UNIVERSITY OF PENNSYLVANIA

The Moore School of Electrical Engineering

Philadelphia, Pa. 19104

FINAL REPORT

Deep Diode Arrays for X-ray Detection

TO

NASA/Goddard Space Flight Center

Code 287

Greenbelt, Maryland 20771

CONTRACT NAS-5-27285 )

PRINCIPAL INVESTIGATOR: Jay N. Zemel

29 January 1984

3
ABSTRACT

Temperature Gradient Zone Melting Process (TGZMP) was

used to form P-N junctions in bulk of high purity silicon

wafers. These diodes were patterned to form arrays for

x-ray spectrometers. The whole fabrication processes for

these x-ray detectors are reviewed in detail.

The P-N junctions were evaluated by

(i) the dark diode I-V measurements,

(ii) the diode Cm-V measurements,

(iii) the MOS C-V measurements.

The results showed that these junctions were linearly

graded in charge distribution with low reverse bias leakage

current flowing through them (few nA at -10 volts).

The x-ray detection experiments showed that an FWHM

of 500 eV was obtained from these diodes with a small bias

of just -5 volts (for x-ray source Fe^). A theoretical

model was proposed to explain the extra peaks found in the

energy spectra and a very interesting point—cross talk

effect was pointed out. This might be a solution to the

problem of making really high resolution x-ray spectrometers.

Few additional studies on materials subjected to TGZMP

conditions were done. They were

(i) anne&Zing of chemically deposited silicon,

(ii) recrystallization of chemically deposited silicon,

(iii) inspection of defects generated by incoherent

radiation annealing.

These provide us with very useful information in understanding the mechanism of radiation heating used in TGZMP.
ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my

thesis supervisor, Dr. Jay N. Zemel for his patient

guidance, suggestions and encouragement during the course

of the work.

I also wish to thank menbers of the Moore School of

Electrical Engineering, especially Dr. Imants R. Lauks,

Dr. Jan Van der Spiegel, Dr. Koriyama (visiting scholar of

Perm, in 1982), Dr. M. Ogita (visiting scholar of Penn. in

1983)» and Dr. D. J. Jaggard for many valuable discussions

and also Mr. T. Carrol and Mr. V. Dominko for their

technical asistance.

My sincere thanks also go to the knowledgeable and

skillful people in NASA Goddard Space Center, especially

Dr. George E. Alcorn and Ms. Kathryn A. Kost for many

useful discussions and experiments.

In particular, my special thanks must go to my brother,

Winston I. Way and his wife, Janney Chen for their constant

encouragement.

Finally, my gratitude is also extended to my academic

colleagues and friends-- S. Azizi, G. C. Chern, J. C. Young,

D. Babic, T. Fare, G. Edelson, P. Hesketh, D. S. Lin,

and Z. Triester.
DEEP DIODE ARRAYS FOR X-RAY DETECTION

TABLE OF CONTENTS

I Introduction ..................................... . . 1

II Principles of X-Ray Detection ...................... 2

II- (a) Preamplifier ............ , ............. , ...... 4

II-(b) Linear Amplifier ............................ 8

II- (c) Multichannel Analyzer ....................... 9

III Fabrication Procedures ......... . ................... 10

III- (a) Sample Preparations ........................ 10

III-(b) Substrate Contact by Phosphorous Diffusion. 10

III- (c) Thermal Oxidation. ......................... 12

III- (d) Aluminum Evaporation ....................... 12

III- (e) Aluminum Migration by TGZMP ................ 14

III-(f) Potassium Hydroxide (KOH) Preferential

Etching .............. , ..................... 23

III- (g) Wet Thermal Oxidation ...................... 25

III-(h) Shape of the P-N Junction .................. 25

III-(i) Metallization and Packaging ................ 29

IV Experimental Results ............................. . . 31

IV- (a) Junction Evaluations. ....................... 31

(i) Dark I-V Measurements ................... 31

(ii) Diode C^-V Measurements ................. 32

(iii) MOS C-V Measurements .................... 4l

IV- (b) X-Ray Detection Experiments .................

IV- (c) 0(-Particles Detection Experiments ........... 59

V Additional Studies on Materials Subjected to TGZMP

Conditions. ........................................ 61

Y-(a) Annealing of Chemically Deposited Si02 ...... .. 6l

V-(b) Recrystallization of Chemically Deposited

Silicon .............. . ....................... 6^
V-(c) Inspection of Defects Generated by Incoherent

Radiation Annealing 67

VI Conclusions 73

VII References 76

Appendix A 79

Appendix B 80

Appendix C 81

Appendix D 82

Appendix E 83
- 1 -

I. INTRODUCTION

Pfann (1) was the first one who used the temperature

gradient zone melting process (TGZMP) to produce P-N junctions within bulk semiconductors. Later Anthony and Cline

demonstrated that droplets of aluminum could be stably migrated through the silicon wafer if the dimension of the

zone is sufficiently small (2,3). .They first recognized

that thermomigration had possibilities in radiation detection (4). The advent of large scale x-ray telescope, such

as the Einstein Observatory, has created a need for an x-ray

spectrometer with broad energy response. Therefore, the

people in National Aeronautics and Space Administration

(NASA) undertook the developement of a TGZMP based detector

using high purity silicon wafers. Workers at this Laboratory successfully used the TGZMP to fabricate ion controlled

diodes (ICD) (5t6). A corperation was initiated between

NASA Goddard and the University of Pennsylvania to develope

a. TGZMP apparatus that then matured into the current contract to make a small x-ray detector array. These diode

arrays have been fabricated, tested, and evaluated. The

results indicate that it is possible to exercise control

over some of the critical processes. The results both here

and NASA Goddard indicate that TGZMP diode arrays have attractive features including very good resolution as x-ray

spectrometers.
' Y . .

- 2 -

II. PRINCIPLES OF X-RAY DTECTION

When a charged particle or photon passes through a semiconductor like silicon, the overall significant effect is

the production of many electron-hole pairs along the track

of it (7»8). The production process may be either direct or

indirect, in that the particle produces high-energy electrons

which subsquently lose their energy in producing more electron-hole pairs. Regardless of the detailed mechanism involved, the quantity of practical interest for detector application is the average energy expended by the primary

charged particle or photon to produce one elctron-hole pair.

This quantity,, often loosely called the "ionization energy"

and given the symbol E. , is experimentally observed to be

largely independent of both the energy and the type of the

incident radiation (9.10). The ionization energy of silicon

is found to be 3.55~3»6o eV at room temperature (9,10,11,

12). At lower temperatures, the energy gap between conduction band and valence band of silicon increases. Thus, the

energy required to produce an electron-hole pair in silicon

increases a little bit to 3-75x*-3.80 eV for temperature

region between 77°K and 90°K (13il^).

From the theory of P-N junction (15)i a. depletion region

almost completely depleted of carriers is formed between P

and N type materials when a reverse bias is applied on the

junction. Across this depletion region there is a large

electrical field due to charge redistribution. When charge

particles or photons are incident on this depletion region,

many electron-hole pairs will be generated. These generated

carriers are then immediately swept by the eledtric field to

the appropriate electrode of the diode where they are collectd.

When a diode is reverse biased across its junction, it

can be treated as a parallel plate capacitor with junction

capacitance
- 3 -

rC - £ 1 1 \ j

 fc

 Si' d u;

where €g. is the permitivity of silicon, A is the area of

the junction, and d is the width of the depletion region

(15).

For an step graded P -N junction, the width of the depletion region can be written as

. _ / x _ ,~ r p

d

 -

 ( q . ND }

 -

 (2

 ' ^ Si ' / N '

(v + <J>))1/2 (2)

where q is the charge of an electron, ND is the doping concentration of the N type material, j^. is the resistivity of

the N type material, UN is the mobility of the electron,

and YD is the reverse bias across the junction (15).

K

is the built-in voltage across the junction. )

From Eq. (1) and (2),

' -

 Cl'£si ' "p 1/2 (3)

When VR is large compared to (B> we obtain

doC(fN . V

Similarly, for a linearly graded P-N junction (15)i

C.oc(VRr (5) J K

The energies of x-rays range from 5 KeV to about 1 MeV.
With these kinds of photons incident on the reverse

biased P-N junction, the number of electron-rhole pairs generated by each photon in the depletion region (If this photon

fully stops in this region. ) is

(7)

Ei (eV)

where E is the energy of the x-ray photon, E. is the

ionization energy discussed before, and f is a factor related to the collection efficiency of the diode

The charge collected by the diode for each photon is

Q = f • N OC Ex_ray (8)

Therefore, the charge generated is proportional to the

energy of the incident photons. By measuring the amount of

charge collected by the diode due to x-ray the energy of the

corresponding x-ray can then be determined.

The whole radiation detection system is shown in

Figure 1. (7).

Besides the detector, the system consists of three main

parts j

(a) Preamplifier

The fast charge pulses generated in radiation detectors will first be collected by a following charge

sensitive preamplifier. In order to assure that complete charge collection occurs, this preamplifier

is normally adjusted to provide a decay time for the

pulse which is quite long (typically 50 to 100 ps).

The leading edge of the output pulse from the preamplifier corresponds to the time over which the

charge produced by the detector is integrated across

the capacitance of the collection circuit. Therefore, the time characteristics of the leading edge

are determined exclusively by the charge collection
- 5 -

eng

Co•H•Pat <H S o <u-P 6 Wro >» (H CO •H O

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- 6 -

time within the detector. But the decay or return

to zero of the pulse is determined by the time constant of the collection circuit in the preamplifier.

This time constant must be large compared with the

detector charge collection time, which might be a

function of the device geometry, so that full collection of the charge from detectors with widely

different collection times can occur before significant decay of the pulse sets in. Therefore, such

pulses generated from the preamplifier as shown in

Figure 2. (a) have a long tail compared with their

leading edge, giving rise to the name "tail pulse".

The preamplifier has a high impedance MOSFET as its

input stage to reduce the noise level. It is cooled down to liquid temperature together with the detector in the detection experiment, which will be

discussed in section IV.

If the rate of interaction in the detector is not

small, thses pulses will tend to overlap one another and give rese to a pulse train which has the

apperance shown in Figure 2,(b). Because it is the

amplitude that carries the basic information (the

charge Q deposited in the detector), the "pile-up"

of pulses on the "tails of preceeding pulses which

have not fully decayed to zero can be a serious

problem. Because the time spacing between nuclear

pulses is random, each pulse can be superimposed

on a different residual tail and the resulting, amplitude no longer is a good measure of Q from that

event. The method used to solve this problem is to

shape the pulses in such a way to produce a pulse

train similar to that shown in Figure 2.(c) by using

CR differentiator - RC integrator shaping networks

(7,8), which is part of the next circuits-linear amplifier. In Figure 2.(c), all the long tails have

been eliminated but the information carried by the
V(t)

FIGURE 2. (a) Output Tail Pulse from the Preamplifier,

V(t)

V(t)

V(t)

(b) the Pile-ip Effect! Of the ts(il pulses

n h h

(c) Only the Relative Amplitude! of the Voltages

Are Detected.

(d) Baseline Shift Effect
- 8 -

maximum amplitude of the pulse has been preserved.

The pulses, have been shaped in the sense that

their total length has been drastically reduced but

in a way that does not affect the maximum amplitude.

"Pile-up" problem also introduces the shift of the

baseline as shown in Figure 2.(d). This can be •

solved by using baseline restorer, which will not

be discussed here.

(b) Linear ampli-fier

The proper choice for the time constant of the

shapping circuits mentioned above depends primarily

on the charge collection time in the detector being

used. In the interest of reducing pile-up, one

would like to keep these time constants short so

that the shaped waveform can return to the baseline

as quickly as possible. On the other hand, once

the shaping time constants become comparable with

the rise time of the pulse from the preamplifier,

the input to the network no longer appears as a

step voltage and some of its amplitude is lost.

This loss is called the ballistic deficit and can

be avoided by keeping the time constants long compared with the charge collection time in the detector. Typical values for the time constants of CRRC circuits vary from a few tenths of a microsecond

for use with very fast semiconductor diode detectors,

to tens of micorseconds, more suitable for some types

of proportional counters in which the charge collection time can also improve the signal-to-noise ratio

by limiting the response of the instrumentation to

those frequency ranges in which the signal has useful

components.

Besides pulse shaping, linear amplifier has another

function of providing enough amplification to match

the input span for which the multichannel analyzer
- 9 -

has been designed. Here a voltage gain of 1000 or

more can "be provided so that the shaped pulse at its

output can easily cover a span of 0-10 volts. It"is

the shaping function of the linear amplifier that

often dominates the performance of the pulse processing system.

(c) Multichannel analyzer

Two sets of circuits are included in "the analyzers

(1) Discriminator

In order to count the generated pulses properly,

the shaped linear pulses must be converted into

logic pulses. A defferential discriminater with

two discrimination levels produce a. logic output

pulse only if the input pulse (from linear amplifier) amplitude lies between the two levels.

By using a se^es of this circuit the bands of

amplitudes or "windows" can be selected in one

of which the input amplitude must fall in order

to to produce an output pulse.

(2) Counter

As the final step in a counting system the logic

pulses must be accumulated and their number recorded over a fixed period of time. The device

used for this purpose is a digital counter.

The counter system is preset to a specified

number. The counting period is terminated when

the number of accumulated pulses has reached

this value. By designing like this a given

statistical precision can be specified before

the start of the measurement and the duration

of counting will be prolonged until enough

counts have been accumulated to guarantee the

desired statistical precision.

Finally, the collected data are displayed on a oscilloscope screen and can also be printed out from a printer.
- 10 -

III. FABRICATION PROCEDURES

Temperature Gradient Zone Melting Process (TGZMP) has

been successfully used for making ion controlled diodes (5.6).

Some of the processes involved in the fabrication were not

well enough controlled. In making x-ray detedtors, essentially

the same procedures were- followed for fabricating ICD, but

better control was exercised over each individual process

step.

(a) Sample preperations

Three kinds of wafers were prepared as shown in Table 1.

N-type silicon was chosen because aluminum doping would form

P-N junctions. (100) orientation was chosen for potasium hydroxide (KOH) preferential etching. The high purity silicon

(I550fl-cm) wafers were provided by NASA. They proved to be

very suitable for x-ray detectors because of their wide depletion region width, long lifetime,and high breakdown voltage. Also it was found that with wafers having both sides

polished, better P-N junctions could be more readily obtained.

t

Each wafer was given an initial and basic cleaning (Appendix A). Considerable care was exercised in these processes

because any thermal stress or scratch during cleaning would

kill the lifetime of the devices.

(b) Substrate contact by phosphorous diffusion

N ohmic contacts were made to the substrate material by

phosphorous diffusion. The spin-on phosphorous dopant solution

used is called Accuspin P-120 and is made by Allied Chemical

Company. First, 7-9 drops of P-120 were spun on to the backside of each wafer to form a source film. The thickness of

this, film was about 2500 8. when spin speeds of 3000 r.p.m.

were used. A 30 minute prebake at 120°C followed in order to

drive off the excess solvent vechicle and harden the film.

Upon completion, the same precedures were repeated on the
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front surface.

The diffusion was done in an open system flushed with N2

and 02. The diffusion temperature was 1100°C and the time was

50 minutes. The conditions for gas flow were

N2 (1.5 liters/min) for the first J*0 minutes

N2 (1.5 liters/min) 5% 02 ^75 cc-/min)

 for the last

 '10

minutes

After diffusion the wafers were dipped in buffered hydrofluoric acid (BHF) solution for 5 minutes to remove the glass

film. The four point probe technique was used to measure the

sheet resistance and the results are listed in Table 2.

The data listed in Table 2. is in agreement with those

provided by the company. The thickness of this diffused layer

is about 2 um, so the surface impurity concentration is apPO ^ proximately 6 x 10 atoms/cm .

(c) Thermal oxidation

Following the diffusion an oxide layer is grown. The dry

oxidation was done at 1080°C for one hour and forty-five minutes. After oxidation, the interference color of the oxide

film was metallic blue. This oxide layer is used to mask the

KOH preferential etching of the silicon, so therefore its

electrical quality is not very important. However, the thickness range which gives the optimal condition is from 1200 A*

to 1500 &.

(d) Aluminum evaporation

The evaporation was done in a vacumn evaporator at a presrj

sure of 6 x 10~' torr. The aluminum slugs were melted in boron

nitride boats by a tungston coated heater. The thickness of

the aluminum is a very important parameter in thermal migration. Usually, the thinner the aluminum is, the longer it
- 13 -

RESISTIVITY OP WAFER

- cm )

SHEET RESISTANCE

(

6

12

1550

5

7

TABLE 2.
takes to penetrate through the wafer. The longer thermal processing time might be bad for devices especially like our radiation detectors because the radiation keeps on damaging the

surface. Conversely, iff the aluminum is too thick, it will

migrate through the silicon very quickly usually causing a

smearing-out effect which will cause large leakage current

through the junctions. The optimal condition is to evaporate

aluminum dots 5 to 6 pm in thickness.

In order not to contaminate the aluminum after it is evaporated on to the wafer, mechanical masks were used to define

the pattern directly. Molybdenum sheets with dimensions 2" "by

2" by 0.003" were covered by KODAK negative photoresist and

then etched by molybdenum etchant at room temperature to get

the patterns (Appendix B). The Al patterns have twenty-five

2 by 2 diode arrays on a two inch wafer. The area of each Al

dot is (150pm)2.

(e) Aluminum migration by T.G.Z.M.P. •

The TGZMP is a process in which a liquid zone in the form

of a sheet, rod, or droplet migrates through a solid in a

temperature gradient. The migration of the liquid zone is

caused by three spatially sequential process: dissolution of

the solid on the hot forward side of the zone; diffusion

transport of the dissolved silicon to the cold rear side of

the liquid zone; deposition of the silicon-aluminum alloy on

•the cold rear surface.

The experimental apparatus used in TGZMP is shown in

Figure 3• It consists of two major parts* the light source

and the heat sink. The light source consists of a water cooled reflector main cavity which houses six QHI 1200T3/CL

quartz infrared lamps (General Electric) in. parallel. The

lamps are cooled by flushing air through the cavity. The cavity is covered by a piece of quartz. The heat sink is a water

cooled copper chamber whose surface is covered by a layer of

carbon paint to avoid reflection. The surface of the heat sink

is 2 cm directly under the light source. On top of the sur-
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- 16 -

face, thin quartz pins 1.5 cm long are used as supports for

the silicon wafer, which is placed directly on top of them.

The surface of the silicon with aluminum pattern is placed

face' down to the cooling base. The temperature was measured

by a chromel-alumel thermocouple (usable range: -180°C to

126o°C) directly contacting to the back surface of the wafer.

The electrical circuit of the TGZMP facility is shown

in Figure *K The three phase AC voltage is controlled by a

three phase variac driven by a synchronous. The output of the

variac is rectified by a diode bridge as shown in the figure.

The DC voltage is then supplied to the lamps.

The temperature gradient across the silicon wafer is

about 200°C/cm. The 150 urn wide aluminum dots can be stably

migrated through the silicon wafer, in eighteen minutes and

fifteen seconds including V50" rise time and 5' 15" fall

time. The slow rise and fall times were necessary to avoid

warping of the wafer and the consequent introduction of defects which would reduce the lifetime. The fall time began

when most of the aluminum dots had migrated through the wafer.

The temperature-time and power density-time profiles are

shown in Figure 5« (a) and (b). When logarithm of power density versus temperature is plotted, a linear relationship

is obtained for both rise and fall periods as shown in

Figure 6. This relationship can be expressed as

Power Density = constant x 10kl temperature) (9)

where k is a constant ranging from 0.52 to 0.57. This expression satisfies the differencial equation ~

= k . p or dT .. p _ 1 (10\

dT dP k

where P is the power density and T is the temperature of the
- 1

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FIGURE 5.

T(°C)

(a) TEMPERATURE - TIME PROFILE

8'10'

4*50" 10'

1

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(b) POWER DENSITY - TIME PROFILE

P/A 2

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- 20 - '

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wafer. The input power will have to be one hundred times larger in order to double the temperature of the sample.

During the thermal migration, the aluminum dot continues

to travel in the wafer along the direction of maximum thermal

gradient . Its concentration keeps on decreasing along the

path. When the migration is completed, the zone near the" back

surface is filled with aluminum-enriched silicon while the

zone near the front surface is filled with silicon-enriched

aluminum. This is why the KOH etchant can etch the back zone

of the P region even when there is no window opened on Si02

layer at the back surface. We can see some mullite covering

the P region on both surfaces after thermomigration. This

might be due to the contamination from the environment.

The pictures preceeding and following the migration of an

aluminum dot through silicon are shown in Figure 7 . (a) and

(b), respectively. The cross section views of the P-N junction after migration are shown in Figures• (a) and (b).

These pictures were taken by Siamak Azizi who was a graduate

student here.

TGZMP is the most critical process step for the fabrication of x-ray detectors. Because of the nature of the heat

source, this is also the hardest step over which to get control. The reasons ares

(i) The thermomigration was done at atmospheric pressure

and the convective heat loss can be quite variable.

Also because of this, the surface region near the

P-N junctions was undoubtedly contaminated. The extent of this contamination was not studied in this

work.

(ii) Since the temperature distribution was not uniform,

thermal stress was introduced and lateral migration

occurred in the wafer. This has been experienced
- 21 -

(a) Aluminum Dot Before Thermal Migration

. .

(b) Aluminum Dot After Thermal Migration

(The background is the silicon wafer.)

FIGURE 1.
- 22 -

(a) Lateral View of the P Region

(The "T" shape lines are due to the crack of

the sample holder.)

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(b) Cross Section View of the P Region

(We. can see a little lateral diffusion from the

P region.)

FIGURE Q,,
- 23 -

from the warping of the wafer after migration and

seen from the shape of the junction as shown in

Figure 8. To solve for the warping problem, thicker

wafers (1^1 to 16 mils) are recommended for processing.

(f) Potassiua hydroxide (KOH) -preferential etching

Before etching the silicon we first defined windows (750

ura) en both surfaces surrounding the P region by using negative photoresist and buffered hydrofluoric acid.

There are two reasons why we have to etch the silicon itself j

(i) The mullite covering on the P region has to be removed for better ohmic contact and smaller diode

leakage current due to surface contamination.

(ii) The N layer beneath the Si02 layer has to be removed

in order not to short the junction.

Different kinds of etching solutions such as aluminum

etchant (Appendix C), isotropic silicon etchant (Appencix D)

(17), and anisotropic silicon etchant (Appendix E) (18 ) were

tried and all failed to remove the mullite. Finally, a. KOH (25

ft) solution in a ^ : 1 ratio with isopropyl alcohol (IPA) was

tried at 6o°C and the mullite was removed completely in one

hour and thirty minutes. In this etchant, KOH has been employed as the oxidant and IPA as a complexing agent. From our

experience, it seems as though IPA does not play an important

role in the etching because the KOH solution does not mix

with IPA. KOK solution etches the 100 surface much faster

than it does thi (111) surface. The ratio of the etching rates

of these two surfaces is about 25 .• 1 at 6o°C as shown in

Figure 9 . The etching rate of the (100) surface is about 0.5

um/min. During the etching process, stirring was used, this

was an important factor in getting good P-N junctions. Upon
1.0

0.001

KOH-IPA-H20

SOLUTION

(100)

'K

FIGURE 9^- Temperature Dependence of the Etching Rate for

(100) and (111) Orientations of Silicon (19).
- 25 -

completion of the etching, a very clean and shining P region was left as shown in Figure10. for the front surface and

in Figurell. for the back surface. The cross section views

are shown in Figure 12.

(g) Wet thermal oxidation

After preferential etching, the Si02 layer was removed

and the samples went through basic cleaning again (Appendix

A). Experience has shown that to passivate the silicon surface an oxide layer of suitable thickness is needed. The purpose of surface passivation is to reduce the leakage current

through the surfaces of the junction. Because the aluminumrich region may melt at high temperature, we should minimize

the oxidation temperature and shorten the oxidation time as

much as possible. It is because of this that we adopted a low

temperature (950°C) wet oxidation process to minimize the lateral diffusion. A 1700 & thick oxide layer was obtained in an

oxidation time of sixty minutes (20 ).

(h) Shape of the P-N .junction

Aluminum is a substitutional dopant in silicon. The temperature dependence of its diffusion constant is

DM= Deo . Exp (-Ea / kfi.T) (11)

2

where D^ = *K8 cm /sec is the diffusion constant of aluminum

in silicon at infinitely high temperature, and E = 3-36 eV cL

is its activation energy (21).

Therefore, for the oxidation temperature and time used

the diffusion length is approximately

L

 =A/ DAI • Time = 0.16 pi (12)

This indicates that the aluminum diffused out to produce a

linearly graded junction rather than an abrupt junction.
- 26 -

(a)

FIGURE 10. Top View of P Region After KOH

Preferential Etching.
- 2? -

K-.siiffl?'r^s«fe-^ ^-^^ wiw-l«i.k F_**WV .t*

fe^Mrr^M5*

5?''?S#*- _5T:-iaii«S"«iglS^

(a)

FIGURE 11.

(b)

The Views of Back Surface of Silicon

Wafers After Preferential Etching.

(P+ regions are at the center.)
- 28 -

—<

(a)

o

M

I

M

X

E-i

O

M

EH

O

Q

(b) (larger magnification)

FIGURE 12. Lateral Views of P+ Region After

Preferential Etching.
- 29 -

This will be discussed further in Section IV. (b).

»

(i) Metallization and packaging

The electrical contacts were made to both the substrate

and the P region by evaporating thin aluminum layer (about

5000 X) on to the wafers. An anneal process for aluminum

at 435°C for 30 minutes in forming gas (92# N2 and Qfo H2) is

needed to improve the adhesion.

Finally, these two by two diode array chips were scribed

and mounted onto ceramic substrates with epoxy. Five gold

pads cut from a piece of gold foil were glued on the ceramic .

substrate. Four of them were connected to the four P regions

and the fifth one to the substrate (the N layer). This completes the assembling of the devices.

The whole fabrication process is shown schematically in

Figure 11.
- 30 -

(1) Wafer Preparation

and Cleaning

<100>, bothside polished

N type, 1550 H-cm

15 mils <

(2) Phosphorous Diffusion

6 x 1020/cm3

2 urn

J

(3) Thermal Oxidation

1500 8 Si09

vS*^yy*vQx;:VCxSN?x>sJ>

Aluminum Evaporation

Aluminum; 5^*6 urn

(5) Thermomigration

Si02 Etching'

(HF or EHF)

(7) KOH Preferential Etching

25% KOH + IPA, 60°C

1-| hours

A ^^^^^^^^^^^^^j A

P region-'

(8) Si02 Etching and

Wet Oxidation

950°C, 50- 60 min

1700&

(9) Metallization and

Gold Wire Bonding

Au

•Al pad

(10) Packaging

*

+

 *+ Si

ft- Au pad -* • wafer

N-Sub.

substrate

P+ region

FIGURE 13. Fabrication Procedures For

X-Ray Detectors
• - 31 -

IV. EXPERIMENTAL RESULTS '

(a) Junction evaluations

Different methods are used to evaluate the diode junctions including diode I-V measurements, diode Cm-V measurements, and Metal Oxide Semiconductor (MOS) capacitor measurements.

(i) Dark I-V measurements

The dark current flowing through a reverse biased

diode is composed of three separate parts:

(1) saturation current due to diffusion of the minority carriers;

Minority carriers of both P and N type materials

near the boundaries of the depletion region can

diffuse into the depletion region. They are then

swept through this region by the strong electric

field. This current can not be controlled by the

process and is proportional to the square of impurity concentration of the substrate for both

abrupt and linearly graded junctions. So, the

higher the resistivity is, the lower this current will be.

(2) generation current in the depletion region: (22)

The generation current is related to the existence of recombination centers, which are due to

the impurity atoms or structural imperfections.

This current is quite process-dependent because

T oc n (P V ) '3 /— MT^

-"•generation

 ni ' V ' V /T UJJ

for a linearly graded junction. P and T are the

resistivity and lifetime of the substrate and

YR is the reverse bias. If VR is increased or

t is decreased due to the process, this current
- 32 -

will be increased,

(3) surface leakage current;

This current is induced by the interface centers

near the intersection of the junction and the

surface. It is quite process-dependent. Because

of the nature of the etching solution used, it is

expected that this current would be the dominant

factor among all different current contributions.

This is one of the reasons why we have to passivate the surface by oxidation.

The reverse bias I-V characteristics of these diodes

had been taken before the surface was passivated.

They are shown in Figure Ik. for three different

kinds of wafers. The same characteristics were also

measured after the surface had been passivated, metallized, and packaged as shown in Figure 15. for

comparisons. For the latter case, the leakage current is always in the nano-ampere region if the bias

is not too large.

The informations shown in Figure 14. and 15. are

again summarized in Table 3. Also listed in the table

are the corresponding depletion region widths at the

approximate breakdown voltages calculated from Equation (2). (page 3)

(ii) Diode C,p-V measurements:

. The trasition capacitances (or space charge capacitances) of diodes under reverse biases were measured

after-surface passivation. Because of the thermal

process involved, such as thermomigration and oxidation, we expect the P-N junction to be linearly

graded rather than of the ideally step graded type.

Figure 16. (a) shows the C-V curve for a diode on
- 33 -

B~ -—ka:ia*ifca»-<ii

(a) Wafer; 8 H-cm.

Breakdown Voltage; -120 volts..

Current Scale; 50 pA/Divisipn.

Voltage Scale; 50 volts/Division.

r?&-^f-&&it*^i'&j3£i£-~r^.^?^^-?l&!S&ffZ3ia1&&&ta,

Wafer; 12 It

Breakdown Voltage; -200 volts.

Current Scale; 100 pA/Qivision.

Voltage Scale; 50 volts/Division.

FIGURE Ifr. P - N Junction Characteristics

Measured Before Surface Passivation.
- 34 -

(c) Wafer: 1550 fl-cm.

Breakdown Voltage: -2?0 volts.

Current Scale: 100 juA/Division.

Voltage Scale: 100 volts/Division.

RESISTIVITY

(fl-cm)

6

12

1550

BREAKDOWN

VOLT AGE (V)

(from Pig. 14. )

120

200

270*300

DARK LEAKAGE

CURRENT (nA)

(from Fig. 15- )

3.4 at -13 v.

0.54 at -13 v.

15. at -30 v.

DEPLETION

REGION WIDTH

(fua)

4.6

8.4

343.4

TABLE 3.
- 35 -

(a) Wafer; 6"8 /I-cm.

(b) Wafer: 12 A-cm.

V0 (VOLTS)

-12

+0.17

-6 -8 -10

VR (VOLTS)

-12

FIGURE 15. P - N Junction Characteristics

After Surface Passivation.
- 36 -

(a) Diode CT - V Characteristic Curve,

-2 -4 -6 -8 -10 -18 ' -20

10 -

5

2 .

1 -

0.5 -

0.2 .

(b) Logarithm of CTM vs. Voltage,

SLOPE = -0.32

-0.2 -0.5 -1 -2 -5 -10 -20 -50 -100

FIGURE 16. ' CT - V and Log (CT) - V Characteristic

Curves for the Case of 12 H-cm Wafer.
- 37 -

the 12H-cm wafer. The Logarithm (CT) - Logarithm (

VR) curve is shown in Figure 16 . (b) which shows a

linear relationship with slope

Logarithm (Cm)

1

 = - 0.32 = - 1/3

Logarithm (VR)

This established that the junctions are linearly

graded rather than abrupt. The CT~V curves for other

two kinds of wafers are shown in Figure 1?. (a) and

(b).

The following model for calculating the space charge

capacitance is proposed*

cylindrical capacitor model

Because during thermomigration the aluminum square

dots are. rounded off in the silicon wafer, the actual shape of the P region is cylindrical but not

square as shown in Figure 18. From the equation for

calculating the capacitance between two concentric

cylindrical plates, we have (23)

2 . 7T . £ «,. . £ n . t

unction ^ - + / A ) (15>

where 2 . A = 150 um is the dimension of the aluminum

dot,

d is the depletion width,

and t is the thickness of the wafer.

t is very difficult to determine precisely due to

the preferential etching, so estimation is used.

Table 4. shows the comparisons between the calculated values and the experimental data. The data

was all obtained at V~ equal

 :Jo -20 volts.

The discrepancies might be due to:

(1) the over-simplified model.

(2) the error of measurements.

(3) "the rough estimation about t.
(a) C - V and G - V Curves for 6 fl-cm Wafer.

(pF)!5.3

C-V Curve

G-V Curve

G(umho)

(b) C - V and G-V Curves for 1550 n-cm Wafer

C ,

 -T"TI>

 tl3.3

(pF)

C-V Curve

-.—~ G-V Curve

12.1

..10.9

G (umho )

-8 -12 -16

VR(VOLTS)

-20

FIGURE 17. C-V and G (Conductance)- V Curves

for 6 and 1550/l-cm wafers.
- 39 -

(150um)

H

 2A h

t (THICKNESS OF

P+ REGION)

P REGION

(DEPLETION

REGION)

FIGURE 18. Simplified Geometry Of The Diode

Used For Cylindrical Capacitor Model,
^ w<o

02

egQM

20

X<

00

CVJ O\ T-H

CVJ rH rH

o\ en

evj en

o o o

o o en

e\j c\j en

\O CVJ o

PQ

<

EH
(iii) MOS C-V measurements;

When the 6 Cl-cm wafers were processed for x-ray

detectors, three other wafers of the same resistivity were processed together with them. However,

the aluminum migration process was skipped for

these three wafers. After KOH preferential etching and basic cleaning, very clean dry oxide films

were grown on both kinds of wafers-the wafers with

P region and those without. Chromium film (100 A)

and gold film (500A*) were then evaporated on to

these wafers in sequence for MOS gate. " ..

After completion, the C-V characteristic curves

were taken for both cases as shown in Figure 19 .

The shapes of these two curves are very similar.

The G-V curves are also shown in Figure 19. The

shoulders on both the C-V and G-V curves correspond

to turn-on of the (111) region at higher voltages

(as expected due to their higher interfacial

state density). The (ill) surface arises because

of the KOH preferential etching. No additional

bumps tha.t might'-be associated with the P region

were found in Figure 19.(b). This indicates that

the P region has no influence on the C-V curve

of a regular MOS capacitor. However, the problem

of growing good quality oxide on top of the P.

region remains. Problems like large leakage current through the oxide or low dielectric strength

appears. The lower dielectric constant also influences the reverse bias dielectric breakdown.

Since the leakage problem occurrs in both cases

o.f Figure 19 when positive bias was applied,

this might be due to "Lhe microcracks in the oxide.

These mierocracks might be introduced by the

roughness of the etched surface of the wafers.
(a) C-V and G-V for Wafer

Without P+ Region.

C-V Curve

G-V Curve

-8

VR( VOLTS)

-12

-•600

-12

VR( VOLTS)

-20

G(umho)

C-V and G-V for.Wafer

With P* Region.

C-V Curve

G-V Curve

G(umho)

-20

FIGURE 19. C -V and G-V for Wafers (a)

 with°ut

(t>) With P+ Region. (6/1-cm wafer)
(b) X-RAY DETECTION EXPERIMENTS

The diode arrays on 1550 H-cm wafers were sent to NASA

for test. The cryostat system for detection experiment is

shovm in Figure 20. The sample was thermally contacted with

a copper tube, which could be cooled down to liquid nitrogen

temperature (77°K). The preamplifier containing a Metal

Oxide Semiconductor Field Effect Transistor (MOSFET) circuit

was also cooled down to this temperature. The importance of

the MOSFET circuit will be discussed later. The temperature

was measured by a standard silicon diode contacting to the

device and a voltmeter. The usable temperature region of

this diode is from ^°K to 400°K. The pressure during mea-

-h. _<5

surements was about 10 — 10 ^ torr obtained by an absorption pump. The devices and circuits were shielded by thin

Be foils and the whole cryostat system was covered with black

cloth.

The x-ray energy of interest is from 5 KeV to 50 KeV.

The x-ray source used in this work was Fe^ with characteristic energies as followst

Koc = 5.895 KeV

Kp = 6.^92 KeV

Since the difference between K^ and K« is only 597 eV, we

really need a detector with Full Width at Half Maximum

(FWHM) less than this difference in order to identify the

two peaks. During the experiment the devices were reduced

to about 86°K to reduce the noise background. The energy

spectra obtained from the detection system for different

bias conditions and operating temperatures are shown from

Figure 21.(a) to (e). In the right halves of these photographs we can see two peaks, one bigger and. the other one

smaller. The -big peak corresponds to the energy level k«

of Fe-^ while the small one corresponds to kg. Also observed from these pictures is that the positions of those

two peaks in the energy spectrum stay invariant through
- 44 -
i^-v*—.. fe«L< -fk^i'Jii^^j'ft?? - V«.-iu.T?>j>A**<*ur tn ;. >«: ^v<h»

^a»?fg»agaffC5?J«gg>fir'an»rf I -it:.- •: .^"^

SfSi^iSKiii^XfS^tf^ii^fi^r^'-

(a) BIAS; -100 VOLTS.

TEMP., i 115 K.

FWHM (K<x)i 593 eV.

(b) BIAS: -40 VOT-JS

TEMP, s 8?°K.

FVfflM (Koc) J 613 eV.

FIGURE 21. Energy Spectra Obtained From Detection

System Under Different Operating Conditions.
(c) BIAS; -20VOLTS,

TEMP, t 86°K.

FWHMi 534 eY.

(d) BIAS; -10 VOLTS,

TEMP. : 85 K.

FWHMj 514 eV.

FIGURE 21. (CONTINUED)
(e) BIAS; -5 VOLTS,

TEMP, i 89 K.

FWHMs 51^ eV.

(f) BIAS; -20 VOLTS.

TEMP. : 130 K.

FV/HMi4-80~500 eV. (estimated)

(NASA Chip)

FIGURE 21. (CONTINUED)
- 48 -

different "bias conditions. The results indicates that these

diode detectors are suitable for soft x-ray detection. The

full width at half maximum (FWHM) was also measured for each

case and the results are listed in Table 5« The best condition found is sample "PENN 5" where -5 volts and 89°K were

used. Its FWHM is around 500 eV. This result is good because

the preamplifier,• which used resistive rather than optofeedback, had a FWHM of about 400 eV with a Si(Li) radiation

detector.

Figure 22 shows the comparison of these results in the

same spectrum. The counts of peaks K^ and Ka were plotted as

a function of bias voltages in full logarithm scale as

shown in Figure 23. and 24. Two linear relationships with

solpes 0.38 and 0.37 for KC* and Kp were obtained, respectively. This means that

(Counts of Ktf or K^ peak) o< vR°-37~0.38 (16)

From Eq. (14) in Section II, we know that

doc(VR)

1/3

Therefore, "e conclude that

(Counts of Krf or Kg peak) oC Depletion Region Width (17)

This is reasonable because the wider the depletion region,

the more charge be collected.

In Table 5- and Figure 21.(f) also show the data and

spectra obtained from one of NASA's device operated at similar

condition except that 130°K was used in stead. Similar results to ours were obtained. The reason for this higher operating temperature will be clear in the last part of this

section.

In the left halves of the photographs from Figure 21.(a)

to (e), we see another peak in the spectrum in each case

which is unexpected. Two phenomena are observed from this.

First, the energy positions of these extra peaks are dependent on the applied biases. The energy increases with the

bias. Secondly, the peak corresponding to -100 volts case
~°~

LU

OO

z: a. >

O O)

CO X

O LU

LO

C3

O

CJ

o

co

=3 UJ

O CL

C3

UJ Q.

Q- 21

O UJ

UJ >

to co

OS CO hUJ <c _I

> "-i O

uj ca =»

UJ

_j

Q.

S

CO

in co in in

-O

V

•P

r«»» o 9 • • • ^

CT» ^— CO r— r— CO 'fj

in 10 in in in ^- 4s

CO

0)

CO

CO CM CM

in vo •

CM i—

CM CM

(ft

*f

CO

CO CO

CM PO

>— I

CNJ *r

CO CM

cr> co en

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vo

in f*

i— CO co

o

co

o o o

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<M

UJ

a.

UJ

a.

UJ

o.

UJ

a.

UJ

o.

o

CM

•H

JC

o

<

co
o o o o

Q\ T-i

K

§s

M

a

o o o o o

V> C^-vO V> ox

tH 00 CO OO 00

CO C3 CO W CQ

-p -p -p -p -p

iH fH H ^1 H

O O O O O

» >»

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I I I I I

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X

o

o

o

o

CM

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tn

o

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o

o

u>

T2NNVHO H3(i SJINAOO
- 51 -

C/2

3000 - -

J?-2000

C/2

E-i

O

O

fa

O

O

s

1000 -•

500

PEAKS

4-

SLOPE =0.3 8

5 -10

LOG. (

—I

-20

—

(VOLTS)

+H-

-100

FIGURE 23'. Logarithm Of K# Peak Counts

vs. Bias Voltage.
PEAKS

500

400

300

200

100

LOG. VR(VOLTS)

-40 -100
- 53 -

split into two. We propose the following model to explain

these interesting phenomena.

In the array pattern there are four P regions sitting

p

at the four corners of a (1000 pa) cubic. When one of these

four diodes is reverse biased at very low temperature, its

depletion region can extend to about 300 to *K)0 micrometers

away from the central P . region. This is schematically

shown in Figure 25.(a). The corresponding energy band diagram is shown in Figure 25.(b). Holes are generated by radiation in the neighbors of the three floating diodes. But

only the holes in the regions of these neighborhoods that

are closer to the biased diode have a chance to diffuse into

the depletion region. From a rough calculation, if the temperature is 87°K and the lifetime of the holes in the bulk

material is in the range of 100 us, which is reasonable for

this high purity (155on-cm) material and low temperature

condition, we obtain the diffuion length as

. rp = 500^1000 urn (18)

which shows that there is indeed a probability for the holes

from those energy band valleys to diffuse into the depletion

region.

If we use a very simple diffusion model assuming that

the probabilityof a hole not being recombined or scattered

during its diffusion process is exponentially decaying as

shown in Figure 25. (c). Therefore, the charge collected

by the biased diode from the nearest neighboring diodes is

d

= 0

IOM-A.

(19)
depletion

region of the

biased diode

built-in depletion

region of the unbiased

diodes

FIGURE 25. (a) the diode pattern when one of them is

reverse biased.

biaseddiode \

P(x')

probability density

function for a

hole to survive

after diffusing

a distance x.

(b) the energy bands diagram for a biased

diode next to an unbiased diode.

edge of the depletion

region

charge collected

(c) the probabilit •i y function of collecting

charge from the neighboring unbiased

diodes.
- 55 -

where k.. is a constant.

Since from Eq. (8) we know that the energy in the energy

spectrum is proportional to the amount of generated charge

in the depletion region. These diffused charges have suffered a distortion in amount before they are collected. This

explains why the extra peak came out in lower energy-region.

When the reverse bias is increased, the'depletion region is

wider which can. then collect generated charge more completely

from the three floating diodes. The holes in the next nearest neighboring diode can be collected in a detectable amount

only when the depletion region is wide enough. This .explains

why the second extra peak came out in even lower energy region for the case of bias equal to -100 volts.

The ratio between the charges collected from the next

nearest and nearest neighbors is

\QOO4-14.

°° = 4-3.7 #

This is very close to the ratio between the energies of the

two unexpected peaks for -100 volts case, which is

(21)

When the logarithm of the energy position corresponding

to the extra peak for each case is plotted as a function

1/3

of VR ' , a linear relationship is obtained, which means

Log (energy position) OC VR (22)

or

k V energy position oC amount of charge oc e 2 * R

oCeK3 ' d (23)

This relationship is shown in Figure 26.
<H

O

CO .

C CO

O W)

•H cfl

-P -P

•H H

CO O

O >

Pi

CO

0

*^W

3

0)

- 56 - r

JS tQ

E-i Jrf

03

(H OJ

O Pi

S <A

X ^

•P -P

VI

so

CM

O

K

S

to

O

* •

CM

II I I I I

O

o

CM

O

O

vHixa SKD jo Haaamw TSMNVHO MI sMoiiisod
- 57 -

This proves our proposed model. The importance of this result is that really good spacial resolution spectrometer

arrays to the dimension of diffusion length of holes in

silicon at low temperature can be available.

Noise

The noise in the energy spectrum comes from several

sources:

(1) any drift of the operating characteristics of the

detector during the course of the measurements, e.g.,

the leakage current. This current produces a high

level of low frequency noise. Such noise can dominate in low-energy x-ray spectrometer systems.

(2) sources of random noise within the detector, e.g.,

the thermal noise. This will be reduced at lower

temperature.

(3) random noise of the instrumentation system.

The important sources of noise occur near the beginning of the signal chain where the signal level is

at minimum. Noise generated at this point undergoes

the same amplification as the signal, whereas noise

generated further along the singal chain is usually

much smaller than the signal. Therefore, discussions

of electronic noise sources generally center on the

preamplifier, and, most important, its input stage.

This is why we choose to use MOSFETs as the input

stage. MOSFETs have low leakage current and high

input impedance.

If we define the equivalent noise charge (ENC) as the

amount of charge which, if applied suddenly to the input termin~lo of the system, would give rise to an output voltage

equal to the RMS level of the output due only to noise. ENC

is translated into the equivalent energy deposition by a

charged particle in the detector. This explains the pile-up

o* counts at the very low end of the energy spectrum.
For MOSFET, it can be obtained that (24)

ENC (XT -4-

where is the transconductance of the channel of MOSFET.

o 13 temperature-dependent and has a maximum between. 110 K

and l4o°K for silicon as shown in Figure 2?. When gm is at

its maximum, the ENC will be at its minimum. This is why

115°K to 130°K was used for some of the experiments.

03

o

•H

H „

•H 7

•H

e

~ 6'

w

o

O

o

o

* 2

1

5 ••

-200 -160-120 -80 -

H 1 1 1 > 4

FET CASE TEMPERATURE ( C)

FIGURE 27. Temperature Dependence Of

The Transconductance Of An MOSFET. (25)
- 59 -

(c) (X-particles detection experiment;

This experiment was done in Physics Department, Universi-cy of Pennsylvania. The detection facility was very similar to the one described before. The source of tf-particles

njt •>

was Am whose peak energy is at 5.^8 MeV. The operating

temperature was higher than liquid nitrogen temperature.

The results of the test are listed in Table 6.

For comparison the data obtained from an ion implanted

detector is also enclosed in the table. From the comparison the deep diode arrays are found not suitable for OC-particle detections. The reasons for this might be that

(i) the penetration depth of ot-particles is too short,

(ii) the passivation film (SiOg) acts as a dead layer

for tf-particles.

(iii) the junction area of the ion implanted diode is

o

large in this case (25 mm ).
- 60 -

o>

CM in «r \o

• • « •

CM <O «»• O

i— F— CM r—

UJ

c:

CO CTi O r—

o

o

CJ3

to in

«si

in in

CM

LU

Q.

O

CO

<

f^

CO

o o CM in

i i

<u

C

Ol CU

Q. O.

c c

<B o ^- co

C C.4J

CU O S- CU

D. f T- T3-
- 61 -

V. ADDITIONAL STUDISS ON MATERIALS SUBJECTED TO TGZMP

CONDITIONS

A series of additional intense incoherent radiation

studies were conduted on the Si wafers in order to obtain

greater insight into the effect of this processing. They

arej

(a) Annealing of chemical vapor deposited SiOp.

(b) Recrystallization of chemical vapor deposited Si02.

(c) Inspection of defects generated by thermal radiation.

(a) Annealing of chemically deposited Si02

Using the results of N. Goldsmith and W. Kern (28,

29) Chemical Vapor Deposited (CVD) SiOp system was

designed as shown in Figure 28.

Silane (SiH^) gas (2.1 % balanced with Ar) was

flushed into the system to react with oxygen gas

just above the surface of the sample in a surface

reaction. The reaction temperature ranged from 300

to 500°C. The reaction equation is

300~-500°C

Si%(g) + °2(g) S102(s) + 2H2(g) (Z5>

Argon or nitrgen was used as the carrier. The flow

rate for each gas species is shown in Table ?.

The optimal ratio between 02 flow and SiH

02 j SiH^ =13- 1 (26)

and the optimal growth temperature was between 320°C

and 350°C. Even for the optimal conditions, it was

difficult to obtain uniform films from this single

wafer system. The Si02 growth rate was from ^00

iVmin to 500 Sl/min. The resulting film could be

etched by hydrofluoric acid (10#) or buffered HF.

In order to study the effect of incoherent radiation
- 62 -

0)

I

3

0)

H)

u

tt>

•p

+»

9^

V

•5

fco

•f*

w

0)

r-l

01

I

VW-SI-2-H

^^

1«.-«

r CM frf U

as o<

C -

O (U

•H -d

•P.H

•H x

co o

o

AC

<D O

Q O

O >H

•H C

10 O

0) o

Q-H

OJ W

•p

CO J3

W O

Q"

> <H

O O

CO

CM
- 63 -

1

c

•H

.e

•

O

»

O

cvj

C

•H

O

*

O

vO

0

CO

O

•

O

•p

•p

H

CO

CO

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CSJ 0

-p

•H

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*~* CM

•a

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ft
- 64 -

annealing, a two inch wafer with a relatively uniform Si02 thin film (about 3000 S) cut into four

quarters was used. It was placed in the annealing

facility (already shown in Figure 3« ) • The temperature was measured as described in section III.(e).

The Si02 films were placed to face directly the radiation source . Different annealing periods were

used for three samples, as shown in Table 8. After

the annealing, thicknesses were measured by an

-step machine and the infrared spectra were taken

. from these four samples. The results are shown in

Table 8. and Figure 29.

The thickness of the oxide film decreased with

annealing time. Wafer 4 is peculiar on its thickness.

There might be some oxide growing during the annealing because of the high temperature and long annealtime used- on this wafer. Th infrared spectra show

an increase in the absorption peaks at wavelength

1075 cm and 813 cm" . These two peaks correspond

to the vibrational energies of the bonding between Si

and 0. This suggests that the number of Si-0 bonds

increased and that the films became denser with

time. From the spectra we also see the change in the

peak height at wavelength 2330 cm . This peak position is very close to the bonding between Si and

H (2240~2300 cm'1) (30).

(b) Recrystallization of chemically deposited silicon

The system for polycrystalline silicon (poli-Si) is

exactly the same as the one used for SiOp deposition. The reaction used is the pyrolytic deposition of silane

SlH4(g) 700~800°C

 Si(s) * 2H2(g) (27)

Different growth temperatures (700,750, and 800 °C)
- 65 -

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were used. X-ray diffraction spectra were taken

from these samples. When the temperature was lower

than ?00°C, the silicon film was "armophous" because

no crystallization peak came out in the spectrum.

When the temperature was from 700 to 750°C, the film

was polycrystalline and it favored the (111) orientation growth rather than (110). However, when the

temperature went higher than 750°C, the growth of

crystal along (.110) was preferred. These results

are in agreement with those obtained by Kamins and

Cass (31).

The sample grown at 700°C was cut into three parts

and two of them were further annealed by the radiation source. The x-ray diffraction spectra were

taken again. The results and annealing condition

used are shown in Table 9 and Figure 30. We see

that during the annealing the films had gone from

armophous status to polycrystalline status mainly

along the (111) direction. In a very short annealing period of 30 seconds at 730°C poli-Si with crystallization peaks even higher than those grown at

730°C for 10 minutes has been., obtained.

(c) Inspection of defects generated by incoherent radiation annealing

The procedures shown in Figure 31« were conducted

on silicon wafers in order to study surface defects

generated by Incoherent Radiation Annealing (IRA).

After completion of the three different processes,

photographs were taken on the surface of the silicon

wafers. They are shown iu Figure 32.

The results indicate that the SiOp film actually

protects the surface of the silicon wafers from

some aspect of radiation damage. This coincides
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- 69 -

FIGURE 31.

Si Wafer, P type

(100), 10-20

Cleaning Procedure

(APPENDIX A)

SAMPLE 3

Thermal

Oxidation

1500 & SiO,

SAMPLE 2

Incoherent Radiation Annealing

Temperature = 980°C, Time = 20 sec

Si02 Removal

SAMPLE 1

Silicon Etching (Wright Etchant (APPENDIX

_0 minutes, Room Temperature

Optical

Microscope
- 70 -

sssvjg*- S~w sjag-iS

^^ii'fess^ssGSfe

WSH£^7K^TMi~C3i^S:?5=*--V^~;=7;-,«W.«.try«i«:r;S£;i£*^»5-1!'-'S,r-^-~- ^-,s!Z3I

^S^^^SfS?S^^^w^^^^-^^Si:®^5?^'-i3?4^-^kC2SSy

fe:7<saJiSS*;Mli.

l*£t'i>M!5tt

(a) SAMPLE 1.

j^^Jg^^fe^f^i^.^?^

(b) SAMPLE 2.

FIGURE 32. Surface Defects Inspection

On Silicon Wafers.
- 71 -

(c) SAMPLE 3.

FIGURE 32, (CONTINUED)
- 72 -

with our experience that migration of aluminum dots

through silicon wafers with oxide always gives us

P-N junctions with ..lower, leakage current. This

is because there are more surface defects generated

by the radiation in the unoxidized case,,
- 73 -

vi. CONCLUSIONS'

The deep diode arrays fabricated by TGZMP have proved

to be very suitable for x-ray detection. A 500 eV FWHM

energy resolution has been obtained by biasing the device

to -5-volts. We believe that better results would have been

possible if more suitable masks and packages were available.

TGZMP has proved to be a very reproducible process for

fabricating the deep diode arrays. Some key points about.the

fabrication process are:

(1) to use both side polished high purity wafer for processing,

(2) to use thick wafer (lb to 16 mils).

(3) to grow 1200 & to 1500 & Si02 for both radiation

shield and etching mask for the silicon wafer.

(^) to use mechanical mask to define the aluminum pattern on the silicon wafer directly.

(5) to use long rise and fall time (3 min to **• min) to

keep the lifetime longer,and also to migrate aluminum in controlled atmosphere.

(6) to grow a low temperature wet oxide for surface passivation.

Thermal migration offers solutions toward extending

x-ray detectors to the 5 "to 50 KeV range and still obtaining good energy and spatial resolution. Some strong features of these x-ray detectors are;

(1) ability to produce detectors with good energy and

spacial resolution.

If those extra energy peaks in the spectrum can be

proved to be the crosstalk between diodes in the

same array in further experiment, the dimension of

these arrays can be shrinked to within the diffusion

length of the carriers in the bulk.

(2) ability to detect high energy x-rays using moderate
bias voltages and moderate resistivity material.

(3) Variable pixel size from the order of tens of microns

to hundreds of microns can be obtained.
- 75 -

TABLE 10

TYPE OF DETECTOR ADVANTAGES DISADVANTAGES

Diffused junctions 1. Easy to make.

2. Less radiation damage.

3. Less sensitive to

contamination.

1. Less sensitive to

radiation (dead

layer effect).

Surface barriers 1. Sensitive to

radiation.

2. Larger collection

area.

3. Room temperature

operation.

1. Poor spatial

resolution.

2. Not easy to make,

3. Sensitive to

radiation damage,

4. Sensitive to contaminations.

5. Larger leakage

current.

Charge coupled

device

1. Good spatial resolution.Poor collection

efficiency

Ion-implanted

junctions

1. Smaller leakage

current

2. Sensitive to radiatior

3. Large collection area

4. Room temperature

operation

1. Low temperature

operation for low

energy radiation or

particles.

Thermally migrated

junctions

1. Good spatial resolution.

2. Smaller leakage

current

3. Less sensitive to

contamination.

1. Fair collection

efficiency (small

junction area)

2. Low temperature

operation
- 76 -

VII. REFERENCES

(1) W. G. Pfann, "Zone Melting", John Wiley & Sons, N.Y. ,

(1966)

(2) H. E. Cline and T. R. Anthony, J. Appl. Phys. , ^3, p. 10,

(1972)

(3) T. R. Anthony and H. E. Cline, J. Appl. Phys. , 4-7,

p. 2325, (1976)

(*0 T. R. Anthony and H. E. Cline, J. Appl. Phys., 4-7,

P.2316, (1976)

(5) C. C. Wen, Ph.d. Dissertation, (1979)

(6) G. C. Chern, Master Thesis, (1981)

(7) G. F. Knoll, "Radiation Detection and Measurement",

John Wiley & Sons, N.Y. , Chap. 11, (1983)

(8) G. Bertolini, and A. Coche, "Semiconductor Detectors",

John Wiley & Sons, N.Y. , Chap. 1, (1968)

(9) G. L. Miller, W. L. Brown, P. F. Donovan, and

I. M. Mackintosh, IRE Trans. Nucl. Sci. NS7, P. 185,

(I960)

(10) W. M. Gibson, G. L. Miller, and P. F. Donovan, "Alpha,

beta-, and gamma-ray spectroscopy", ed. K. Siegbahn,

North-Holland Publ. Co., Amsterdam, p. 3^8, (1965)

(11) K. G. McKay, and K. B. McAfee, Phys. Rev. 91, p. 1079,

(1953)

'(12) L. M. Halbert, and J. L. Blankenship, Nucl. Instr. Meth.

8, p. 106, (I960)

(13) C. W. Fab j an, M. Kenawy, and H. Rauch, Z. Physik 188,

p. 378, (1965)

R. H. Pehl, F. S. Goulding, D. A. Landis, and

M. Lenzlinger, Nucl; Instr. Meth. 59, p.^5. (1968)

(15) A. S. Grove, "Physics and Technology of Semiconductor

Devices", Chap. 6.
- 77 - "

(16) the sane as (9).

(17) 3. Schwartz, and K. Ro latins, "Chemical Etching of

Silicon", J. Electrochem. Soc., , 123, p. 1903f (1976)

(18) H. M. Finne, and D. L. Klein, "A Water-Amine-Complexing

Agent System for Etching Silicon", J. Electrochem. Soc.,

11^, p.965, (1967)

(19) J. B. Price, Report of Motorola Semiconductor Product

Division.

(20) R. M. Burger, and R> P. Donovan, "Fundamentals of

Silicon Integrated Device Technology", Vol. I.,

Prentice-Hall Inc., Chap. 2, (1967)

(21) H. F. Wolf, "Silicon Semiconductor Data", Pergamon Press,

p. 138.

(22) the same as (8), p.111.

(23) L. C. Shen, and J. A. Kong, "Applied Electromagnetism",

Brooks/Cole Engineering Division, p.304, (1983)

(2*4-) the same as (8), Chapter 3.

(25) T. V. Blalock, IEEE Trans. Nucl. Sci. NS-13, p. 1*57,

(1966)

(26) J. Kemmer, Nucl. Inst. and Meth., 169, p.^99, (1980)

(27) J. Kemmer, P. Burger, R. Kenck, and E. Heijne, "

"Performance and Application of Passivated Ion-Implanted

Silicon Detectors", IEEE Trans. Nucl. Sci., p.733,

(1982)

(28) W. Kern, "Apparatus for Chemical Vapor Deposition of

Oxide and Glass Film", RCA Review, Dec., p.525, (1968)

(29) N. Goldsmith, and W. Kern, "the Deposition of Vitreous

Silicon Dioxide Films from Silane", RCA Review, March,

p.153, (1967)

(30) W. A. Pliskin, and H. S. Lehman, "Structural Evaluation

of Silicon Oxide Films", J. Electrochem. Soc., Vol. 112,

p.1013, Oct., (1965)
- 78 - ;

(3D T. J. Kamins, and T. R. Cass, "Structure of Chemically

D^posited Poly Crystalline-Silicon Films", Thin Solid

Films, 16, p.1^7, (1973)
- 79 -

APPENDIX A

I. Initial cleaning:

(a) Boil wafer in mixture of H2S04/H202 (4/1) for 10 minutes.

(b) Rinse in deionized H20.

(c) Etch wafer in 10% HF for 1 minute.

(d) Rinse in deionized H20.

II. Basic cleaning:

(a) NH4OH/H202/H20 (1/1/5) at temperature 75 -v 85°C for

10 minutes.

(b) Rinse in deionized H20.

(c) HC1/H202/H20 '(1/1/5) at temperature 75 * 85°C for

10 minutes.

(d) Rinse in deionized HgO.

(e) Spin dry the wafers.
- 80 -

APPENDIX B

Molybdenum etchant:

(a) H3PO^ : 38 parts (by volume)

(b) HN03 : 15 parts

(c) CH3COOH: 30 parts

(d) H20 .: 75 parts
- 81 -

APPENDIX C

Aluminum etchant;

(a) H3P04 150 ml

(b) HN03 5 ml

(c) CH-jCOOH 5 ml

(d) H20 10 ml
- 82 -

APPENDIX D

Anisotropic silicon etchant;

(a) C6H^(OH)2 b.O m/o

(b) NH2(CH£)2NH2 46.4 m/o

(c) H20 ^9.6 m/o
- 83 -

APPENDIX

Wright etchant;

(a) HF 60 ml

(b) HN03 30 ml

(c) Cr03(5M) 30 ml

(d) Cu(N03)2 . 3H20 2 g

(e) CH^COOH 60 ml

(f) H20- 60 ml