wire contact These detectors have been used primarily for beta-ray and electron measurements which are described in detail elsewhere~-12).
Fig. 14 FImshed detector with top contact shown markable improvement. Where the reverse current is very high and remains so even if the crystal is re-etched, the p r o b l e m is generally one of lineage in the crystal. It should be emphasized that while low reverse currents are desirable, the change in current with a change in voltage between 100 and 400 V should also be low and steadily increasing. In fact, for detectors greater than 5 m m in thickness and over 1 c m m diameter, the reverse currents may be as high as 5 #A with detectors whose resolution is quite good. In these cases, an increase in voltage from 100 to 400 V produces an increase in current of less than 2 / t A . Finished detectors are usually stored in darkness away from excessive heat. If the storage is prolonged, h t h l u m ions will occasionally disappear from the surfaces and the diode structure may be destroyed In these cases, good performance may be obtained again by lapping and sanding the crystal and using stare etching techmques. If the j u n c t i o n between the lithiumrich region and the intrinsic material is restored and
The a u t h o r s are most grateful to J. R. Gigante for his cooperation and assistance at every stage of this work. They have also benefitted greatly from the generous advice of H. M. M a n n of A r g o n n e N a t i o n a l Laboratory.
References 1) j H Elliot, Nucl Instr and Meth 12 (1961)60. 2) j L Blankenshlp and C J Borkowskl, IRE Trans Nucl Scl NS-9, no 3 (1962) 181 3) H M Mann, J W Haslett and F. J Janarek, IRE Trans. Nucl. Scl NS-9, no 4 (1962) 43. 4) G L Miller, B D. Pate and S Wagner, 1EEE Trans Nucl. Scl NS-10, no 1 (1963) 220. 5) C Chasman and J Allen, Bull Am. Phys. Soc 8 (1963) 398 6) F S. Gouldmg and W L Hansen, IEEE Trans Nucl Sck NS-I1, no 3 (1964) 286 7) R P Lothrop and H. E Smith, UCRL-16190 (unpublished). s) C S Fuller and J A Dltzenberger, J Appl Phys 28 (1957) 40 9) j W Harpster, D L Bennett and K J Casper, Nucl Physics 47 (1963) 443 10) K. J Casper and R H Thompson, Bull Am Phys. Soc. 9 (1964) 17 11) K. J Casper and R H Thompson, Bull Am Phys Soc 9 (1964) 688 12) R H Thompson and K J. Casper, Nucl Physics (to be published)
INSTRUMENTS
AND
METHODS
40 (1966) 330-336;
~) N O R T H - H O L L A N D
PUBLISHING
CO.
FABRICATION M E T H O D S FOR L I T H I U M DRIFTED SURFACE BARRIER SILICON DETECTORS* H M MURRAY, J W HARPSTER ~ a n d K J CASPER Department of Phystcs, Western Reserve Umverstty, Cleveland, Ohto
Received 21 September 1965 Techniques for the fabrlcatmn of llthmm surface barrier slhcon detectors are presented All important chemical procedures are described m detail The detectors made by these techniques
exhibit resolutions of 15-30 keY on internal conversion electrons when operated at room temperature.
1. Introduction
3. Crystal specification and evaluation
A n u m b e r of papers I 7) have appeared describing methods of fabricating h t h l u m drifted sohd state silicon detectors. The detectors described tn these papers either have not been of the surface-barrier type or else requtre coohng for best operatton The methods described here are consistently dependable for the preparation of surface barrier types which exhibit good r e s o l u n o n at room temperature.
The sihcon crystals are obtained as wafers shced and lapped to the desired thickness from Geoscience Instruments Corporation.§ The silicon is specified to be p-type, b o r o n doped, float-zone refined, 50-200 ohmcm, with a d t s l o c a n o n c o u n t less than 20000/cm 2 The orientation Js usually [l l 1] although crystals cut with different orientations have been used. As a first step the d M o c a h o n pattern is determined. A single slice from each ingot ts etched in the following manner' 1. The crystal is lapped first with 20 [lm and then l0/2m a l u m i n u m oxide powder, cleaned ultrasomcally m trlchloroethylene and washed successively in alcohol and distilled water 2 A 2'1 mixture of H N O 3 and H F ts prepared in a Teflon beaker and cooled in ice water. The wafer is
2. Chemical specifications Electronic grade chemicals are used t h r o u g h o u t the fabrlcatmn process Either absolute ethanol or m e t h a n o l ~s statable where the use of alcohol Is specified. The acid concentrations are 7 0 ~ H N O 3, 48% H F and glacial acetic acid and all etches are prepared and used in a Teflon beaker Baker Atomex I m m e r s i o n Gold S o l u t m n ++ is used in the electroless gold plating. The h t h l u m is obtained in the form of rods ~ and is kept stored m mineral oil For evaporation, the desired a m o u n t of h t h l u m is cut off the end of the rod with a diagonal wire cutter and cleaned with t n c h l o r o ethylene Vinyl gloves tt are worn whenever the crystal must be handled and the etched sides of the crystal are kept scrupulously clean. Generally, stainless steel tweezers are used to hold the crystal wherever possible. In discussing the crystal after h t h m m diffusion, the term " f r o n t surface" refers to the surface on which h t h m m has been diffused and "backsurface" refers to the surface toward which the lithium is drifted
1 10 Beckman Street, N e w York, N Y
* Supported in part by a grant froth the N a t i o n a l A e r o n a u t i c s
and Space Administration + Present address Department of Electrical Engineering, Ohto State University Columbus, Ohio +Engelhard Industries, lnc, Baker Platinum DtvlsJon, 113 Astor Street, Newark, N J Lithmm Corporation of America, Bessemer City, North Carolina V+Wllson Tru-Touch, Becton, Dickinson and Co, Rutherford, New Jersey 330
FJg 1 Slhcon wafer after dislocation etch, showing the high dislocation density ring.
LITHIUM DRIFTED SURFACE BARRIER SILICON DETECTORS
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4 h at r o o m temperature, after which the etch is again quenched with distilled water. In most of the crystals, an outer ring of high density dislocations is observed. Such a ring is evident m the crystal shown In fig. 1. The size of this ring is approximately the same for all of the slices of the ingot. Therefore, before any further processing, this high dislocation ring must be removed from each crystal. If the dislocation density is high even in the center of the crystal, the ingot is rejected as unsuitable
Fig 2 Lapidary wheel used for sanding sides of wafers.
then etched for 45 sec in this solution. The etch is quenched by placing the beaker under running distilled water until all o f the etchant has been displaced by water. In all of the polish etches, no air is permitted to come into contact with the wafer while it is being activated by acids. 3. The wafer is then placed immediately into a Teflon beaker containing a 10:3:1 mixture o f H O A c , H N O 3 and H F in sufficient a m o u n t to immerse the wafer. The crystal is etched in this solution for at least
F~g 4. Slhcon crystal after diffusion m furnace of fig 3 The dark tuner circle is hthmm hydroxide coating and indicates the area of diffusion
i
Fig 3 Schematic drawing of pyrex diffusion furnace. The diffused area of the crystal is limited by the size of the aperture m the evaporation table
4. Preparation of the crystal for diffusion Each crystal is sanded down to remove the high dislocation ring using the small lapidary wheel* shown in fig. 2 and cloth backed, # 320 silicon carbide discs. The crystal and the disc are kept moistened to prevent overheating of the crystal. The sides are then smoothed by light hand sanding with silicon carbide paper. The faces are lapped and the wafer is ultrasonically cleaned in tnchloroethylene and washed successively with alcohol and distilled water 5. Diffusion of lithium Two diffusion furnaces have been used. The first, shown in fig. 3, is operated in a vacuum of about 10 -5 mm. Lithium is evaporated from the basket onto the surface o f the crystal. The furnace is then raised to a temperature of 370°C as measured with the platinum * Model N B M D , Lee Lapidaries, 6617 Loram Ave., Cleveland, Ohio
332
H.M. MURRAY et al.
resistor*. After 5 min, the current is t u r n e d off and air is let into the system immediately. The crystal is then cooled by placing it on a c a r b o n block. A crystal diffused in this m a n n e r is shown in fig. 4. The d a r k lithium h y d r o x i d e circle indicates the area of diffusion. Diffusion of l i t h i u m on the sides o f the crystal is minimized, b u t the final detector size is restricted by the aperture in the m o u n t i n g plate. Tungsten w~re evaporation basket
-I Stainless steel plate
Mma~
Plat+num resmtor
~\N,chrome w,re furnace
Fig 5 Diffusion furnace The heating element 1s a coil of wire between two stainless steel plates and insulated from them with mica The second furnace, shown in fig 5, is similar to the one used by L o t h r o p and Smith 7) and is also o p e r a t e d m a vacuum. Lithium metal is e v a p o r a t e d onto the surface of the unheated crystal. The crystal is then heated to a diffusion t e m p e r a t u r e o f 370 °C, as measured by m o u n t i n g the p l a t i n u m resistor on the heating plate. The valves for the p u m p s are closed and app r o x i m a t e l y 1 liter of nitrogen gas is let into the bell jar. U n d e r these conditions, the diffusion rate is 0 1 r a m / m i n u t e for silicon, and a diffused layer o f 0 1 m m for each m m o f crystal thickness is used. After diffusion, the current for the heating coil is t u r n e d off, air let into the v a c u u m system i m m e d i a t e l y a n d the wafer cooled by placing It on a c a r b o n block. After the crystal has reached r o o m temperature, the lithium h y d r o x i d e coating is washed f r o m the surface o f the silicon with distilled water. The face is l a p p e d and the sides are sanded to remove all traces o f this coating. The crystal is then scrubbed with cotton swabs moistened with distilled water. * C.E. # 30707-7A, Engelhard Industries, Inc., Instruments and
Systems Dwmlon, 850 Passaic Ave. East Newark, New Jersey.
6. Stain etehing
Stain etching techniques 8) are used to reveal the extent o f the h t h m m diffusion. The stain etch is an accurate 1000 l mixture of H F and H N O 3 in sufficient q u a n t i t y t h a t the wafer is c o m p l e t e l y covered by the solution in a small Teflon b e a k e r The beaker containing the wafer with the h t h l u m diffused face down is placed directly under an incandescent light until h y d r o g e n bubbles are seen on the sides and surface o f the wafer. After the back surface has stained darkly, the wafer is removed f r o m the etch and washed m distilled water. The p-type material should stain d a r k l y in c o m p a r i s o n to the n-type, h t h l u m - r i c h region The usual practice has been to ignore the results o f the first etch o f a freshly p r e p a r e d solution and to l a p and sand the crystal and etch it again in the same solution Stain etch solutions can be used with satisfactory results for app r o x i m a t e l y an h o u r after mixing.
Fig 6 Slhcon crystal after diffusion in the furnace of fig 5 and after stain etching The lightly stained regions are the lithium diffused areas and the darkly stained region is the undlffused p-type material Diffusion on the sides of the crystal is qmte evident
The crystal shown in fig. 6 was diffused in the furnace of fig 5 a n d has been stain etched. Thin figure shows the i m p o r t a n c e of the stain etch for determining the q u a h t y o f the j u n c t i o n Side diffusion of lithium has occurred on this crystal and must be removed before a g o o d diode can be obtained. The crystal is sanded a n d checked with staining until the sides of the wafer show a sharp, even j u n c t i o n with no evidence of diffused lithium on the sides. Fig. 7 shows the crystal of fig. 6 after this processing With the crystal shown in fig. 4, the sides are usually sanded down to the h t h l u m h y d r o x i d e circle. A t this
Fig 7 Sihcon crystal of fig 6 after sanding and stain etching. The side diffusion of hthlum has been removed, and a sharp, even junction can be seen
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DETECTORS
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cotton swabs moistened w~th distilled water. In order to provide electrical contacts to the faces of the wafer, an electroless gold solution consisting of 8 drops of A t o m e x Immersion Gold Solution, 6 drops H F and 40 ml of distilled water is prepared in a pyrex beaker. The wafer is placed in this solution and is heated on a hot plate until the surfaces of the wafer are covered with gold. It is then washed in distilled water and the flat surfaces are masked with Apiezon Wax W*. The sides are cleaned of any excess wax by using cotton swabs moistened lightly with trichloroethylene. After rinsing the wafer successively with alcohol and distilled water, the gold is removed from the sides with aqua regia and the crystal is washed in distilled water.
8. Etching and drift
Fig 8 Back surface of slhcon crystal after drifting. The dark area ~s the uncompensated p-type material point, the stain etch is used and the crystal is sanded further, if necessary, untd a g o o d junction is observed.
7. Electroless gold contacting The crystal is now lapped and sanded to remove the stain etch. As before, after sanding it is cleaned ultrasonlcally with trlchloroethylene, rinsed successively wtth alcohol and distilled water and scrubbed with
The wafer is now etched m CP-4A consxstmg o f a 5 : 3 : 3 solution of HNO3, H F and H O A c in a Teflon beaker. The etching time is 21 m m at r o o m temperature. The solution is agitated constantly during etching and the etch is quenched in distilled water. After the wafer has been washed free of etchant, it is placed in a beaker of trichloroethylene to remove the Apiezon wax. The wafer is then washed successively in trichloroethylene and alcohol and dried with nitrogen gas. The reverse current is checked at 100 V bias and, if less than 10 /~A, the wafer is drifted. If the reverse current is greater than 10/~A, the wafer is left under a * James G Biddle Co., Plymouth Meeting, Pennsylvama.
F~g. 9. Back surface of sihcon crystal, after drifting as seen under a low-power microscope. The dots are rather blurred and indistinct.
334
n . M . MURRAY et al others, have been used with no noticeable differences in the final results. The drift m a y be s t o p p e d at some desired depth, b u t it IS usually continued until the b a c k surface is reached. Some o v e r c o m p e n s a t l o n or excesswe c o n c e n t r a t i o n o f l i t h i u m m a y occur at the back surface, but, with the stain-etch procedures described in the next section, this presents no problem.
10. Preparation for final etch
Fig 10 Back surface of silicon crystal of fig 9 after additzonal lapping and stare etching reverse bias of 400 V for a p p r o x i m a t e l y 12 h. If the reverse current at 100 V is still high after this time, the front and back surfaces are r e m a s k e d and the wafer is re-etched. If the current is not reduced to 10 p A after a few etches, the trouble is usually a t t r i b u t a b l e to lineage and the crystal is discarded.
9. Drifting Three drifting m e t h o d s 3' 4, 6) described in detail by
After drifting, the wafer is l a p p e d to remove the gold and the sides are sanded to remove the etched surface. The crystal is then stain etched and two effects are observed. The first, shown in fig. 8, is t h a t the c o m pensated region is stained a tone intermediate between that of the n- a n d p-type regions. The r e m a i n d e r of the p-type material, in this case the dark, irregular region in the center o f the back surface, can be removed by l a p p i n g and sanding the surfaces and sides. The second effect is p e r h a p s the m o s t i m p o r t a n t for the f o r m a t i o n of windowless, surface barrier detectors As shown in fig. 8, small circular, d a r k l y stained regions a b o u t 50/~m in d i a m e t e r can be observed on the surface of the crystal in the c o m p e n s a t e d material. If the back surface is inspected under a low power microscope, it can be seen that the dots are not p a r t i c u l a r l y clear and distract (fig. 9). This is the result of the pile-up o f l i t h i u m on the back surface d u r i n g the drifting process. These regions are usually strongly n-type and m u s t be r e m o v e d in o r d e r to achieve a windowless detector.
Fig 11. Surface of fig 10 as seen under a low-power m~croscope The dots and circles should be compared to fig 9 to note the improvement m sharpness
335
L I T H I U M D R I F T E D S U R F A C E BARRIER S I L I C O N DETECTORS
The back surface is therefore lapped until sharp, clearly defined dots or circles such as those shown in figs. 10 and 11 can be observed. In fig. 10 a screw dislocation, not observable before drift, has become apparent as a result of the h t h m m - i o n compensation. This crystal was discarded since such a dislocation prevents the formation of a good diode structure. The front surface of the crystal is now lapped and again plated with electroless gold. A flat head stainless steel screw is mounted on the lithium side using D u P o n t Silver P r e p a r a h o n 8030 which hardens overnight at 30 °C. The front surface and the head of the screw are then masked with Apiezon Wax W. Some care must be taken since the heat required to melt the wax may loosen the screw. However, the silver cement will harden again immediately after cooling. The excess wax is removed from the sides and other faces o f the crystal using cotton swabs moistened in trlchloroethylene The
/Evaporated Silverp r i n / gold layer point c . ~ /~"-.. / / /Etched s,hcon . r ~ c:.~.,~ //.~./~ crYsial I~ / I ~-~Du Pont ~1-t~1 I I_--~ I silver Aquadog/ [ ~ ~ ~ _ -~--;7--~ [ Ipreparahon
Gold wire(dla
175p. )
0,ezon uc,,e
I
I ,
"'oc"
Fig
13. Schematic drawing of detector showing method of
mounting. crystal is then rinsed in alcohol and distilled water and the gold is removed from the exposed portion wlth aqua regia The wafer is lightly lapped and sanded and scrubbed with cotton swabs moistened with distilled water. 11. Final etch
Fig 12 Detector mounted m plastic pipette shown just after the final pohsh etch
As shown in fig. 12, the screw is inserted m a plastic pipette m order to hold the crystal m the polish etch. The crystal is etched for 2½ m m in CP-4A at r o o m temperature and the etch ~s again quenched with distilled water. After the etch, the detector is m o u n t e d in a Lucite holder as shown in fig. 13. Gold is evaporated on the face of the detector with the sides of the detector protected by a mask. An ion p u m p is usually preferred for this v a c u u m evaporation, since backstreaming of oll onto the detector may occur with oil diffusion pumps. After evaporation, the detector is stored in darkness for two to three days. The contact to the evaporated gold layer is made with a 0.175 m m gold wire as shown in fig. 14. A tiny drop of Aquadag* is placed on the surface and the wire ~s then bent at an angle to contact the detector at this point. A small drop of General Cement Silver Print (with most of the solvent removed) covers the w~re at this contact and bonds it to the Aquadag. After the contact is dry, the reverse current is measured. In general, the best detectors will have reverse currents less than 1/tA at 100 V. If the current is higher than this, further storage in darkness under bias voltages of 200 400 V will occasionally give re* Acheson Colloids C o , Port Huron, Michigan.
336
H.M. MURRAY et al. well-formed dots are obtained on the back surface, the crystal can then be etched and finished as before. This has restored good detector properties in almost every case. The detectors fabricated by these means have resolutions between 15 and 30 keV for the 975 keV conversion line of 2°7B1 when operated at room temperature. For alpha particles, resolutions between 45 and 60 keV on the 8.78 MeV alpha-ray of mesothorium have been measured. The diameter of the detectors is greater than 1 cm and the thickness ranges between 2 and 6 mm. If these detectors are operated at low temperatures, care must be taken not to expose the detector to mr while cool, since moisture will form on the surface of the detector and may loosen the gold wire contact These detectors have been used primarily for beta-ray and electron measurements which are described in detail elsewhere~-12).
Fig. 14 FImshed detector with top contact shown markable improvement. Where the reverse current is very high and remains so even if the crystal is re-etched, the p r o b l e m is generally one of lineage in the crystal. It should be emphasized that while low reverse currents are desirable, the change in current with a change in voltage between 100 and 400 V should also be low and steadily increasing. In fact, for detectors greater than 5 m m in thickness and over 1 c m m diameter, the reverse currents may be as high as 5 #A with detectors whose resolution is quite good. In these cases, an increase in voltage from 100 to 400 V produces an increase in current of less than 2 / t A . Finished detectors are usually stored in darkness away from excessive heat. If the storage is prolonged, h t h l u m ions will occasionally disappear from the surfaces and the diode structure may be destroyed In these cases, good performance may be obtained again by lapping and sanding the crystal and using stare etching techmques. If the j u n c t i o n between the lithiumrich region and the intrinsic material is restored and
The a u t h o r s are most grateful to J. R. Gigante for his cooperation and assistance at every stage of this work. They have also benefitted greatly from the generous advice of H. M. M a n n of A r g o n n e N a t i o n a l Laboratory.
References 1) j H Elliot, Nucl Instr and Meth 12 (1961)60. 2) j L Blankenshlp and C J Borkowskl, IRE Trans Nucl Scl NS-9, no 3 (1962) 181 3) H M Mann, J W Haslett and F. J Janarek, IRE Trans. Nucl. Scl NS-9, no 4 (1962) 43. 4) G L Miller, B D. Pate and S Wagner, 1EEE Trans Nucl. Scl NS-10, no 1 (1963) 220. 5) C Chasman and J Allen, Bull Am. Phys. Soc 8 (1963) 398 6) F S. Gouldmg and W L Hansen, IEEE Trans Nucl Sck NS-I1, no 3 (1964) 286 7) R P Lothrop and H. E Smith, UCRL-16190 (unpublished). s) C S Fuller and J A Dltzenberger, J Appl Phys 28 (1957) 40 9) j W Harpster, D L Bennett and K J Casper, Nucl Physics 47 (1963) 443 10) K. J Casper and R H Thompson, Bull Am Phys. Soc. 9 (1964) 17 11) K. J Casper and R H Thompson, Bull Am Phys Soc 9 (1964) 688 12) R H Thompson and K J. Casper, Nucl Physics (to be published)