Silicon semiconductor needle transducer to utilize piezojunction effec

Stockard, R. R.

Wortman, J. J.

English

NASA Technical Reports Server

Silicon 
R. R. Stockard and J. J. Wortman 
Research Triangle Institute, Durham, North Carolina 
Paper presented at 1966 International 
Solid - State Circuits Conf.erence, Philadelphia, Pa. 
b 
m N66 2 9 4 5 7  
(NAsA'cR OR TMX OR AD NUMBER' (CATEl%ORYl 
This work was supported by the Office of Advanced Research and 
Technology, National Aeronautics and Space Administration, under 
Contract No. NASr-222. 
https://ntrs.nasa.gov/search.jsp?R=19660020167 2020-03-16T19:22:46+00:00Z
* 
It has been known for some time that large mechanical stresses 
(-lo8 - 10l1 dynes/cm) have significant effects on the electrical 
. These changes in character- characteristics of p-n junctions 
istics, for uniaxial stresses, result from stress-induced changes in 
the energy band structure of the semiconductor material . 
effect of stress is to decrease the average energy gap of the material. 
This affects the junction parameters in two ways: 
increase in the minority carrier concentration and, second, it can 
increase or decrease the carrier recombination rate. 
1 - 3  
4 The net 
first, it causes an 
In diodes, stress causes the saturation current to increase. At 
high stress levels, the forward diode current is 
I = Is [exp qV/kT - 13 
where 
Is = Iso exp ca , 
c is a constant depending on material and orientation and a is the 
magnitude of the stress. 
The practical utilization of this piezojunction phenomenon is 
limited primarily by two factors. First, the stress levels required to 
produce the effect are extremely large--on the order of 10 
In order that the associated force levels remain reasonable, it is 
4 atmospheres. 
r a  
z necessary to apply the force over a very small area (-mils ). 
this stress must be very accurately coupled to the junction area of 
the semiconductor device of interest. Care must be taken to avoid over- 
stressing of the device since the stresses required to produce the phe- 
nomenon are near the fracture strength of the semiconductor material. 
Also 
-2- 
The major d i f f i c u l t y  i n  fabr ica t ing  p r a c t i c a l  transducers based 
on the  piezojunction phenomenon has been the  appl ica t ion  of t he  re-  
quired stress t o  the  proper posi t ion on the  semiconductor. 
the  l imi ta t ions  described, i t  is a l s o  des i rab le  t o  stress t h e  e n t i r e  
junct ion t o  avoid unstressed regions of the  junct ion from cont ro l l ing  
the  e l e c t r i c a l  p roper t ies .  
Along with 
Heretofore, the  most successful method of s t r e s s  appl ica t ion  has 
been by means of a spher ica l  indentor point  such as a s t e e l ,  diamond, 
o r  sapphire phonograph needle. This helps  t o  avoid overstressing s ince  
t h e  s t r e s sed  area increases  with appl ied force. Also, t h e  f r a c t u r e  
s t rength  a p p e a r s  t o  be higher when only a small area of a l a rge  sample 
i s  s t ressed.  
t h e  c r i t i c a l  alignment between the point  and the  small junction. 
alignment must be accomplished with t h e  a i d  of a microscope. 
l a t e r a l  movement of the  indentor po in t  a f t e r  contact is made can dras-  
t i c a l l y  change the  device cha rac t e r i s t i c s  o r  damage i t  permanently. 
The major disadvantage of the  indentor point  method i s  
This 
Any 
The search fo r  a more r e l i a b l e  and less tedious method of stress 
appl ica t ion  l e d  t o  t h e  semiconductor needle approach t o  t h e  problem. 
The semiconductor needle transducer i s  simply an inversion of t he  in-  
dentor point method i n  t h a t  t he  indentor point  i s  now made of s i l i c o n  
and the  p-n junct ion device i s  formed on i t s  apex. S t r e s s  is  appl ied 
t o  the  junct ion by forcing the  apex of t he  needle onto a hard, con- 
duct ive surface.  Figure 1 is a sketch of the  needle s t ruc tu re  with a 
diode on the  apex. 
The needles a r e  formed by electroetching one 
s i l i c o n  bar. The radius  of curvature of the  apex 
end of a rectangular  
clan Se csctrolled to 
as low as one micron. Figure 2 is a photomicrograph of a typical 
needle along with a straight pin for comparison. The p-n junction 
or junctions are formed on the needle by using conventional photo- 
, lithographic and diffusion techniques. Both mesa and planar types 
have been made. However, the planar types have oxide passivated 
junctions and are inherently more stable. 
A photograph of the forward I-V characteristics for three stress 
levels in a planar silicon needle is shown in Figure 3 .  The curve to 
I 
I 
the far right represents zero stress with stress increasing for each 
curve in the direction of the origin. The horizontal scale is 0.2 
I V/cm and the vertical scale is 0.01 mA/cm. In addition to increasing 
the forward and reverse currents, the breakdown characteristics are 
~ 
softened. Both the junction depth and the radius of curvature of the 
I needle tip are critical parameters in determining the sensitivity of 
diode needles. 
Basically the silicon needle is a transducer of force and 
displacement. 
fulness, the silicon needle transducer has been incorporated into a 
For the purpose of demonstrating its practical use- 
I laboratory accelerometer. A photograph of the accelerometer is shown 
I 
in Figure 4. The needle is attached to the housing and the seismic 
mass is pressed against the needle tip by a spring. The spring pro- 
vides a dc stress bias on the junction. Acceleration acting on the 
mass varies the stress level about the dc point and changes the 
electrical characteristics of the junction. The accelerometer was 
calibrated by rotating it through 360" in the vertical plane. The 
calibration curve is shown in Figure 5 .  
- 4- 
In sumary, the silicon needle transducer is a unique method of 
utilizing the piezojunction effect. Its major advantages over other 
known methods are that it eliminates critical alignment problems and 
can be made more sensitive to stress. 
-5- 
Footnotes and References 
This work w a s  supported by t h e  Off ice  of Advanced Research and 
Technology, National Aeronautics and Space Administration, under 
Contract No. NASr-222. 
H a l l ,  H., Bardeen, J., and Pearson, G., Phys. Rev., p. 129; 84, 1951. 
Sikorski ,  M. E., Physical  Acoustics, Vol. 1, P a r t  B, Chapter 12, 
Edited by W. P. Mason, Academic Press;  1964. 
Rinder, W. J., Appl. Phys. 36, 2513; 36, 1965. 
Wortman, J. J., Hauser, J. R., and Burger, R. M., J. Apple Phys. P a  1222; 
35, 1964. 
-6- 
p.020”q 
n-type Si 1 4 - -___ h i N i  
0.5” I I 
d L- si 0, 
Tj 
Diffused 
p-type Si 
t i  ‘ i  V 
-A 1 to 50  microns 
Figure 1. Silicon needle transducer, 
planar type. 
Figure 2. Photomicrograph 0f.a silicon 
needle transducer compared to 
a straight pin. 
Figure 3. Fokard I-V characteristics 
of a planar silicon needle 
transducer. 
I I I I I 1 I 
7 L J- ( - lg )  
1 I 1 I 
0 120 240 360 
e- 
Figure 4 .  Photograph of an accel- Fig. 5. Accelerometer calibration curve. 
erometer using the silicon 
needle transducer. 


Silicon Needle Transducer

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server