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-TU i^i i um (.nj i i , iiMKtMiiiijwinvin'flnwPivpiiiw.iw'M am m m*m***mmmiimif>iim'*i*^> " ' utiumnniwumi tiMmmmm^^^m
AD-A017 400
DAMAGE PROFILES IN SILICON AND THEIR IMPACT ON DEVICE RELIABILITY
G. H. Schwuttke
International Business Machines Corporation
Prepared for:
Advanced Research Projects Agency
1 July 1975
DISTRIBUTED BY:
urn National Technical Information Service U. S. DEPARTMENT OF COMMERCE
o ©
DAMAGE PROFILES IN CltlCON
tind THEIR IMPACT ON DEVICE RELIABILITY
G. H. Schwuttke, Principal Investigator (914) 897-3140 International Business Machines Corporation System Products Division, East Fishkill Laboratories Hopewell Junction, New York 12533
TECHNICAL REPORT No. 6 July 1975
Contract No. DAHC15~72-C-0274 Contract Monitor: Dr. C. M. Stickley
Sponsored by Advanced Research Projects Agency ARPA Order No. 2196, Program Code No. P2D10
328162
il" JTIQN STATEMEWf ^
Approved for public ralen—| DiscihuUcn Unlimited
Reproduced by
NATIONAL TECHNICAL INFORMATION SERVICE
U S Deportment < f C >mmtrc« ngfietd VA JJIS'
Kikuchi Map of (001) Silicon. (i*ft) Computer generated. (Right) Micrograph taken with
200 keV electrons.
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* Unclassified
Security Clas^ifiration
DOCUMENT CONTROL DATA ■ R&D (Srrunly </>mi/irn(ii>n of tillr, hnjy of nhstracl and >n<ii\inf, anmilution nuvf br rnlrtnl uhm thr oirrnll HIHUI n ./(nw/i.rf/
la. nEPOIT SFCUBITY cu»ssiric «IION 1. ORIGINATING *CTIV'TV (Corp-tnttr ituthnrl
International Business Machines Corporation System Products Division, Kast Fishkill Hopcwcll Jmction, N.Y. 12533 I REPORT TITl t
DAMAGE PROFILES IN SILICON AND THEIR IMPACT ON DEVICE RELIABILITY
Unclassified '55r~ÖR0ÜP
4 DF.SCRiRTivE NOTES (Typr of rrpnrt and inrlustvr datesl
Scientific 1 January 1975 to 30 June 1975 S. AUTHCRISI i h'H none, ruddle initiai, tail nrmr)
G. H. Schwuttke
$ REPORT D»TE
1 July 1975 • O. CONTRACT OR GRANT NO.
DAHC 15-72-C-0274 i. PROJECT. TASK, WORK UNI.- NOS.
*■ 000 CLEMENT
d. 000 SUBELEMENT
7a. TOT Al ND OF PACES TTT (osr
NO. OF REF5
23 «a ORIGINATOR'S REPORT NUMBEWV
TR-22.1921
9fc. OTHER REPORT HOIS) (Any other numbtrs Olai may be assigned Inn reporl)
10. DISTRIBUTION STATEMENT
II. SUPPLEMENTARY NOTES It FPONSORINC miLI' ARY ACTIVITY
Advanced Research Projects Agency
13. ABSTRACT
Thii report summarizes investigations done during the cor :ract period of January 1, 1975, to June 30, 1975. It describes work dealing »ith improvements of advanced measurement techniques. Chapter 1 deals with the computer generation of Kikuchi patterns needed for complex structural analysis of crystal defects in silicoi. The program is applicab;« to a large variety of problems and can be used to generate KikucKi maps for different crystal structures, earh desired crystal orientation, and electron enerqy. The program can also be used to generate channeling patterns for scanning electron microscopy application. The report provides a complete set of computer gt.ierated Kikuchi maps for silicon ard 200 keV electrons. A complete program in Fortran IV using an IBM 1800 computer is also given. The second part describes the application of MOS C-V and MOS G-V measurements for the evaluation of epitaxial films on silicon or inrulator substrates. It is shown that the presence of an underlying junction requires important precautions with use of the MOS C-V measurement technique. The junction requires an increased number of components in the equivalent network, which impedes the analysis. This chapter shows how to solve the problem. Values for MOS dot diameter, layer and substrate resistivity, oxide thickness, etc. are given and refer to ranges wwere meaningful lifetime measurements can be carried out.
'
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Uiiclassified
x-ciirily (.liissi ficntion
KEY »ones
Silicon Defect analysis Kikuchi patterns Transmission electror. microscopy Epitaxial layers Lifetime measurements
lüi all i ■«■■i Mfcllll I I t an. !■ u
" ■ ' ■" •" • II -II ■ Ml IMI|I«IIP|ISI II IM
IBM Reference No. - TR 22.1921
CONTENTS PAGE
List; of Investigators
Summary
Chapter 1
Computer Generation of Kikuchi Maps to Transmission Electron Mirroscopy (TE'^) and Scanning Electron Microscopy (SEM) Investigations
by H. Kappert
Intrcduc t ion
Geoi etry
Orientation
The Program
Selection of Diffraction Planes Transition to Desired Orientation Rejection of High Index and High Orier Planes Calculation of Coordinates Determination of Radius and Center of Kikuchi
Circle Provision for an (x.y) Array for the Plot Plotting Section Plotting the Pole Map
Results
References
Appendix: Program in Fortran IV for IBM 1800 Computer
ii
lii
1
2
6
17
21
24
25
«' "'"IP i i ■■■■■i mi. mim^m0f*m*m^~^~~*™*™^^ tmrnmnfm^^immmmmm
Chapter 2
Electrical Characterization of Quasi MOS Structures on Silicon
by W. R. Fahmer, E. F. Gorey, and C. P. Schneidet
Introduction
Analysis of Equivalent Network
Spreading Resistance vs. Dot Diameter
Range of Quasi MOS Capacitance Technique
Comparison With Other Techniques
Results and Discussion
Summary and Conclusions
Refe/ences
48
50
51
52
55
LIST OF INVESTIGATORS
The project is supervised by Dr. G. H. Schwuttke, principal
Investigator. The following people contributed to the work
in this report:
Dr. W. Fahrner
Dr. H. Kappert
Dr. G. H. Schwuttke
Investigator (Visiting Scientist)
Investigator (Visiting Scientist)
Principal Investigator
Mr. E. F. Gorey
Mr. C. P. Schneider
Mr. H. Ilker
Technical Support
Technical Support
Teclinic.il Support
11
mm -- ■ ■ -■- - ■
—, 1 I 11 i II ■i <'» ■"»"«l wmmmmmwm
SUMMARY
This report summarizes work done during the contract period
of January i, 1975 to June 30, 1975. It describes research
programs dealing with improvements of advanced measurement
tacnniques. Such improvements became imperative during the
course of the contract work and are needed for subsequent
structural and electrical characterization of impact sound
stressed (ISS'ed) silicon wafers before and after oxidation
and epitaxy.
The first chapter advances the analytical capabilities of
transmission electron microscopy through the application of
computer-generated Kikuchi patterns. Kikuchi lines in
electron diffraction patterns are used for complex crystal
defect analysis basr.d on two-beam orientation of the specimen
Since our Hitachi microscope permits seeing only about 4° of
a diffraction pattern it is impossible to index Kikuchi
lines without a detailed Kikuchi map. Therefore it was
necessary to computer-generate Kikuchi plots. A program for
the generation of such plots was written. The program was
used to generate Kikuchi maps for the three main orientations
(001), (Oil) and (Ul) for Si and 200 keV electrons.
iii
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An additional benefit of this computer program is
that it is applicable to a large variety of problems.
By changing proper parameters the same program can be
used to generate maps for all kinds of crystal structures,
each desired crystal orientation, electron energy,
crystallographic order and maximum index-number of lines.
The program considers also parameters such as map-scales
and camera le-.igth of the microscope.
In addition the pr^jtrim can be used to compute and print out
channeling patterns used for crystal orientation analysis
in the scanning electron microscope. The report provides
a complete set of Kikuchi maps for silicon and 200 keV
electrons as well as the complete program to generate other
patterns.
The second part of the report describes the application of
MGS C-V measurements to the evaluation of epitaxial silicon
films on silicon or Insulator substrates. It is shown that
the presence of a semiconductor junction under the MOS struc-
ture requires certain considerations to be made if meaningful
measurements are to be obtained. The junction requires an
increased number of components in the equivalent network,
which Impedes the analysis.
iv
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9
A solution to this problem Is given. If jne side of the
Junction Is o. <dized, a q-.asl MOS structure Is obtained. The
equivalent network of such a structure is discussed and the
conditions for MOS C-V and G-V measurements are given. When
the MOS admittance can be measured the following Information
can be obtained:
1. Surface-state density, the corresponding
capture cross sections, the charge density
In the oxide, and deep energy levels.
2. The doping concentration
3. The minority carrier lifetime.
The discussion concentrates on the measurement of minority
carrier lifetime In epitaxial silicon films. Values for
layer and substrate resistivity, dot diameter, oxide thickness,
etc., are given to establish the range for this "Quasi MOS
Capacitance Technique."
"■ ■" "'■ I "I I«»ll »UJ|IM|»»IJIII*l I H l««W|l ■ .. ............. I m.ii i .... mi < •<■ <<i <<< ««^pmmDMmiPn^nMII^^HWIII |lll|
I
Chapter 1
COMPUTER GENERATION CF KIKUCHI MAPS FOR TRANSMISSION ELECTRON MICROSCOPY (TEM) AND SCANNING ELECTRON MICROSCOPY (SEM) INVESTIGATIONS
by H. Kappert
INTRODUCTION
Electron diffraction patterns of samples of very good
crystalline perfection and of a thickness such that inelastic
scattering of electrons is fairly high, show a distinct line
structure superimposed on the background intensity. This line
pattern is known as the Kikuchi pattern. A similar pattern (known
as Coates or channeling pattern) appears in the SEM when the
intensity of the backscattered electrons is recorded as an
angular dependence of the incoming beam in reference to the
sample orientation.
Several applications of the TEM technique such as Burgers vector
determination, crystal orientation determination, md indexing of
unknown spot pattern make use of such Kikuchi patterns (1-8).
For such applications it is convenien: to have Kikuchi maps which
show the geometrical configuration of the pattern and give the
indices of all lines in it.
One way to obtain such maps is to take many images in the
TEM of overlapping parts of the diffraction pattern for different
tilt angles of the specimen and assemble these images in a
composite map. Another way is to generate plots of indexed
1
J^MM^—^B" ~ . < .
p«pnpap«««nM«p< •"P" "
Kikuchi maps through a computer. The advantage of the second
way is that once a program is available it is very easy to
select any desired crystalline structure, sample orientation,
dil F2rent wavelength of electrons or even x-rays, scale of
map, or order of >xkuchi lines, just by changing some parameters
in the program.
To generate Kikuc.ii maps ver> good irstructions can be found
in the literature (U,7,9). We followed mainly the recipe
given by C. T. Young and J. L. Lytton (9).
GEOMETRY
Kikuchi lines are originated by inelastically scattered
electrons in the sp^^'men. The energy losses of these electrons
are small enough tha^ they still can be considered coherent,
but they have changed direction in reference to the primary
elecl^on beam. Therefore, in spite of the primary beam being off
Bragg angle e for a certain set of net planes, a particular
fraction of the inelastically scattered electrons makes a Bragg
reflection at these planes, e.g., (h, k, 1) in Fig. 1. This can
be regarded as a virtual incoming electron beam for
each set of net planes, which is split below the specimen into
a transmitted and a diffracted part. We find this situation
not only in the plane that corresponds to the plane of the
drawing in Fig. 1 but in all directions whenever the Bragg
- - lUiMM l
—^-^ ^mrmvmmm iniiiiiiiiiijiiqpM«]win«i*P!«nwM^^«iipp«ppiK«^ppainN*mmnM«^p^i »n Jm. WP-.JIII - — "' *m
mm
f
[hkl]
Direction of Primary Beam
Direction of Inelastic Scattered Electrons for Bragg - Reflection On
inelastic Scattered Electrons
Reflecting (h,k,il) Plane
( h k £) Plane
(hk£) Line (hk£) Line TEM Screen
Fig. 1. Geometry for formation of Kikuchi lines.
Kikuchi Patterns 3
BBC
condition la satisfied for the (h, k, 1) plar.M. Therefore
all the directions of the particular fraction of elastically
scattered electrons in Bragg conditions for a certain set
of net planes (h, k, 1) are represented by a cone (Fig. 2),
wh^re the cone axis is in the direction of the plane normal
(h, k, 1) and the half opening angle is 9Oo-0 'Kossel cone).
The same geometry can be used for channeling patterns ir the
SEM. In the SFM the primary beam itself has to be tilxed along
a cone surface to get a signal of the backscattered electrons
with less • more intensity than tor 1 ^e background intensity
that appears as dark or bright lines on the TV display.
The real Kikuchi line patte n is th( intersection of all
the cones produced by the different sets of net planes, with the
image plane in the TEM below the specimen. The real channeling
pattern is the intensity modulation of the backscattered
electrons above the sample, as seen by the collector and
displayed on the TV screen, dependent on the tilt angle of
the primary beam in reference to the sample orientation.
The usual way to generate Kikuchi and channeling maps is
to make a stereographic projection of all the cones, which are
thus represented as circles on the projection sphere (Fig. 2).
This may be done because the projection keeps the angles
between intersecting lines or circles on the sphere constant,
results in only small distortions of the distances especially
(h,kf£]
Direction of Inelastic Scattered Electrons in P'agg Condition for the (h,k.£)0<ane \
Primary Beam
Reflecting Plane
ih.k.i) K,ne
Projection Sphere
Intersecting Line of Kossel Cone and Projection Sphere
Fig. 2. Geometry of Kossel cones and th&ir intersection with the projection sphere.
Kikuchi Patterns 5
i ii i—i
ms
close to the center pole , and has the advantage .hat
standard stereographic pole maps can be used for indexing
purposes. The stereographic projection of the intersecting
lines of the diffraction cones with the projection sphere
appears as circles in the projection plane, which are called
Kikuchi circles. The radius and center of each Kikuchi
circle has to be calculated. Subsequently the part of the
circle that is within the plotting area hat to be determined
and to be drawn.
ORIENTATION
Standard stereographic projections are made from G to 0 and
are observed from the top, as shown in Fig. 3. The channeling
patterns in the SEM are observed in the same way. To find
the correct orientation between standard stereographic pole
maps, channeling map and channeling pattern on the screen,
we have only to consider which lenses are used to focus the
primary beam onto the sample surface and how the tilt angle
is related to the TV display.
The G to 0 direction of the standard stereographic projection,
as shown in Fig. 3, is just the opposite to the direction
in which the electrons go in the TEM to form the Kikuchi
pattern on the screen. One gets around this orientation problem
^ „_.._,..
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73
u 3
i c (0 M 3
1 8 « 8 s p
I
CO
iZ
Kikuchi Patterns 7
1MB
by turning the projection upside down (1). Therefore
standard stereographic projections have to be inverted
for indexing purposes. Another possibility s to use the
standard stereographic projection without inversion. This
requires an adjustment of the Kikuchi maps such that the indexed
reflections in the diffraction pattern correspond to the planes in
the crystal. From the geometry shown in Fig. 2, one can see that the
cone representing the directions of diffracted electrons
intersects the projection sphere at the lower hemisphere as
well as at the upper hemisphere. A downward projection—that
is, the unconventional projection from 0 to G (Fie. 3)--
would result in the real Kikuchi map as seen on the
TEM screen.
An upward projection—that is, the conventional stereographic
projection from G to 0 (Fig. 3)--results in the same
map, except that this one is rotated by 180° in reference to the
real Kikuchi pattern on the TEM screen. To do it this way
was recently proposed by Head et al. (10)• We decided to use
this method because a rotation of 180° is more convenient than a
mirror ii.version, and we can use standard stereographic
projections as in x-ray studies.
The correct orientation between the sample, the TEM
micrograph, the diffraction pattern, standard pole map
and Kikuchi map is found as follows:
» 6 ^ ^^ A«^ <>% * ^ja
^ ;«> '^^ # w '*^#
Fig. 4. (001) map for Si 200 keV, xo = yo = 11cm. R = 11cm, RMN = 2. SMSQ = 5.
Kikuchi Patterns 9
aamam^a
■■■■■■■■■■■■■■iHMHMH
#....«. & ^
Reproduced from best available copy. ®
Fig. 5. (001) map for Si 200 keV. xc = yo = 20cm, R ■ 32cm, RMN = 4, SMSQ = 9.
s$fi #
V ß i HI ^ .1«
Fig. 6. (011) map for Si 200 keV. xo - yo - 12cm. R - 11cm. RMN « 4. SMSQ » 5.
Kikuchi Patterns 11
u
r^ ^H ^ f B^
Fig. 7. (Ill) map for Si 200 keV, xo = yo = 12cm. R = 11cm, RMN = 2. SMSQ - 5.
12
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a* e sn
Reproduced from best avdiiable copy.
Fig. 8. (001) map for Si 200 keV, xo = yo = 12cm, R = 32cm, RMN ■ 4, SMSQ = 9.
Kikuchi Patterns 13
mmmmm
i
^ ^^ ^ -
Fig. 9. (001) map for Si 30 keV, xo = yo = 12cm, R = 32cm, RMN = 4, SMSQ = 9.
14
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Fig. 10. (011) map for Si 200 keV, xo = yo = 12cm, R = 32cm. RMN = 4, SMSQ = 9.
Kikuchi Patterns 15
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Fig. 11. (111) map for Si 200 keV. xo = yo = 12cm, R = 32cm. RMN « 4. SMSQ - 9.
4
16
- - - - ■
1. Consider the rotation between micrograph and
diffraction pattern that depends on the number of
lenses used and their excitation.
2. Orient the Kikuchi plot ■'"he same way as the TEM
Kikuchi pattern and take negative val les of the
pole indices as plotted in the map.
3. Rotate a standard stereographic pole map with the same center
pole as the Kikuchi plot by 180° in reference to the pole
map plotted within the Kikuchi map.
To avoid any other inversion or rotation we have to use both
negatives and prints of the micrograph as recorded in the
TEM on the screen.
THE PROGRAM
The program is written in Fortran IV for an IBM 18Q0 computer.
The time used to generate one plot depends on the number of
lines that have to be plotted. For example, the plot shown in
Fig. M takes less time than the plot shown in Fig. 6, ^ "lere
higher order and higher index lines appear. The time for the
plot in Fig. 6 was about 50 min. The attached program was
used for this plot. Figures 6-11 show different plots
generated with the same program when parameters used are as
described in each figure caption.
Kikuchi Patterns 17
MM m^^mm
I""" ipwgM^H.ii ipippMRi«np< ji . jyiiiiimiiiniutiiiiM'Uiiiii i warn p •^^I^PP^OT^^VB«^
Steps of the program:
1- Selection of diffraction planes
The usual structure factor equation:
n F = Z f. exp C2W (u. h + u. k + w. 1]
j=l - J D 3 (1)
is used to calculate the structure factors for silicon so
that the planes can be selected which give the Bragg
reflections. For this calculation the coordinates u, v, w of
the position of the 8 atoms in the Si unit cell have to be put
into (1). The Bragg equation is used in the form
e hkl " 2a ^-"fh2 i k2 + l2'= 0.0023 l^h2 ♦ k2 ♦ I2' (2)
for 200 keV electrons and Si samples to calculate the
Bragg angles ekhl for each plane.
2. Transition to desired orientation with coordinates
X31, X32. X33
The coordinate transformation
XH XU XL
Vfi ki'1*i V^i h2//s^ k2/ys^ i2/ys7
hj/fij *3/iir3 i3/^/
XHS XUS XLS /
CO
18
mfßjiti um iin*miMmniwmv i * i niipavw i i >■ ■iiiuuifiiii 11 ■ ■ui^v^^i^miMiniOT^nMHpnPM"«'1^1"!^*^ ^"■WV««P^W«I
is used where (h3, k3, 13) = (X31, X32, X33). The other
vectors (h1, k^, 1-) and (h2, k2, 12) have to be chosen in
accordance with a right-handed Cartesian system.
si = hi + ki + lr
3. Rejection of high-index and high-order planes
Planes of high-index numbers larger than a maximum number
y2 2 2" h + k + 1 have to be rejected; the same is true for
planes of higher order than maximum order RMN.
■+. CalculdLJon of coordinates in the projection plane
The coordinates of one intersection point of the diffraction
cone with the projection sphere have to be calculated. For
this purpose spherical polar coordinates are introduced. Then
the calculation of the coordinates within the projection plane
has to be performed by the equation
Px = 2R tan y/2 (p '2 + p '2 1/2
x y
P = 2R tan y/2 (p '2 + p '.1/2
y y
(4)
where Y = cos (P /R) , as can be seen from Fig. 3
Kikuchi Patterns 19
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UPPPlPi^ um wmmimm^^Bmqm1* "mm&vmmmmmm* ■ »wm'mmimmmmm* ■in11 »I■■■«
5. Determination of the radius and center of the
Kikuchi circle
From Fig. 3 it can be deduced that the radius of the
Kikuchi circle is
CA = 1/2 (BO + OA)
and the coordinates of the circle center C are
(5)
CX = Px (CO/PO)
CY ■ P (C^/PÖ)
&. Provision for an (x,y) array for the plot
Only a small part of the Kikuchi circle in an area with
radius RI=SQRT (XO x XO + YO x YO) around the center pole
has to be plotted. Therefore it has to be checked whether a
particular Kikuchi circle has a part of it insice this
area. Then the circle equation
[X - CX]2 + CY - CY]2 = R2 = [CA] (6)
is used to provide an array of 50 (x,y) pairs for the plotter.
7. Plotting section
A final check is made to find out which values of the array
are inside the plotting square given by XO and YO. The
20
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.. U.W, ^p.. . . . muwrnimm« < i >■■■ ,. IWPP'P'«»'*1 '" ■ l"' «i.mv'^mmmmmm***' ^-—,
slope of the lines near the border of this square is determined
in order to plot the indices of this Kikuchi line pair at
the end of the line with the same slope.
8. Plotting the pole map
An extra program was written to produce pole maps of the
same scale as the Kikuchi map. The steps are essentially similar
to steps 2, 3, and 4 above.
RESULTS
The size of each plot is about 24 x 24 cm2, which is the maximum
usable size of the plotter used. The numbers inside the map
represent the indices of the pole where the location is indicated
by a +. The numbers outside the map represent the indices of a
certain Kikuchi line pair. The indices are plotted at one end of the
respective Kikuchi line with the same slope as the lines
at this end. Figures 4, b, 6, and 7 represent (001), (Oil), and (111)
Kikuchi maps with the same scale for Si with 200 keV electrons.
Figures 8, 10, and 11 again represent <001)1 (011), and (111)
Kikuchi maps for Si with 200 keV electrons but for about three
times larger scale and with higher-order and higher-index lines.
Figure 5 is a larger scale plot of the lower-right quadrant of
Fig. 4; and Fig. 9 is the same plot as Fig. 8 but with 30 keV
electrons and can be used as a map for channeling pattern
for the Sli. Figure 12 gives several examples of Kikuchi poles
as shown on the maps and in the TEM.
Kikuchi Patterns 21
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wmmmmm**^i^m ^^^^m^m^^mm*~- u\ ..■ ■ . ■will mmmmmmmmmm
ACKNOWLEDGMENT
The author wishes to thank R. Anderson and G. Das for helpful
discussions
REFERENCES
1. P. B. Hirsch et al., Electron Microscopy of Thin Crystals,
Butterworths, 1965.
2. S. Amelinckx et al,, Modern Diffraction and Imaging
Techniques in Material Science, North Holland, 1970.
3. L. E. Murr, Electron Optical Applications in Materials
Science, McGraw Hill, 1970.
4. E. Levine et al., J. Appl. Phys. 32» 21ni (1966).
5. P. R. Okamoto et al., J. Appl. Phys. 38^, 289 (1967).
6. M. V. Heimendahl, Phys. Stat. Sol. (a) 5, 137 (1971).
7. J. C. Bomback and L. E. Thomas, J. Appl. Cryst. U, 356 (1971)
8. W. K. Wu and J. Washburn, J. Appl. Phys. ij_5, 1085 (1974).
9. C. T. Young and J. L. Lytton, J. Appl. Phys. i4_3, 1408 (1972).
10. A. K. Head et al.. Computed Electron Micrographs and
Defect Identification, North Holland, 1973.
24
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APPENDIX-FORTRAN IV PROGRAM
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< ct •(D X a: ~~ IVJ C n a o 0i l- » — (M X -> a. 1^ < 3 • UJ a. CM -J X ~i o •■ ̂ ■ < U ~ I--
UJ a 13 — a CO ^) tt • a • k a a a s CD X < •• » u. -J •-I
IM Cl o 'U •■ o » —» u. UJ ■M -0 o K » a k *• •i z PM II C <I UL • >- O •* 0* LU < c X 2 m UJ — ^ m t- CO u. IL X » --« «4 ^^ H •— a v^ •—• u. — r<~ u _J ^4
J a o at _ III O ~ O- H .• «a OJ \s\ • >- C H I '1 n II U1 O ii — o 1^. lU ^ —1 UJ a. UJ • i>n <J II ^-« UJ «: ►- »- a — v »-<M a IM u\ <r> (- J X -J • I n u ♦ 3 C ► 3 3 3 a M O X t— Of ü Uj L/t •. >- ■■ h- n: u. w. ►- !/! * • or P^ »4 pg UJ o e c u; + N 2 ." M i X , • n ii 1/1
o » r- H- u O 2 — t— o < O < 3 (\j IM a UJ » i •l 1 a- 1 »— • • tl a rj —' u. — 3 * >- _i -J o *o a C UJ UJ o < — Q r t- C X '-< ~* w. y •-■ z rv V m —1 <I e a 1- ir • iL r - O X i- h- i^ < ■: _l IM
a ct _J a ►- -J O i- 1 a _J < oc 7 ■ < a c. 1 II N H ■ ■ ^ H it UJ « 1! il ^ Of — ^ ^ 2 o i - -1 Q. < a. UJ UJ -J a 2 — - C UJ O C UJC «i e O — c c —• c ^1 C — 3 '-,
IM I c M --' fM C u 14 o o a l/l i/i < o O w u. ■ < C — Q X e da. u. o C a u. a. K >- or o a x a x B -1 o U. U. 1- c « LL u. u u. oo (_) Ll O X >- o o
(O IK <^. X 2 •-< < 4 n CL -1
o o »<n in ;• 1- v- UJ 1"- o UJ ec ~4 1 4 o o> » 0> -> u. o o «: ■— VI LU a: ^ ■ ac o m m IM ^
B ^3 rir cc •—• z •s. "x •" • •
< • _J •
a • L> OO o o OO o o o uoo o oo
e • o o • oeoe€«»o ■ "■ ■ ' ""' - Kikuchi Patterns 25
BMB
w.^i JIWPWPMMPÜM ■■niipuiw*mpm(np|ipRV^^«nMnnp> ■i> IP -•«j.ia^iiiiwuiiuK.n mill i
O (M r» o
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0>< O^fl 0><IMOC o o I -. C I — O I — - I O Z II Xiim*ii-r_iiiu.u.>/i < >- II < II < ii «3 on — ii uj MMOMMCMMMLLO O I X XO**O_J-JU.MV0
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cc a
CJ M M M I X X _l z o « « « c "1 Of OC QC M — II II II I-
iT l/l l/> M U. I * -I «^ M X X X z
< O M ac
i*i (*\ (*> M |\( |*1 XXX • • • C*l 1*1 1*1 M CM (*1 XXX ■♦ ♦ ♦ NMM Mfg (»i XXX f • 1 <M rg uj M CM (*» XXX • ♦ ♦ r* i-l *m M fS. f«l XXX • * • M M M MfMf*lMMMrsJf\.f\jl*»(*l xxxi/^i/ii/ii/ii/'-i/ii/ivn — — — •vV'v.'V'v'N.'vv
• ►-»-►-M(\;r*iMf\,(*iM,M • • • • »M • • •ofa'af^^—^-^fMfMfvjf^m
MOOCM i OMMOOCXXXXXXXX II II II !l II M II II II 1/? 1^0 Vi || II II II It II n l| MtNjm'-«fNif*,Mf\-fn ii ii ii Mrs]rTiMf\,r^'-4<NJ MMMrvjr\>(Mrrir«^rr>Mrgf<iMMMr\jr\jrv,(<i(n XXXXXXXXXt<01/>l/)<<<I4<<<I<
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Kikuchi Patterns 27 "^^
J piiuiiiiiiii luiiiiiivvipp^paiiwppmpaMMwpp^aMwpii^
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c o w O
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O m _) ^ o in C O »* ~4 (V (\i u> vr» m u^ m m oooooo 3 3 3 3 3 D ^1 Ü Ü i- ^ ^
O m o .< o i/> Jf^ l*> -< ^ l/\ (^ m iT1 »n in IT u"i OOOOOO =33333 * ^. ^ ^ ^ ^
O m o ir\ o u^ < 'O ^ ^- oo <o tn in m m trv ip oooooo 3 3 3 3 3 3
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■_, *■ xj •- < « — <J c -< + < 8 • • M l « «3 -It « N + < <~ n ,y r- >o — — « 1- — (NJ • x *- «- O l^ o — O i« i. J
a M >- ? i\) DC 2 PI P ^ to « » « t- z to » It i> ■• *- _j m or < C < — ♦ II •— O cc ac ec II ~ O QC n: a: I •—• o ä K o; -: x • a t> •— a l^ 1- I I rg i/i U II II II « m
O D w — —* i- to < i- !<»-►- »< H ii rg rg rg rg n II — -• • o n 11 11 11 — 11 O M (V. -i « -« O ^* •^ rsj CM i\i o ^- r-< « ^ ^.
II o ii U. X CJ rx J. c rg X LL I a — -> I ^ _j a rg rg X ü Of .-« »H I ^ J < (/) O X X H o 1/1 o U l/> ■I ►- O < (- O < »- ■- 1- < U) U X X X < i/) U X X X
c _ tf rg *\ in « * * >0 m <o •c ■c o ■0 INI M M fvj Tt rg rg rg
! O r~ rg rg m rg CM rg rg • O O rg rg
^ oc c£ tn */) rg I « <l V * rg I — — O «rt o P C M • • 1 n — O cc: of o:
rg i/) u II ii n rg ii II rg rg rj O rg rg CM rg CM a rg rg X if _J < «1 u X x x
j 2
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rg r>- z • rg < ♦ ► t- — O < rg <-• | »« r- m _< rg a- x . m
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M ♦ ** rg * *" •I « ffe IM ♦ z oc OJ ** «-• z — *— •-• X «—1 — rg JL CM X #-* • rg < rg • ^4 <• «-H • rg < rg o CO ^4 z p4 < O I — < O I -< rg < O I oc < 3 X 1 ll> ■■ — X 1 0> — <v X 1 CD — X o ^-< •
rg ~ ■■ rgr- z U) ~- rar- z I« •v rg rg 2 M o ,;
t- * «M < • O ^- • rg < • o 1- » - «i • o t- o x X cc <
^ oc .*• ■ t- (M o oc « •>- rg l_> a « o »- rg 1_) a. r- • , • • • o wo
— O < »» •v o — o < v. ^ o — — < >v «^o o < rg f\i ^* ^ 1 ■■ -* %/i — a> i MM —* uO rg eo 1 0* ■M IS, z ^, -^ v.. ^ ^ I •^ CNJ r* m ~* -< V «(^ m ^- •—• -v rg rg m (M (M ^ < K (-- r\i -J rvj o- (M oc rg M j rg c f-< CD —t $m _i »a. IM cc INJ rg cc ><■ > ty ■X- ■>- —» •-* x » in < <? t (Nl x »in < c~ X. ** x — m < CT- I (M IM un a ^^ o r\j «v. m r (M IM S >!■ ^^ *- HI < z - -« >. c -• — ro < Z — —• ^ cc -- r^ < r IM (M 2 X > X V CE c: (C a: «o + ♦ cr < -* —* a |i -J z in O < rg rg a r- * ^ m c; < — •-< a rg -t z IT c «.: U. CM BH -< —• rg c\. n + -*• ^ N u. u. — <M *~* < CD "- *- u. a — rj ^ vl cr. — t- it u. ^ ^^ < M i- U. Q < < < <1 IT X > n w %. ^t • • V ■i • ^5 •- >C r- z « ti » . «i • c t- o r- z v • V — ^. . m ►- m i^ i' » CM <)• •c I i ; 1 • • (Si r\j » 1
cc og rg t- -~ m r- < f^- • "■ o: -• — I- — c. co < ex: • »— OC — —4 K l\i f-i OO «J CD • nc IM r\j a: X > X > c < < o •3 -t iTi ft ^H r* OC P< — CM IM o tsi a <si r\ a -< — rvi (XJ c V If »— r-4 OC rg w CM rg C i^n i< X ex ^ ~* (M fM II II • X St C r\j N 11 N H • T 5^ O -< II 11 it II • x >C C _l II II II H t M X ai a: cc cc <c 11 n II 11 <M X X (^ _) —' O —' c —• •-< rg x x U) _J -4 a —< o t-H ^4 CM X X i/1 X rg O CM a fM CM N X K- II II -i II .-« PvJ ~ — UJ It ii II II X (NJ l- fM ►- tM (M II II II II x ^ h- r-< — ^^ r-« II II | II — <M 1- rj H rg M II II II <l " — CM fM w ^ .^ rg r\j >— rj K >- .—' w i 1 ■ Z — X > — —I I 1 Z ■" X > iM s: s. > r\j X > _i X >- X >- (_) ^j _ o •— -v •—• I-H *** ^-» (Nl U_ < c <3 c <J «1 rg ~ r* — a < C? 1 c 4 < •-< nj rg rg u. < c < B i < IM rg (M 3 tn tn tr. (T ^ X > X > OC U. « « < >~ 130000HU.«0<D «J — C C: || c O •- u. < ■a < — o o ooo ►- u. CD X o < < < < a o u
«r ir OvO a) a> o o .-» rg o * 4 f- i^ «r r- f^ r- CO CD CO oc -< » O rg rg r-fvj rg (M r- rg rg rg 00 rg
rg rg rg o , O c; OOO o o .
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"~ Kikuchi Pattern i 29
■H
mmmm mm.m ■»W^^flW JIHI jiii|i«iiMipi «m^KIH^nr-'aiiiw
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OOOOOOCCOOOOOOOOOOOCOOOOOOCOOOOOOOO-^.-«!-!^»* 333333333333333333333333333 = 333333333333
— cc
X —
• - — X m o H — v * rg •M U X * • >t u • » (X </> —
o 1 w -^ «M
K " 3 I — »- >- > K > OC V o ^^ o >. 1 * X • —
— X > # > - ai — u / *— ** 3 — I/I ♦ ♦ o > • ~ rg
oc O — I • • 5 !£ 5 ♦ — o • • CO S CD Ä o — w 1/) -■ ph o —i m »* C\J — > 1 -* o -^ >»■ to 7 m • K _ o •- n r<j ■-• * CO ai
rsi o X * oc V ■I- — » rg <-- rg
LL a. o — »
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1-4
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* o + m X X gri ■»• M p in ^1 -* — ** rg •^ HI {*> ^^ .-« x o 1 » • X X f4 f* ->■ in m to i^ in i/i m in to XSI
f\J X + — •—' o i- »-< >-4 -^ f\J ** o -— -r • » * • •-* •^ • »■H • » rg rg
z 0
o — — >J- in »-• (_i _l ^-< X X V rn «* >»• o a o •e x X m X c ^ X X
r^ + — >- I -1- LU «« K a: 1 1 1- * 1- i >t »-< -< v >« (Si S IÜ -n >x V r~t UJ U0 »-• N. > f— —- cc QC M — ■^ m in i/i to IT £ a t^ in tfl IA to
m *- (Nj ac x 4 c • c a I — ^* a a o — O •— (\j » ^ ^ «« ^» ^ » I-I •-« * c- a '■i * * rj rg
» o • * o —* ^J■ i^ 9 a. O Hi •-• it « \r. •■ pri 1/0 ■■ c > c- ^- -^ ^- ►- m i^ > >- o f^ rr* >- IT r- > > X tu
a' '^ ^ H i^ v _-i «-« 1 *~* C | ru « i J «v — — o t-* «^ *- rv. c N ^ rg n- ** o w
II ^^ ^^ X X QC or M 1- • M i- o c ^* ^ X X > > m m z <C Z n u- t~ a: a •-< c z S\ u> z m z oc P4 X (M O p^ 1- u i-' M « c M — 1 LT > o O l u < n. -u rg ir. ru rg r- rg < •J ^ •c
» w0
l/N X —' T Ü 01
X • ct • i »- i- ^ t-. 1 i* L/l < 4 ■w — l 1 1 1 -» *- t— in ►- in -^ • IT. • IT • m i- •^ — h- m H »— o o c HI O X I cr or u n < M o CC r\j rNj 1/1 > AJ .-> rg .. CM t/1 I/I < < w. •-* c ^ < lO ^0 •I < < z c • O II
11 II •-* \S) II ^ II )■ i/: o u a O B < —• V at ». i- >- a CO ^ >- X x >- V "« •■H II o n O ^4 n C 11 C II O II nj TM 11 a II
1 s o u.
t/> <Ni ^ < ii 1/1 01 i/) on QC II t— 1 QC II — 1 i- --• •* 1 II n n II X >- •« ►- «N >- >• •"• h- "^ h- w4 ►- P< M >• rg (- ra
o ^ o UO QL Wl
T x _j I/) II II H II N « — II II < n ii 11 i/i */) m to ~- w U. U. *— u. Li. U- Ii *- ** u- u.
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LL i l .i (_' 3 o ^t rj M I\J —^ M u. -^ (Nj rg u. rg O > x f\J #^ IV^ ■-< r\- u. U. -J C -1 c tL ■J Q -1 C «J O -J a. U- _1 q -i
M — 00 O Ml X X >- > 1- (■ — >- K 1- »-• V o oc M V X X >- >- — — < o < c •""■ •- U •C O < o « w •" < 3 <
00
1 ■0
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«r « >r ^ ■* -t ■»* in m m u-. in m i^ in in ir. m m
oou o u o
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o o o o v4 <VJ »'I ^ IM 1\; <SJ (M tn m i/x m 3 3 3 3 X * * *
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m IT» in tri m in .ri in if» (f\ 3333 3 33333 x i ^ .- »: ;£ x J£ x X
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# • • • Kikuchi Patterns 35 •
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Kikuchi Patterns 37
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Chapter 2
ELECTRICAL CHARACTERIZATION OF QUASI MOS STRUCTURES ON SILICON
by
W. R. Fahrner, E. F. Gorey, and C. P. Schneider
INTRODUCTION
Junctions are present In Important devices, e.g., In burled-
channel charge-coupled devices (CCDs) or In silicon on sapphire
(SOS)-based Integrated circuits. In order to optimize the
performance of such a device—speed, transfer efficiency, etc.--
the characterization of the electrical properties of the material
Is desirable. For homogeneous silicon, there are some standard
techniques based on metal oxide semiconductor (MOS) capacitance
measurements for this purpose. The presence of underlying
Junctions requires Important precautions to be made with the
us« of MOS-CV measurement techniques.
Owing to the Junction, the number of components of the equivalent
network Increases and the analysis Is Impeded. It will be
shown In this paper how this problem can be solved. When one
side of a Junction described above Is oxidized, a quasi MOS
structure results. The equivalent network of such a structure
Is discussed In this paper, and the conditions for MOS C-V
and G-V measurements are given. In the case where the MOS
admittance can be measured, the following Information ctn be
ob talned :
38
MM ■ ,,.-..
-"""-"-■ "■■ mammimmmmimmmui nwmmtwn mimMtM* iiiiBimii i i i mi ym •
!
1. The surface-state density N (1), the corresponding s s
capture cross sections (2), the charge density in the
oxide (3), and deep energy levels (4).
2. The doping concentration ND.
3. The lifetime of the minority carriers (5).
Measurements on epitaxial Junctions according to (3) have
already been carried out (6) without an adequate the retical
basis.
In this investigation, we are primarily interested in the
measurement of lifetime, since this parameter is more sensitive
to defects and impurities in the crystal than any other '.lectrical
parameter (7). Thus, in the section "Range of Quasi MO 3
Capacitance Technique," the values for layer and substrate
resistivity, dot diameter, oxide thickness, etc., refer to
ranges in which such lifetime measurements can be carried out.
ANALYSIS OF EQUIVALENT NETWORK
In Fig. 1, a cross section of an oxidized n epitaxial layer on
a p' substrate is shown. The network of the structure consists
of the oxide capacitance C , the distributed layer resistance
R of the n-layer, the distributed Junction admittance
Y » G + Jw CT, and the distributed substrate resistance Rs. J J J
2 A net consisting of buried layers each of 2.5 times 2.5 mm
area with a spacing of 8 mm is diffused into the substrate
prior to epitaxy. The doping profiles underneath A (off the
Epi Characterization 39
iiin iipiiip^^iwiiiBiMPia^iFwi^Mlpnmnn «. ■ i —^^r^^mfri^mitmmw^i^i^mm^m^mr'f'^w^r*^'^7mmmi^^r^^imK^ >■> ' ■■■.upww^«»»
m nTimTurnrii]
B
S.02
n - Epi .31Acm
n +0.001 XX cm
[4 0.2S mm J
P - Substrata
ICiicm
1 1200 &
T T 4.6 inn
i.
300^m
i
Fig. 1. Cross section of a n-p- junction. The n layer waa grown by regular gas phase epitaxy. It is covered by 1200 J? thermal oxide.
40
- -^ - ■
mm" ■ i. \uHimmmiim^ mtmw^mff ■ " ■■»— •—> ^»W^^P^MPPPP^WWIfV^W—■
buried layer) and B (on the buried layer) are obtained by
the spreading resistance technique (Fig. 2). The C-V curves
for two dots off the buried layer are shown in Fig. 3. The
2 dots have areas of 0.01765 and 0.00196 cm . The same
measurements are repeated for a <115> and a <100> oriented
wafer with epitaxial layer resistivity of approximately
0.9 ohm-cm and of approximately 4-ym epitaxial thickness. The
substrate resistivity is 8 and 6 ohm-cm respectively (Fig. 4a b).
In these wafers, no buried layer is present.
The analysis of the data given in Fig. 3 reveals that the
measured low-frequency capacitance in accumulation is identical
with the oxide capacitance. This is confirmed through measure-
ments on control wafers, by measuring the oxide thickness and
the capacitances for different dot areas and comparing the
ratios with the area ratios.
The dc voltage drop (v ) across the junction is 0 since there
is no dc current flow through the MOS structure.
The ac admittance is given by Gj (Vj) + juCj (Vj) with Vj = 0.
G can be derived as Gj - dlj/dVj " I0 x (q/kT) x exp (qVj/kT).
In anticipation of the results, the lifetime of the epitaxial
layer (which is heavily doped compared with the substrate and
can be expected to give the major contribution to the dark
current) is measured now to be approximately 1 psec. With this
Epi Characterization 41
pjiJULiiiimu ll|ill.JJi<,r~^K>MinmMmP ■•"■
^94
N0.NA
lern"3)
1018-
lO".
Epi-Region
1016-
I —aq^aaattu
1015
Distance from Interface
Si02-ln)Si
T 2
T" 3 pm
Fig. 2. Doping profiles underneath the dots A (triangles) and B (circles) in Fig. 1.
42
.^■.aMMMan^M_
■ qillll.lMHIIII . 1 II.IW II.IPHII pi i Uiimni NIJIIIW*II»I Mi' MI iiniiii. ..in HI inMJiiiuawiiigi imnim mm' '
Slow Ramp % 0.002 Hz
0.01765 cm2
dot
1MHz
0.Ü0196 cm2 dot
-8 -4
532 pF
R = 159a
T T 0 4
VOLTS
415 pF
58.5 pF
T 8
C [pF]
C
[pFl
Fig. 3. C-V curves for two dots off the buried layer (A in Fig. 1). Note that for thr smaller dot no dispersion is seen.
Epi Characterization 43
(■■^BB:
w^MapnvmvMip mmmmtmmmrmimmm^iim^HWmmm'mm^ .ILM, ijai^aHMiWMippipnpmM^pvvMpipiwvpK^E/* aiawiiiwRivg^wfqM
i 536pF ■f
0 01765 cni2 dot
0.001961...2
dot
-4
220a
C
IpFj
345 pF
C
[pFl
1 0
VOLTS
r 4
T 8
Fig. 4a. The same C-V curves as in Fig. 3 for a (llö) oriented substrate wafer. The epitaxial resistivity is three times larger than that in the samples characterized in Fig. 3.
44
■ - _
pn mi m>™*m^mii^immmtmiW> mmmmm~ ■~*^m^^^*mm^m*^^m i wimmm*~~*mmm*m^*^*mr
0.01765 cm2
dot
0.00196 cm2 dot
1 536 pF
323 pF
759 a
-T 0
VOLTS
c [pF)
C
IpF]
Fig. 4b. The same C-V curves as in Fig. 3 for a lOO) oriented substrate wafer. The epitaxial resistivity is three vimes larger than that in the samples characterized in Fig. 3.
Epi Characterization 45
KI^HHHaM
' ^ ' " ' " ''" »F—— '- " ■ "■■ I.HIPI i- . i. I.I.....I -—— u ......i ....... ii..m in . iiuiiKiMjwpmiLiiiiijii «•<
value and the given wafer data, one obtains I 'v 10 A, and o
— 8 thus GI 4 x 10 exp (qv^kT) mhos. The capacitance
Cj is ^ 20 nf at 0 Volt and CüCJ ^ 10 mhos for a frequency
_3 of 2 x 10 Hz. (I and CT have been calculated by means of o J '
Shockley's equation I = qA (p vD /T + n VD /T ) and the 0 NDP- NA 1/2 depletion approximation CT - eA (-s^— . — ) . v,. is
D D A the diffusion potential. The other symbols have the usual
I
meaning.) The ac admittance is resistance-controlled up to
% 1 Hz. Even in range f > 1 Hz, there is no Influence of the
junction admittance, since uiC T >> wC . These considerations J ox
explain why a voltage-independent accumulation capacitance is
measured over the usual frequency and bias range. The validity
of the assumption on G (v.), C (v ), and the numeric values
are found in good agreement with C (Vj) and differentiated I (vT)
curves measured after oxide removal and ultrasonic cutting
(e.g., C - 35 and 40 nf for dots cut at A and B, respectively).
The bulk resistance R of the substrate can be neglected as
long as the resistivity is not too large. Experimentally, the
upper limit was found to be approximately 50-100 ohm-cm and is
theoretically given by the comparison of the RC time constant
with the measurement frequency of 1 MHz.
From the data given in Fig. 3 (i.e.: C - 532 pS
C„_ (ace). ■ 415 pf), the lumped series resistance can be HF
determined:
46
aaaa^-_B
■ l«)«»»«»««»!!! .1 ■ II Uli ■ I »««IM I • - UHHipi luiwi» 11 tßm^uy^iTijr^m—--*^ ■ i m> •^n^mmmi
2 2 CHF (ace) - Cox/(l + B*T*) [1]
RC ox [2]
This gives R - 159 ohms. For the case of a buried layer
underneath the dot, this value is reduced to R - 88 ohms.
(Note that in Fig. 2 the buried layer extends into the
epitaxial layer due to outdiffusion.) When the dot area is
2 reduced to 0.00196 cm and the capacitance to 58 pf, no
dispersion is observed in accumulation up to 1 MHz. With
the same values of R, one obtains now (WT) - (27r.l06'R'C )2
ox _3 _3
3.4 x 10 and 1 x 10 , respecti- ely; C can be measured
with 1 MHz.
Since the time constant of the surface states is usually
observed in the kHz range, G(V) and r u) measurements (2) are
also feasible. (A G(V) and a G((JO) curve is a plot of the total
conductance G vs voltage or frequency at a fixed
frequency or voltage respectively.) This is roughly checked
by measuring G(V) in the kHz range. The typical peak due to
surface states and its shift with frequency is observed.
From Fig. A, one obtains R values of 220 ohms and 240 ohms for
,15 15 -3 N - 5 x 10 and 4.8 x 10 cm , respectively.
Epi Characterization 47
WWP..UH111II * i^jupRiMpi \mmfmmmmmim'''^^mm
SPREADING RESISTANCE VS DOT DIAMETER
For frequencies co < 1/RC , the equlootentlal lines of the n — ox ^ r
structure are schematically drawn In Fig. 5a. Therefore, one
can assume a disk-like volume representing the epitaxial layer,
In which the current flows (Fig. 5b). The voltage v Is
applied at the Inner area 2I\T »d ., d . being the epitaxial o «pl epl
layer thickness; the outer area at r Is grounded. A voltage
dv drops across a volume Increment 2 r ^ dr • d .: r epi
dv - -1 dR o
(1 Is the current caused by v ).
dR Is defined by
dR - p-dr/A(r) - p • dr / (2-TT • r • de ^
dv - -i •D*ir/(2*v*r*4 ,) o epl
v - - (i 'P^-TT-d .) In (r/r.). o *pi i
Since v(r ) " v : o o
(vo/lo) - R - - (p/2 d ) in (r^r^ [3]
As a result, one obtains an Increase of the spreading
resistance for smaller dot diameters. For the epitaxial wafers
of Figs. 2 and 3, we are not able to observe this change, first
because the spreading resistance Is too small, second, because
the existence of the junction and oxide capacitance heavily
Interferes with the assumption of a simple radial current.
For the SOS wafers shown here, however, the effect Is clearly
visible. For an oxide of 1300 X, and an \ lO-ym-thlck
epitaxial layer of 1 ohm-cm, one obtains R ■ 500, 1380,
2 and \ 2450 ohms for dots of 0.01765, 0.00196, and 0.00049 cm ,
48
MM.
■IPMI I ■ui^iJ'i"PWPW^FP-«T^ipp«^pppipi«liBP^ppp)WWi«.u.(iiiiW»i .P"P"ti '"»'■i«»VI,|<iiW)lilM .1 II I ■anmpq^Hl.il I iimmumwrnvmnf.
-nM*
bulk
Fig. Ga. Equipotential lines for frequencies * 1 MHz. The current flow in the epitaxial layer is practically radial.
Fig. 5b. The epitaxial layer in a simplified disk-like geometry. For r < r , the potential is assumed to be constant (= v ).
Epi Characterization 49
mmmmmmmmmmmmmmm mm^^mmmmmmm
I
respectively. Since, for these structures, contact witn
the backside is made by overlapping Al at the edge of the
wafer, the spreading resistance is measured without distortion
by the Junction capacitance. Though we do not observe an
R a (-Inr ) dependence but, rather, an R « 1/r one, the
model qualitatively describes the behavior of the spreading
resis tance.
RANGE OF QUASI MOS CAPACITANCE TECHNIQUE
In the preceding sections, we have shown that the contribution
of the junction impedance can be neglected, if appropriate
values for the resistivities, oxide thickness, dot diameter,
etc., are selected. In tne following, we give approximate
ranges for those parameters. Within these ranges, the junction
impedance is small compared with the interface, oxide, and
layer impedance.
For a 5-ym layer of p ^ 0.3 ohm-cm resistivity and 1200 A SiO?,
a dot of d < 0.5-mm diameter should be used. The substrate
might have p £ 20 ohm-cm. For other oxide thicknesses d , d
can be chosen to be d 0.5 d . For higher layer resistivities 1200 ox'
the layer thickness should be increased according to the
resistivity ratio. For epitaxial p values >^ 15 ohm-cm, care
must be tiken that the condition C << CT is still valid, ox J
e.g., by increasing the wafer area. For the insulator-silicon
structure, with an oxide thickness of 1300 Ä and a layer of
50
■MMMHM ■M^BMMa mm
i i^niiiiiiPL iiiji i ji.imiji JU.I\ nummfmmmKmK^^^m" .m . •um'»"*« »* .-..-..—...■n ,. i ■. «• ■"
16 — 1 a, 10 ym and N , N 'v 1-2x10 cm , we consider a dot of
0.25-mm diameter as appropriate. The same changes in dot
d'ameter and epitaxial layer thickness should be done for
a variation in oxide thickness and liyer resistivity. The
diameter of the wafer should be at least 1-1/4 inches. Since
these values refer to the oxide capacitance (x ■ RC ) and r ox
the lifetime measurements are carried out with the inversion and
deep depletion capacitance (8), thu accuracy increases by
(Cox/Cf)2. (Cf. Eq. [1].)
COMPARISON WITH OTHER TECHNIQUES
Finally, this technique should be compared with some alter-
natives, namely, the MOS emitter device and the buried-layer
SOS structure, described, for example, in Reis. (9) and (10).
Though some restricting conditions are implied in our
technique, such as dot diarerer, epitaxial layer thickness,
etc., its advantages are greater versatility and measurement
without distortion of a conduction layer. For example, t' e
technique of Jones and Barber (9) is restricted to lifetimes,
whereas a buried conduction layer (10) might cause out-
diffur,ion and thus a reduction of lifetimes even in the
epitaxial bulk.
Measurements using a guard ring as a counterelectrode were
made. The substrate was at the same potential as the guard
ring. The equivalent network consists of the oxide and space
charge capacitance, C and C , respectively, of the dot or 'OX 8C
Epi Characterization 51
iiVi»nuiw*<i um .■. ; fvm^n^mmimimm^™*'**« " ' j u, iltiniiiiWip^jp^^wi.^^^wi»^-wj^pW mmimm i^i^^^mmmmm
in series with the oxide capacitance C , the space charge
capacitance C of the ring, ar.cS the spreading resistance of S C K
the layer. The netv^ik is shunted by the coupling capacitance
C between the dot and the ring. (We neglect here the shunt
to the substrate, because this has been discussed above.)
The condition C << oxR 8cR can be fulfilled by the ox C n + C _ '
oxR sell appropriate choice of the ring area. The condition
C • C OX s c
Cc << p ^—r is always valid since oxide thicknesses of ox sc
500...5000 A are used. For a dot diameter of 60 mils, an
oxide of 1000 A, and a spacing of 150 ym, one obtains
-4 C ■ 10 pf. Thus dispersion-free C-V curves are feasible.
This was confirned by the experiment. However, a disadvantage
is encountered. with the present state of the art, photoresist
deteriorates the oxide and silicon properties, especially the
lifetimes and the oxide stability. We observed reductions in
lifetimes from 500 ysec to 0.1 ysec and flatband shifts.
(The lifetime data of the paper were obtained by the technique
given in Ref. 8.)
RESULTS AND DISCUSSION
The results of the measurements can be summarized as follows:
Flatband voltages and surface-state densities are found in
the ujual ranges ■> f % -IV and < 1.4x10 eV~ cm' , respective.-
52
■MMMB
.II1IWI !>■ wmw^1fHff*^mprpjji mm\ i"i inv'iwi.-li JUUii ^q^rnvn^»^"'. "''V lP»1" I», in« i.u.„in^»nB|»(«p»jH|Bi«n»n«(ipww»M««i
1 Since the peak of the G-V curves Is found at depletion
surface potentials for frequencies In the kHz range, the
capture cross »ctlon can be expected to be 10 to 10 cm .
No deviation compared with "normal" MOS structures can be seen.
Liferiu^ü are measured to be 1 ysec In the epitaxial waferr,
-3 10 psec in the n-SOS wafers, and 0.1 ysec in the p-SOS
wafers.
The SOS low-frequency curves for wafers of layer thicknesses
between 1 and 10 ym do not show any trace of a donor or an
acceptor level (Fig. 6). This is in contradiction to earlier
reports (11, 12), where an acceptor and a donor level at
E - EA - 0.25 eV and £„ - E - 0.30 eV were published. This c A D v
might be attributed to different measurement techniques. The
densities of the levels are reported to be 10 to 10x cm
A good resolution for surface states (or bulk impurities
11 -2 seen as surface states) for the slow rnrnp technique is 10 cm
If this value is divided by an effec.lve distribution width
of 10 to 100 A, one obtains bulk concentrations of 10 to
18 - 3 10 cm . The slow ramp technique, however, measures states
localized at the surface, whereas the techniques used in
Refs. (11) and (12) cover the cotal depth of the film. Thus,
it might be concluded that the reported levels are located
near the Si-Al-O» interface.
.'
Epi Characterization 53
-- i
^■i.....t...».i.iii,u.iiH,|,i jiMiiJinim^twqfWW^HWWP—"-^—^
c/c
n-SOS
C-V curves by ilow "Kamp"
If ä 2x 10-3Hz)
-2 "T 0 r
2
VOLTS
T- 4
-r 6
ox
-0.9
h0.8
F-l.
^0.9
-0.8
-0.7
0.6
Fig. 6. Enlarged portion of the low frequency C-V curve of a p- and an n- SOS wafer. No indication of a deep lying level is visible. For comparison, the ideal low frequency curves are shown too.
54
MM. ■
ii ■ i MI .i \m^m***^^mmmmm'i'*m*mm'''mmmm\i i . ^^WPBPH^^BHI^WP •WP ■ ■"■ '— ^^i^w»wi!ii^p^^«|m
!
another possible explanation Is the fact that one deals with
extremely deep levels. Their time constant Is controlled by
the surface potential u : r s
(o th NA) exp (-u8)
Since the levels are defined to be either acceptor or donor
levels, they exchange carriers only with the valence or
conductance band, respectively. With the given data and an
— i c 2 assumption of a capture cross section o * 10 cm and a
thermal velocity v , ■ 10 cm/sec, one obtains for the hole
2 dispersion time constant In the n-type wafer T - 2x10 sec /f p
This value Is comparable to our measurement frequency of
-3 -15 ? 2x10 ^ Hz. Capture cross sections of 10 CTQ have been
measured (13); in this case, however, higher values are
more likely, and the first explanation should be preferred.
SUMMARY AND CONCLUSIONS
The measurement of the admittance of an MOS structure
over a p-n Junction or on SOS devices is discussed. »t
is shown that by appropriate choice of the wafer data, the
contribution of the junction to the total admittance can
be neglected and lifetime measurement can be carried out.
No deep levels can be seen for the SOS system.
Epi Characterization 55
1 " - " in lui iiiiii iijwi iiiitJitB igmmmm*m'*m*m'i - m.i " - ■"
REFERENCES
1. R. Castagne, C. R. Acad. Sc. (Paris), 267. Serie B,
866 (1968).
2. E. H. Nicollian and A. Goetzberger, Bell Systeir Tech. J.,
46, 1055 (1967).
3. D. R. Kerr, Int. Conf. on Properties and Use of MIS
Structures, J. Borel, Editor, CNRS-LETI, Grenoble, 303 (1969).
4. W. R. Fahrner and A. Goetzberger, Appl. Phys. Lett.,
21, 329 (1972).
5. M. Zerbst, Z. Angew. Phys., 22, 30 (1966).
6. P. Rai-Choudhury and D. K. Schroder, J. Elec t rochetn. Society,
119. 1580 (1972).
7. Technical Report No. 4.
8. W. R. Fahrner and C. P. Schneider, ESSDERC, Nottingham,
England (1974).
9. J. E. Jones and H. D. Barber, Und Int. Symp. on Silicon
Materials Science and Technology, March 13-18, Chicago,
561 (1973).
10. D. K. Schroder and P. Rai-Choudhury, Appl. Phys. Lett.,
2_2, 455 (1973).
11. F. P. Heiman, ibid., 1J,, 3.32 (1967).
12. D. J. Dumin, Solid-State Electron., 12, 415 (1970).
13. W. Fahrner and A. Goetzberger, Appl. Phys. Lett., 17_
16 (1970).
56
mm
Recommended