Method of forming thin window drifted silicon charged particle detecto

Fessler, T. E.

Smith, A. B.

Vincent, J. S.

English

NASA Technical Reports Server

REPLY TO 
AlTPd OF: GP 
TO : 
FROM : 
SUBJECT ,D 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
October 16, 1970 
USH/Scientific & Technical Information Division 
Attention:: Miss Winnie M, Morgan 
GP/Office of Assistant General 
Counsel for Patent Matters 
Announcement of NASA-Owned 
U.S, Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8, 1970, 
the attached NASA-owned U , S ,  patent is being forwarded for 
abstracting and announcement in NASA STAR, 
The following information is provided: 
U,S, Patent No, : 3,310,443 
Corporate Source : Lewis Research Center 
Supplementary 
Corporate Source 
NASA Patent Case No.: XLE-00808 
Gayle Parker 
Enclosure: 
Copy of Patent 
A 
(CODE) 
s 
9 ES) 
t -I I z/jJ 
(NASA CR OR TMX OR AD NUMBER) ( C A T E ~ O R Y )  d 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19710001086 2020-03-17T02:27:55+00:00Z
AL 
METHOD Ob' FORMING THIN WINDOW DRIFTED 
SILICON CHARGED PARTICLE DETECTOR 
Filed Sept. 1963 _-.I.- 
FIG. I p-type SILICON 
p-type SlLlCORl FIG. 18 
12 
n-9ype SILICON I4 
FIG I C  1 6  
FIG. 2 
THE 
Y 
The invention described herein may be manufactured 
and used by or for the Government of the United States 
of America for governmental purposes without the pay- 
ment of any royalties thereon or therefor. 
The present invention relates, generally, to semicon- 
ductor radiation detectors and, more particularly, to an 
improved ion drifted radiation detector and the method 
for making it, the detector providing linear ssponse to 
,high energy particles. 
Solid state radiation detectors offer a simple and effec- 
tive means for detecting nuclear radiation and have been 
found very successful when used for measuring the inci- 
dent energy of charged particles because of their high 
energy resofution. However, for these devices to pro- 
vide a linear response to the longer-range particles, the 
required thickness of the depletion region in the p-n 
junction has increased. Resort has been had to both 
higher reverse biasing voltages and to higher res 
silicon material to increase the thickness of this 
but both of these steps suffer from inherent limitations. 
The maximum voltage that can be applied to the junction 
is Iimited, for example, by avalanche breakdown of the 
material or noise and interference introduced by surface 
leakage of the induced current. High resistivity material 
is not easily obtainable, Thus, resort has been had to 
ion drifted semiconductors to  obtain an essentially in- 
trinsic silicon and thereby a thickened depletion region 
at this junction. 
Achievement of an intrinsic region by the use of the 
ion drift process relies on the drift of mobile donor or 
acceptor ions in the depletion layer of a reverse-biased 
n-p junction, The general results of such drift is a tend- 
ency to create a-thick intrinsic semiconductor layer in the 
junction region in which donor and acceptor ion impuri- 
ties exactly compensate and provide the semiconductor 
with an exceedingly high effective resistivity. 
Although many n-type materials are available, lithium 
has been found most practical for the drifting process 
sin& the rate of drift is dependent in large degree upon 
the smallness and the mobility of the impurity donor 
ion in the host crystalline structure. Semiconductors 
drifted with lithium have been successfully utilized to 
measure extremely high incident energies of radiation 
particles (up to 24 mev. protons and 96 mev. a particles) 
but semiconductors produced by prior known processes 
require extremely close control during fabrication to in- 
sure that the intrinsic region closely approaches the semi- 
ing contact surface for the detector giving uniform par- 
ticle resolution. 
It is still another object Qf the invention to  provide a 
process whereby a particular thickness of electrical con- 
6 tact and particle entry material may be electively appfied 
to the radiation detector to suit any desired experimental 
purpose. 
A more complete appreciation of the invention and 
many attendant advantages thereof will be readily a g  
10 preciated as the same becomes better understood by 
reference to the following detailed description when can- 
sidered in connection with the accompanying drawing 
wherein : 
FIGS. l A  through 1F show in schematic cross-section 
16 the major steps in the fabrication of a lithium drifted 
semiconductor radiation detector from an original slice 
of a p-type silicon semiconductor material in accordance 
with the present invent,ion; 
FIG. 2 is a curve graph showing impurity distribution 
FIG. 3 is a graph showing impurity distribution after 
drifting. 
According to the present invention, the foregoing and 
other objects are obtained by providing an improved 
25 lithium drifted detector and process for producing the 
same. In particular, the invention includes the forma- 
tion of a layer of aluminum on one side of a ptype silicon 
material, the diffusion of lithium into the opposite side 
of the ptype material by the application of heat, drifting 
30 of the lithium impurity by the: combined use of heat and 
an electrical field applied to the silicon material utilizing 
the aluminum coating as one of the electrical contacts, 
removal of the surface layers at the opposite sides of the 
semiconductor by lapping or the like to provide an 
35 essentially intrinsic silicon semiconductor, and the de- 
position of a thin layer of rectifying contact to these 
sides of the semiconductor. 
Referring now to the drawing, an appreciation of this 
invention may be had by describing its use in the fabrica- 
40 tion of a thick junction lithium drifted radiation detector. 
The starting material for this process, shown in FIG. 1, 
is a slice or wafer 10 of readily obtainable low resistivity 
p-type silicon. The starting materials actually utitized 
in practicing the invention were wafers of p-type, 50 
45 n-cm. silicon, doped with boron, 3 to 6 mm. thick, and 
approximately %” in diameter. Preparatory to  ap- 
plication of the alumlinum coating, the flat sides of 
the wafer 10 are carefully lapped with a grinding com- 
pound to a satin finish to remove mechanical surface im- 
Io perfections and is carefully washed in a series of baths to 
remove any chemical impurities from the starting mate- 
rial. A grinding compound suoh as an 800 mesh Alsoa 
suspended in a light mineral oil has been found satis- 
88 factoty for the lapping process to  thereby obtain the req- 
ui&e smooth surface. The series of baths which follow 
lapping may then include she following to insure removal 
20 after diffusion; and 
conductor surface. Aaditionally, the semiconductors Of 
formed have contact surface areas which are thick, pitted (1) Water and a detergent 
and nonuniform thus decreasing the performance of the 00 (2)  Acetone 
detector and detrimentally effecting energy resolution. ( 3 )  Dichloroethylene 
Accordingly, it is an object of the present invention to (4) ~~l~~~~ 
provide a detector comprised of intrinsic SikOn, coated ( 5 )  Acetone 
with a rectifying contact material that gives uniform 65 (6) Methyl alcohol 
(7) Distilled water response to incident particle energies. 
(8) Equal parts of concentrated I t  is yet another object of the pwsent invention to pro- 
vide an improved process for making a thick junction (9) Distilled water 
drifted radiation detector that does not require close 
control. After cleaning the slice 0 is then etched in a bath of 
It is an additional object of the invention to provide a concentrated acetic, nitric, and 40 percent hydrofluoric 
fabrication method that produces a thin uniform rectify- acid for the short time (1-4 minutes) required to remove 
8,510,445 
all pits in its surface and is then again washed in distilled FIG. 3 also illustrates the effects of the drifting process 
water. and shows the distribution of the donor and acceptor im- 
After final cleaning a fairly thick coating 12 of alumi- purities and the formation of the intrinsic region W after 
num (approximately 0.1 micron thick) is applied to one a portion of the drifting process is completed. 
flat side of the slice IO. This coating (FIG. 1B) serves 5 Upon completion of drifting, the flat surfaces of the 
as one of the electrical contact surfaces for the drifting wafer are lapped to remove the impurity regions, approxi- 
of the n-type impurity ion and, advantageously, may be mately a 5 mil thickness being removed from each side 
applied by a standard vacuum deposition process. After of the wafer. This providcs a wafer which is stoichio- 
removal of the aluminum coated silicon slice from the metrically balanced since any uncompensated silicon or 
vacuum coating apparatus, a suspension of lithium in oil excess lithium has been removed. It should be under- 
(e.g., 30% lithium, 69% mineral oil, and 1 %  oleic acid) stood that grinding, etching or the like may also be used 
is painted on the opposite side of the slice with a glass rod to rcmove these surfaces. The lapped semiconductor is 
or  the like. illustrated schematically in FIG. 1 E and consists, cssen- 
Diffusion of the lithium impurity ion into the p-type sili- tially, of an intrinsic silicon layer 24 providing a uni- 
con i R  then obtained by placing the coated wafer 10 in  16 formly thick depletion region giving cffective resolution 
a small furnace preheated to 450" C., with the lithiiim of high energy particles. Additionally, since the outer 
side of the wafer up, and allowing it to rcmain there for layers of the silicon wafer are removcd, no close control 
approximately four minutes so that thc lithium diffuses of the final stages of the drifting process is required to 
into the crystalline structure of the silicon wafer. A nitro- insure that the depletion region extend to the surface of 
gen atmosphere is maintained in the furnace to prevent 20 the silicon semiconductor. 
oxidation of the lithium during this diffusion step. After After lapping the wafer is cleaned to remove all DC 200 
removal from the furnace, the excess lithium on the sur- Oil and rinsed in distilled water. The wafers may then 
face of the detector is removed, for example, by dropping be rinsed in the series of baths utilized above for the ini- 
the heated wafer into a beaker of oil which causes the tial cleaning to insure that the requisite final etch given 
majority of the excess lithium to flake off and remain be- 26 to the wafer removes any remaining pits or surface irregu- 
bind in the oil. One oil found satisfactory for this pur- larities due to lapping. The white etch described pre- 
pose is "Silicone Oil" (DC 200), sold by Dow-Corning viously for the initial preparation of the silicon slice may 
Corporation. be utilized to obtain this final etch. 
The lithium diffused wafer then has the structure gen- The rectifying contact material for the radiation de- 
erally shown schematically in FIG. IC and comprises an 30 tector is next applied. to provide a surface through which 
n-type silicon layer 14 and a p-type silicon layer 16. The radiation particles enter the detector and surfaces for 
donor-acceptor profile farming these two layers is illus- electrical contact. Vacuum coating of this material has 
tmt& in FIG. 2. The solid horizontal line Nd indicates been found best for the purposes of this process since a 
the level of acceptor impurity (boron) that exists through- thin and uniform layer may be obtained. A material 
out the silicon wafer while the curve ND indicates the 35 which has been used in practicing this method is gold and 
declining level of diffused donor impurity (lithium) X it may be applied in very thin layers to both the lapped 
distance from the diffusion surface. surfaces of the wafer, of course, thicker layers of gold 
Following the diffusion of lithium into the silicon wafer, may be applied to suit a particular experimental purpose. 
the lithium is drifted under the influence of heat and a fd practice it has been found that a thin gold layer hav- 
reverse biasing voltage. This driit occurs because the 40 ing a 20 Percent transmission characteristic to  light is sat- 
lithium ions (positive charge), when made mobile by isfactory for most research uses. The radiation detector 
the application of heat, move from the Li-excess side then appears as schematically shown n FIG. 1F wherein 
of the junction to the Li-deficient side owing to electric the rectifying contact layers are indicated by the numerals 
field at the junction region. 26 and 28. 
The requisite electrical connections for the detector 
by utilizing a temperature between 100-150" C. and a may then be applied by utilizing a silver paint connection. 
reverse biasing voltage varying between 200-600 volts im- This connection is best formed by placing a small spot of 
pressed across the wafer. It is, of course, essential that epoxy cement over the gold contact layer, applying a smdl 
the drift rate greatly exceed any diffusion effects due to circle of gold over the epoxy spot, and then applying the 
heating in order to form a thick-junction region. A gen- 60 silver Paint to the small circle. This arrangement pre- 
eral description of the drifting process may be found vents the silver paint from affecting the gold-silicon inter- 
in an article ,by J. H. Elliot in Nuclear Instruments and face. The completed wafers can then be mounted in any 
Methods 12 (1961) 60, this article clearly indicating the suitable holder and are ready to be utilized. 
proper procedure and parameters that may be utilized Obviously many modifications and variations of the 
in order to obtain a lithium drifted semiconductor. In 55 Present invention are possible in the light of the above 
the practice of the instant invention a current of 15 milli- teachings. It is, therefore, to be understood that within 
amperes at 250 volts was initially supplied to the heated the scope of the appended claims, the invention may be 
silicon water (approximately 150" C.) by a current regu- practiced other than as specifically described. 
lated power supply. As the drifting process progressed, What is claimed is: 
the current and the voltage were maintained essentially 80 1. The method of producing a thick junction radiation 
at these constant values by reducing the amount of SUP- detector comprising evaporatively depositing a thin coat- 
plementary heat supplied, sufficient ion mobility being ob- ing Of aluminum to one side of a p-type silicon body; 
tained by the combination of supplementary heat and heat Painting a suspension of lithium and oil to the opposite 
generated by the electric power imposed on the wafer. side of said p-type silicon 'body; heating said p-type silicon 
After Beveral hours (the number of hours required at 15 86 body in a nitrogen atmosphere at 450' C. to diffuse the 
ma. and 250 volts has been found to be Proportional to lithium into the p-type silicon body and form an n-type 
the cube of the thickness of the slice), the drifting process region; applying heat and an electric Aeld at  250 volts and 
i s  complete and the semiconductor has the structure 15 milliamperes to initiate drifting of the lithiitm in the 
shown schematically in FIG. lD, comprising an intrinsic silicon body; maintaining thc amperage and voltage at the 
or depletion region 24 and impurity regions 20 and 22 70 constant rates of 15 milliamperes and 250 volts for a time 
consisting of a heavily doped area and the aluminum proportional to the cube of the thickness of the silicon 
coated area, the surface area of the lithium side being body to complete drifting to form a thick depletion m- 
pitted and uneven due to the diffusion proceess and the gion: lapping the opposed sides of the silicon body to the 
aluminum coated area including a silicon-aluminum alloy depth of at least .005" to form an inlrinsic silicon body; 
impurity. 76 etching the lapped sides of the intrinsic siilcon body to 
 the^ drifting of lithium may be adequately performed 45 
3,3 10,443 
remove surface imperfection depositing a thin gold coat- 
ing to the etched sides of the intrinsic silicon to provide 
reactive contacts; and applying electrical contacts to the 
gold coating by layering a spot of epoxy cement and silver 
paint thereto. 5 silicon body, 
5. A method of producing a thick junction radiation 
detector comprising. 
coating one side of a p-type silicon body with alumi- 
num, 
applying a suspension of lithium and oil to the opposite 10 
side of said p-type silicon body, 
heating said p-type silicon body whereby the lilhium 
diffuses thereinto and forms an n-type region, 
removing excess lithium from said opposite side of said 2,4629218 21 1949 O k n  -------------- 148-191 
p-type silicon body, l5 3,009,840 11/1961 Erneis _ _ _ _ _ _ _ _ _ _ _ _  148-185X 
3,084,078 4/1962 Anderson _ _ _ _ _ _ _ _ _  148-186 X plying heat and an electric field thereto, 
3,225,198 12/1965 Mayer _ _ _ _ _ _ _ _ _ _ _ _ _ _  148-188 
stant for a time proportional to the cube of the thick- 20 HYLAND BIZOT, Primnry Examirler. 
ness of the silicon bod to complete drifting thereby 
forming a thick depletion region, 
removing at least .005 inch of material from the op- 
posed sides of the silicon body to form an intrinsic 
removing surface imperfections from the opposed sides 
of said intrinsic silicon body, 
depositing a thin gold coating to said opposite sides of 
the intrinsic silicon to provide reactive contacts, and 
applying electrical contacts to the gold coating. 
eferences Cited by t 
UNITED STATES PATENTS 
initiating drift of the litkium in the silicon body by ap- 3*016,313 --------------- 148-188 
maintaining the amperage and voltage substantially con- 


Method of forming thin window drifted silicon charged particle detector Patent

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