A 12 bit A/D converter was designed and developed which will operate at 200 C with .05 linearity, 1/accuracy, 350 WSec conversion time, and only 455 mW power consumption. This product also necessitated the development of a unique three metal system in which aluminum wire bonding is done utilizing aluminum bonding pads, gold wire bonding to all gold areas, and employment of a nickel interface between gold and aluminum connections. This system totally eliminates the formation of a intermetallics at the bonding interface which can lead to bond failure. This product represents an advancement in electronics as it proved the operation of integrated circuits at high temperature, as well as providing information about both the electrical and mechanical reliability of hybrid circuits at 200 C

Sullivan, M. R.

Toth, J. B.

English

NASA Technical Reports Server

HYBRID A/D CONVERTER FOR 200'~ OPERATION 
Mark R. Su l l i van  
Je f f rey  B.Toth 
Micro Networks Company 
324 Clark Street 
Uorcester. Massachusetts 01606 
ABSTRACT 
This paper describes the design and developmei~t o f  a h iqh performance hybr id  I 2  b i t  analot: t o  d i g i t a l  converter. 
which w i  l l operate re1 iab ly  a t  200'~. A product o f  t h i s  type was found t o  be necessary i n  areas such as 
geothermal probing, o i l - w e l l  logging. j e t  engine and nuclear reactor  monitoring. and other appl icat ions where 
the environment: may reach temperatures o f  up t o  200°C. This product repre--nts an advancement i n  e lec t ron ics  
as i t  prov-d the operat ion o f  in tegrated c i r c u i t s  a t  h igh  temperature, as we l l  as prov id ing information about 
beth the e1ectri:al and mechanical r e l i a b i l i t y  o f  hybr id  c i r c u i t s  a t  200°C. Because the c i r c u i t  design o f  the 
A/D converter involved both d i g i t a l  and l i n c a r  c i r c u i t r y .  t h i s  produced an opportunity t o  evaluate the 
performance o f  both technologies a t  2 0 0 ~ ~ .  I n i t i a l  mechanical f a i l u r e  modes led  t o  researching mure r e l i a b l e  
methods o f  wi re bonc'ing and d i e  attachment. The r e s u l t  o f  t h i s  work was a 12 b i t  A/G converter which w i l l  
operate a t  ZOOOC w i t h  . O j Z  l i n e a r i t y ,  I% accuracy, 350 pSec conversion time, and orlly 455mU power consumption. 
This product a lso  necessitated the development o f  a unique three metal system i n  which aluminum wire bondinq i s  
done u t i l i z i n g  aluminum bonding pads. go ld wi re bonding t o  a l l  gold areas. and employment o f  .I n icke l  i n te r face  
between gold and aluminum connections. This sytem t o t a i l y  e l iminates the formation o f  i n te rmeta l l i cs  at  the 
bonding in ter face which can lead t o  bond f a i l u r e .  
Recently the e lec t ron ics  indust ry  has been made aware 
o f  the need fot electronic components and systems which 
w i l l  operate a t  temperatures as h igh as 200°C. These 
appl icat ions include geothermal probing. o i l  we l l  
logging. j e t  engine and n r~c lear  reactor monitor ing ?nd 
other h o s t i l e  environments where the temperature {nay 
reach 200°C o r  higher. I n  some a f  these appl icat ions.  
as i n  o i l  we l l  logging and geothermal probing. i t  i s  
necessary t o  transmit data through long lengths o f  
cable which run from deep i n t o  the ear th t o  the sur- 
face. These appl i c a t  ions are where a h igh tempera- 
ture A to  13 converter becomes h igh ly  desirable. Trans- 
m i t t i n g  l o w  level  analog data over a long distance 
such as t h i s  dould be very d i f f i c u l t  without i n t r o -  
ducing s i g n i f i c a n t  extraneous errors .  Through the 
use o f  an A t o  0 converter i t  becomes possible t o  take 
outputs from s t r a i n  gauges and thermocouples, convert 
them t o  "ones" and "zeros" and then transmit t h i s  data 
d i g i t a l l y  t o  the surface. 
ADVANTAGES OF HYBRIDS 
An A t o  D converter can be fabr icated i n  many d i f f e r -  
ent forms such as a module. p r in ted  c i r c u i t  board. o r  
hybr id  c i r c u i t .  Hybrid r e l i a b i l i t y  a t  12s0c has been 
proven t o  be excel lent  through many thousands o f  hours 
o f  q u a l i f i c a t i o n  tests .  This reputat ion makes hybr id  
technology a wise choice fo r  200°C operation. A 
hybr id  c i r c u i t  can conta in several d i f f e r e n t  1.C.s i n  
one small package. which i s  advantageous i n  appl ica-  
t ions where space i s  l im i ted .  
A TO D CIRCUIT DESIGN 
An A t o  D converter proved t o  be a chal lenging 
product to  design and evaluate a t  200°C due t o  the 
fact tha t  1 i t t l e  informat ion coricerning the d i f f e r e n t  
types o f  components avd t h e i r  proper t ies a t  h igh 
temperature was ava, lable. Passive components. such 
as res is to rs  and capacitors and ac t i ve  components 
inc lud ing t rans is to rs  and in tegrated c i r c u i t s  required 
extensive analysis and evaluat ion. The f i n a l  A/D 
design employs both l i near  and d i g i t a l  c i r c u i t r y .  
I n  the design o f  the MN5700, r e l i a b i l i t y  whs consid- 
ered o f  prime importance. Two factors  that s . g n i f i -  
cant ly  e f f e c t  the r e l i a b i l i t y  o f  any c i r c u i t  are 
power and leve l  o f  complexity. Research i n  h igh 
4 
8 
temperature e lect ron ics has shown that the ra te  o f  
aging. those factors that produce chanqes i n  para- 
meter o f  key components. w i l l  ap rox imatr ly  double 
w i t h  each IOOC temperature r i s e .  5 
For a hybr id  c i r c u i t  the substrate temperature w i l l  
increase as the power consumption increases. As a 
design goal the t yp ica l  subbtrate t o  ambient temper- 
a ture r i s e  was not to  exceed 10°C. The 32 p in  double 
PIP Package, selected p r imar i l y  fo r  i t s  form factor .  
has a t yp ica l  substrate t o  ambient r i s e  o f  270c /Uat t .~  
Thus t o  keep t h i s  r i s e  under IO°C, the typ ina l  
power consumption was l im i ted  tc  311 m i l l i w a t t s .  To 
reduce the complexity. as few 1.C.s were used as 
possible. 
There are several d i f f e r e n t  approaches t o  A t o  D 
conversion which are cu r ren t l y  used. The MN5100 uses 
the successive approximation method. This al lows a 
converter t o  be made using few cnmponents and has 
good charac te r i s t i cs  i n  speed. resolut ion.  and 
accuracy. A successive approx;mation A t o  0 converter 
consists of four sections, D t~ A converter, reference, 
comparator. and successive approximation reg is te r  
SAR). See Figure& Each o f  these sect ions w i  l I 
be discussed showing the design considerat ions fo r  
200°C operat ion. 
D t o  A Converter 
The D t o  A sect ion o f  the MN5700 u t i l i z e s  a vottagc 
switching R-2R ladder network. The switch i s  CFOS 
and connects each leg o f  the ladder e i t h e r  t o  qround 
o r  a reference voltage. See Figure-. A CMOS switch 
was chosen because o f  i t s  low power consumpt icm and 
evaluat ions showed i t  t o  be r e l i a b l e  at  200°C. 
Reference 
-- 
The reference c i r c u i t  shown i n  F i g u r e 2  consists c f  
a temperature cmpensatrd Zener diode and a die1ec- 
t r i c a l l y  iso lated op-amp. I h z  Zener was found ta be 
accurate t o  about 1Oppm/O~ from 25 '~  t o  1 2 5 ~ ~ .  From 
1 2 5 ~ ~  t o  200'~ the temperature c o e f f i ~ . z n t  iqcreased 
t o  40ppm/O~. F i g u r e 2  shoks a grann o f  zener 
voltage vs. temperature. 
https://ntrs.nasa.gov/search.jsp?R=19820007456 2020-03-21T15:51:00+00:00Z
AID Block Diagram 
s t a r t  >----I \ EOC 
SAR 
Clock I I 
O/A CONVERTER I out 
MSB 
Vref 
6 
FIGURE 2 
V- Reference C i r c u i t  
Reference 
Vol tage 
6 
FIGURE 3 
Research and evaluat ion showed that a d i e l e c t r i c a l l y  
i so la ted  op-amp was the hest choice for  ZOO°C opera- 
t i on .  Host s i l i c o n  b ipo la r  1.C.s use junc t ion  
i s o l a t i o n  between t rans is to rs .  These types o f  
c i r c u i t s  show t rans is to r  in te rac t ion  a t  200~c .3  1.C.s 
,which are manufactured using d i e l e c t r i c  isol2:;r;~ have 
the ac t i ve  areas separated by an insulat i r lg  layer o f  
mater ia l .  This reduces t rans is to r  i 7terac.t ion and 
a lso  reduces leakage current t o  the substrate under 
h igh temperature condit ions .4 
Zener Vol tage 
e 4 1  vs 
4 Temperature 
25 50 75 100 125 150 175 200 
Temperature (OC? 
Figure 4 
The chdnge i n  the reference voltage a t  200~: was 
fouqd t o  be t y p i c a l l y  . 2 9 .  The c i r c u i t  was evaluated 
t o  see why the change i n  reference voltage was less 
than the change i n  zener voltage. Evaluat ion showed 
that the o f f s e t  o f  the op-amp had i t s  largest  change 
between 15CDC and 200°c, as shown i n  F i g u r e 2  Th is  
change was i n  the opposite d i r e c t i o n  t o  the d r i f t  o f  
the zener. and therefore the accuracy o f  the reference 
became the d i f ference o f  the two .  
Offset Pffset 
Change 6 vs. 
v 1 Temperature 
25 50 75 IOC 125 150 175 200 
Temperature (OC) 
Figure 5 
The centra l  component c the D t o  A c i r c u i t  i s  a 
r e s i s t o r  network. The network used i s  a t h i n  f i l m  ch ip  
using nickel-chromium res is to rs  deposited on s i t i con .  
The change i n  resistance over temperature wi ! l  deter 
mine the accuracy anr l i n e a r i t y  o f  the device. An 
absolute change i n  resistance resu l f s  i n  an accuracy 
change and a change i n  r e s i s t o r  r a t i o s  w i l l  r e s u l t  i n  
a l i n c a r i  t y  change. The graph i n  F i g u r e d  shows 
typ ica l  changes i n  resistance from 2S°C t o  200°C. 
F i g u r e 1  s:~ows changes i n  r e s i s t o r  r a t i o s .  I n  order 
t o  meet design requirements o f  f l / Z  LSB t o  the 10 b i t  
l eve l ,  the reszstor r a t i o s  must t rack t o  b e t t e r  than 
2.05% from 25OC t o  200°C. 
The t h i n  f i l m  r e s i s t e r  ch ip a lso  has the advantage o f  
being a c t i v e l y  laser trimmed. This r e s u l t s  I n  an A t o  
D which w i l l  meet a l l  spec i f icat ions without any exter-  
na l  adju:;tmcnts. Any external  components added 
would be another source o f  e r r o r  when ra ised t o  200%. 
? 
Change 
Resistance 
vs . 
Temperature 
I 
25 50 75 100 125 150 175 200 
Temperature OC 
Figure 6 
---
-I Resistor R a t : ~  
.014 vs. 
% change A Temperature 
. c i  
25 50 75 100 1:s 150 175 200 
Temperature i 'c) 
Figure 7 
Comparator 
I n  most hybr id  A t o  D converters, the comparator i s  a 
s ins le  I . C .  chip. These are usual ly  b i p o l a r  devices. 
Tests done on most ava i lab le  b ipo la r  1.C.s indicated 
they were not the most re1 iab le  choice f o r  200°C 
operation. This i s  due t o  the problems o f  junct ion 
i s o l a t i o n  prev ious ly  stated. Because o f  \ i s .  a 
comparator was designed using a d i e l e ~ t r i ~ ~ l l y  i so la ted  
op amp and d isc re te  t rans is to rs  which operated re- 
l i a b l y  a t  200'~. 
Successive Approximation Resister 
The SAR used i n  t h i s  design i s  a CflOS I . C .  This was 
chosen because o f  the good charac te r i s t i cs  o f  CflOS a t  
h igh temperatures and the low power consumption. CflOS 
1.C.s have been constructe which were funct ional a t  
)OO°C f o r  over 1000 h r ~ r s . ~  Uhi l e  leakage current on 
CflOS devices operat ing a t  2 0 0 ~ ~  may be large when 
compared t o  +2s0C operat ion, t h e i r  volt;-e thresholds 
do no t  change appreciably. Thus devices operated from 
low impedance sources work very re1 i a b l  y a t  2 0 0 ~ ~ .  
ELECTRICAL TEST RESULTS 
The f i r s t  prototype u n i t s  were evaluated f o r  confor- 
mance t o  the 200°C specifica'.ions. Test r e s u l t s  show- 
ed that  these performed as expected. These u n i t s  were 
then put on a 203'~ burn- in  w i t h  frequent m n i  t o r i n g  t o  
observe changes o r  s h i f t s  that occured. A f te r  approx- 
imately 25 hours, large s h i f t s  were seen i n  l i n e a r i t y  
and accuracy. The cause o f  a s h i f t  such as t h i s  i n d i -  
cated a chacge i n  res is to rs  o r  s change i n  the outpui  
resistance o f  the switches. The par ts  were burned-in 
longer and catastrophic f a i l u r e s  were seen. Visual 
inspect ion showed tha t  go ld b a l l  bonds were l i f t 1 ~ 3 g  
o f f  o f  the a!uminum pads on the I . C .  chips. 
BONDING FAILURES 
The bonding f a i l u r e s  which occured a t  the aluminum/ 
gold in ter face arose from the formation o f  an i n t e r -  
m e t a i l i c  compound at  t h . ~ t  po in t .  As the time a t  h iqh  
temperature increases. these compounds do not e x h i b i t  
s u f f i c i e n t  mechanical strength t o  insure bond i n t e g r i t y .  
As a resu l t ,  t t ~ e  bonds have a tendency t o  break and 
cause dn o w n  c i r c u i t .  
CEVELOPMENT OF HETALlZATlON SYSTEN 
I t  was concluded that the most r e l i a b l e  hybr id  could 
be fabricated i f  a l l  w i re  bonding was done to  s im i la r  
metals. This was a problem hccause ava i lab le  I . C .  
chips use aluminum boading pads. whi le the substrate, 
res is to r  chips, and posts have gold bonding areas. 
To accomnodate t h i s  bonding schcne, a substrate was 
needed w i t h  both gold and aluminum bonding areas. 
T= Metal-Metal izat ion Process 
In order t o  construct the type o f  substrate described, 
i t  was necessary t o  use three metals - gold, alumir.um, 
and r i c k e l .  The gold i s  used for  conductor runs and 
bonding areas, and the aluminum i s  used on ly  f o r  bond- 
ing areas. a t  the 1.C.  chips. The aluminum bonding 
pads s i t  on top o f  gold pads, but have a layer s f  
n i cke l  i n  betweeta the gold and aluminum layers t o  act  
as a d i f f u s i o n  bar r ie r .  which e l iminates the formation 
o f  i n t e r n e t ~ l l i c  compounds. 
Figure 8 depicts the major process steps. S t a r t i n g  w i t h  
a wholly meta l l ized A12 O3 ceramic p l a t e  (Fig.&) a 
gold conductor pat tern i s  def ined using standard photo 
l i thographic  and etch ing techniques (Fin. 8b). Next 
a n ickel  layer and an aluminum layer  are vacuum depos- 
i t e d  (Fiq.8c). F ina l l y .  the aluminum pads are formed 
by se lec t i ve  removal o f  unwanted f i l m  (Fig. 8d). 
0 
-- '250 A NiCr 
t o3 
Substrate 
c-- Au 
.: NiCr \ b) 
Figure 8 
The prccess st .os,  th icknesses, and ma te r i a l  s e l e c t i o n  
have been chcsen on the bas is  of  c o m p a t i b i l i t y  w i t h  
present f a b r i c a t i o n  techniques, as w e i l  as perforlnance 
c r i t e r i a .  
Au/Ni /AI Subst r a t e  Eva1 ua* 
For eva lua t i on  purposes, a subs t ra te  was made which 
had a p a t t e r n  a l l ow ing  go ld  and a l ~ r ~ ~ i n u m  w i r e  bonding 
t o  be done between p ins  o f  a h y b r i d  package. Connec- 
t i o n s  were made which c o r ~ s i s t e d  o f  26 bonds (13 w i res )  
between p ins  O F  the package. The bonds rons i s ted  o f  
aluminum w i r e  on go ld  pa&., aluminuln w i r e  on aluminum 
pads, g o l d  w i r e  on aluminum pads, and go ld  w i r e  on 
go ld  pads. The aluminum pads were deposi ted on go ld  
us ing  a n i c k e l  b a r r i e r  as descr ibed i n  the  prev ious 
sect ion .  The res i s tance  was measured between the p ins  
o f  the  package a t  var ious  i n t e r v a l s  o f  2 0 0 ~ ~  bake. 
Th is  measurement inc luded the bond res is tances along 
w i t h  the  r e s i s t ~ n c e  through the aluminum/nickel /gold 
i n t e r f a c e .  F igure  9 shows a graph o f  change i n  
res i s tance  versus time a t  200°C f o r  the f o u r  d i f f e r e n t  
bond i n te r faces .  i t  can be seen t h a t  the best 
r e s u l t s  a r e  obta ined when bonding i s  done between 
s i m i l a r  metals.  
~ o n d  :r:istance 
Temperature 
.8 
- I I I I 
10 20 50 100 200 530 
Time a t  2 0 0 ~ ~   ours) 
F igure  3 
- 
U! 
Figure 10 
Tests have shown t h a t  u n i t s  w i l l  opera te  r e l i a b l y  and 
remain w i t h i n  200°C s p e c i f t c a t i o n s  i n  excess o f  
500 hours. Beyond 500 hours.  some u n i t s  w i l l  e x h i b i t  
a slow s h i f t  i n  l i n e a r i t y  and accuracy. This appears 
t o  be caused by r e s i s t o r  ag ing and changes i n  the  
c h a r a c t e r i s t i c s  o f  the CMOS switches i n  the  D/A sect ion .  
L i f e  t e s t s  have shown t h a t  most u n i t s  r?main w i t h i n  
s p e c i f i c a t i o n  i n  s r c t s s  o f  1000 hours. Tests have 
a l s o  shown t h a t  mcst ca tas t roph i c  f a i l u r e s  and u n i t s  
w i t h  l a rge  s h i f t s  w i l l  show up i n  the f i r s t  24 hours 
o f  ope ra t i on  a t  200'~. To he lp  assul-c. r e l i a b i l i t y ,  a11 
u n i t s  a re  tes ted,  burned- in f o r  2: hours a t  200°C, and 
re tes ted.  
A1 1 200°C s p e c i f i c a t i o n s  are  a l s o  yuaranteed a t  -55OC. 
The MN5700 i s  a v ~ i l a b l e  w i t h  h igh  r e l i a b i l i t y  screening 
according t o  MI L-STD-383 f o r  M i  1 i tary/Aerospace 
Appl i c a t  ions. 
REFERENCES 
I. A.F. Veneruso, "High Temperature E l e c t r o n i c s  f o r  
Geothermal Fnergy. "IEEE C i r c u i t  and Systems 
Magaz ine, "01 . 1 , NO. 3, 1979, p. 12. 
2. P. R. Prazak, "Hybrid Converters L i ke  i t  Hot !200°i) 
F igu re  10 SIIOWS a s e c t i o n  o f  the subst ra te  used i n  the and Cold (-55°C)1', E l e c t r o n i c  D e s k ,  VOI .28, 
MN5700. The shaded areas i n d i c a t e  aluminum pads No. 23, Nov. 1980, p.125. 
which a r e  prope r l y  located f o r  aluminum w i r e  bonding 
t o  the I . \ - .  Chip. 
Other F a i l u r e  Modes 
3. M. H. Shepard, "Thermal Eva luat ion  o i  Ceramic 
Packages", Micro  Networks Q u a l i f i c a t i o n  Report, 
Mar. 1979, p.1. 
The next  group o f  ca tas t roph i c  f a i l u r e s  were seen i n  the  4. 0.w. palmer, and R.C. Heckman, O E ~ ~ ~ ~ ~ ~  Temperature 
500-750 hour range. When these u n i t s  were inspected, Range Microe lec t ron ics" ,  %'Transact i o t , ; ~  
i t  was seen t h a t  some o f  the cp3xy mounted ch ips  had Contponents, Hybr ids,  and Manufactur ing Technology, 
l i f t e d  o f f  the  subst ra te  and caused some bonds t o  Vol.CHMT-1, No. 4, Dec. 1978, pp.334-335. 
break. Th is  was co r rec ted  by us ing - d i f f e r e n t  type o f  
epoxy w i t h  b e t t e r  h i g h  temperaturz c h a r a c t e r i s t i c s .  5. H a r r i s  Semiconductor Products D i v i s i o n ,  Analoq. Eva luat ion  cf t h i s  epoxy a f t e r  1000 hcurs a t  200°C Data Book. p.1-5 
showed l i t t l e  o r  no degradat ion i n  i t s  bonding and 
adhesive c h a r a c t e r i s t i c s .  6. J .  D. McBrayer, "High Temperature t-lnplementary 
Metal Oxide Semiconductor ( c M ~ s ) " .  SAND 79-1 487. 


Hybrid A/D converter for 200 deg C operation

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