188 IEEE TRANSACTIONS ON PARTS, HYBRIDS, AND PACKAGING, VOL. PHP-10, NO. 3, SEPTEMBER 1974

A Hybrid Integrated Silicon Diode Array for Visible Earth-Horizon Sensing

FRANK J. BACHNER, MEMBER, IEEE, RONALD A. COHEN, MEMBER, IEEE, ROBERT W. MOUNTAIN,

WILLIAM H. MC GONAGLE, AND ARTHUR G. FOYT, MEMBER, IEEE

Abstract-An earth-horizon sensing device which operates

principally in the visible portion of the. spectrum has been

designed as a hybrid integrated circuit. The circuit was fabri-

cated and tested for use in the LES-8/9 communications

satellites as part of the system which maintains the satellites’

orientation with respect to earth. The hybrid circuit consists

of four silicon chips mounted on a high-density alumina ce-

ramic substrate. Each silicon chip contains eight photodiodes

which are shallow junction (approximately 1.5 micrometers) n

on p type, and each diode is surrounded by a one-micrometer

deep p+ channel stop. The individual diodes in the array have

as typical ‘parameters dark currents at five volts of less than 5

nanoamps, breakdown voltages measured at 10 microamps in excess of 70 volts, and a uniformity of photoresponse across

an array of + 5%.

The choice of the techniques used in the hybridization of

this circuit was dictated by the requirements that the four

silicon chips be aligned to one another within -I 0.001 in., and that the entire assembly survive unpackaged and unencap-

sulated in a potentially corrosive prelaunch environment. For

precise silicon die alignment, the dies were mounted with a

gold-filled epoxy. To provide maximum corrosion resistance, a Ti/Pd/Au metallization system was used on both the silicon-

chips and the alumina substrate, and interconnections from

the chips to the substrate were made with thermocompression bonded gold wires.

Environmental tests indicate that the complete assembly will survive a Mil Std 202D immersion test with no degrada-

tion. The diodes will survive extended aging at 2OOcC, but the epoxy deteriorates badly after a few hundred: hours. A completed sensor aged at 125’C for 720 hours has shown no

significant changes.

INTRODUCTION

An earth-horizon sensing device, operating principally in

the visible portion of the spectrum, has been designed, fabri- cated and tested for use in M.I.T. Lincoln Laboratory’s LES

8/9 communications satellites as part of the system which maintains the satellites’ orientation with respect to earth. The

complete hybrid circuit, shown schematically in Fig. 1, con- sists of four -226” X .338” silicon chips mounted on a 2.0” X

2.0” X 0.025”, 99.5% alumina substrate. Each of the four silicon chips contains eight photodiodes whose active area is .032” X 0.170”.

Manuscript received May 14, 1974; revised July 28, 1974. This work was sponsored by the Department of the Air Force.

The authors are with the Lincoln Laboratory, Massachusetts Insti- tute of Technology. Lexington, Mass. 02173.

j---2Din.

SUESTR/\TE

/

DIODE ARRAY

i+-0.170 in.4

Fig. 1. Schematic of the earth horizon sensor illustrating the ceramic substrate, the location of the four diode arrays on the substrate, and the eight diodes on each array..

The techniques used in the fabrication of the photodiodes

and the thin-film ceramic substrate and in the assembly of the hybrid were dictated to a great extent by the rather difficult requirements placed on both the diodes themselves and the completed circuit. The requirements on the silicon photodiode

arrays are listed in Table 1. In addition, the completed hybrid circuit had additional restraints which led to the selection of a complex metallization scheme on the thin film substrate and the particular choice of assembly techniques. These require-

ments are listed in Table 2. TABLE 1

Photodiode Requirements

1. Spacing between diodes in an array accurate to within *O. 1 x-nil.

2. Uniformity of photpre sponse within *5% on each array.

3. Diode Leakage less than 50 nA at 3 volta.

TABLE 2

Requirements on the Completed Hybrid

1. Alignment of mounted diode arrays to within +I mil (each diode array to any of the other three arrays).

2. Solderable thin film metalization terminating at plated-through holes in the ceramic.

3. Entire assembly must survive prelaunch environment as an unpackaged device.

BACHNER eta/.: ARRAY FOR VISIBLE EARTH-HORIZON SENSING 189

The succeeding sections of this paper will discuss how these

varying requirements were satisfied. In addition, various en-

vironmental and electrical test data will be presented to verify the correctness of the solutions chosen.

DIODE ARRAY FABRICATION

The individual arrays are fabricated on high resistivity (5

ohm-cm) p-type, borondoped silicon wafers which were 1%

in. in diameter, polished one side with a <I 1 I> orientation.

The active area of the diodes is formed by a conven.:ional two- step phosphorus diffusion which results in a n+.-p photo-

sensitive junction. However, in order that the diodes have

adequate sensitivity the depth of the n+-p junction was held to

as small a value as possible (1.3pm) consistent with adequate device yield. To do this, it was necessary to use ,a diffusion

schedule that would not only result in such a shallow junction,

0

n+ , I n+ ;--.-Sic,

I

p-Si

contact

but would also provide sufficient gettering so that the diodes . . would have the required low leakage current and high break-

Fig 3 Earth horizon sensor photoresponse scan showing the edge response of diodes not incorporating the metal shield.

down voltage. Response generated using a scanned laser at h= 0.6328 /Am.

To prevent surface inversion of the high resistivity, p-type wafer which would result in unacceptably low resistance be- extending the contact metallization around the entire active

tween adjacent diodes in the same array, a “heavily doped”, area of each device on an array (see Fig. 2). Since this metal-

p+ channel stop between each diode element was formed. The lization was isolated from the siltion by the passivating oxide,

channel stop was a 3-micron deep boron diffusion. A sketch of therewas no change in the electrical performance of the diodes,

the cross-section of a portion of an array is shown in Fig. 2. and the edge response was eliminated as shown in Fig. 4.

CONTiCT

Fig. 2. Cross-section of a portion of an eight diode array showing the n+ active region, the p+ channel stop, the passivating oxide and the metal edge-shields.

,I Sh!eld

The metallization chosen for the diode arrays was electron-

beam evaporated Ti/Pd/Au (300 A, 1000 8, and 5000 A, respectively) [I 1 . This metallization system was used for both the top side of the devices and the back side of the arrays. The back side of the array was lightly boron diffused to insure

ohmic contact. The Ti/Pd/Au system was selected to insure corrosion resistance for the completed hybrid, and the results of environmental and stress tests on these devices will be presented in later sections.

A last consideration in the design of the silicon arrays was the elimination of edge response from each of the cliodes in an array caused by the n+-p junction’s intersecting the surface of

the wafer. Fig. 3 shows the response curve of two adjacent diodes on a single array. These curves were generated with a laser scanning system described in detail elsewhere [21. The large edge response of the two diodes, which is undesirable in the present application, is clearly illustrated in the figure. To reduce this effect, the portion of the junction which curves upward to the surface was shielded from incident light by

I p-s1

Fig. 4. Earth horizon sensor photoresponse scan showing the elimination of edge response with the incorporation of an edge shield. Response generated using a scanned laser at h = 0.6328 pm.

HYBRID FABRICATION

The thin film interconnect substrate for this hybrid circuit

was fabricated in a four-step process. First, the holes (shown in Fig. 5) were drilled in the 2” X 2” X 0.025” high density, 99.5% alumina ceramic. Second, both the front and the back of the ceramic were metallized by sputtering 300 8, of Ti, 1000 a of palladium and 3000 8, of gold. Sputtering was used rather than electron beam evaporation as with the diode arrays to insure that the side walls of the holes would be coated with

metal. Third, the front and back of the substrate were electro- plated with gold to a thickness of 2.5 pm with sufficient agitation to insure plating the side walls of the holes. Fourth,

190 IEEE TRANSACTIONS ON PARTS, HYBRIDS, AND PAl:KAGING, SEPTEMBER 1974

both sides of the substrate were etched leaving the inter-

connect pattern shown in Fig. 5 and 0.064” squares (for soldering) surrounding each hole on the back side of the

substrate. Etching was done using a dry-film, photoresist

which was simultaneously laminated on the front and back of the substrate and the desired pattern exposed into the resist.

The resist kept its integrity over the holes insuring that con-

ductivity was maintained through the hole after etch back.

Fig. 5. Complete earth horizon sensor vyith interconnecting wires soldered into plated-through holes.

The diode arrays were mounted onto the substrate using a

gold-filled single component epoxy (epotek H43). Although better thermal characteristics and greater elevated temperature

stability could be achieved by mounting the arrays eutectically

or with a solder preform, the requirement that the arrays be aligned relative to one another to within f 1 mil precluded the

use of either of these techniques. Using epoxy, the operator

was able to locate all four arrays on the substrate in the uncured resin. The alignment of any or all of the arrays could be correct in X, Y or o until precise alignment was achieved.

Only then was the epoxy cured.

Interconnections from the diodes to the substrate were made by thermocompression bonding gold wires using a ball

bond on the diode and stitching to the substrate. These bonds

can be seen clearly in Fig. 6. During bonding the substrate was held at 200°C and the bonding tip at 3OO’C. Although the

epoxy will not survive extended periods at 200°C (see below)

no deleterious effect was observed during the short time of the

bonding operation.

Fig. 6. Single eight-photodiode array showing epoxy mounting and thermocompression gold-wire bond interconnections.

DIODE EVALUATICN RESULTS

A. Current-Voltage Characteristics

The primary requirement on th,? electrical performance of

these diodes is that the leakage current not exceed 50 nA at 3 volts. Thus the breakdown voltage lleed only exceed the 3-volt

operating level of the device. Holvever, because the devices

were made on lightly doped substrates to increase the photo-

response, a well made device had a much higher breakdown

voltage. Table 3 lists the limiting parameters for the 32 diodes of the four arrays mounted on a completed sensor. From the

data in Table 3 it can be seen tha: diodes greatly exceed the

required levels for leakage and breal:down.

TABLE: 3

Diodes I-V Characteristics for the 32 I)iodes of a Complete Sensor

Reverse leakage current at 3 V (min.)

= 2.5 nA

Reverse leakage current at 3 V (max. )

= 6.6 nA

Reverse leakage current at 3 V (ave.)

= 4.7 nA

Reverse breakdown voltage at 10 pA (min.)

= 60 V

Reverse breakdown voltage at 10 )IA (max.)

= 79 v

B. Optical Characteristics

Optical evaluation of the diode arrays was done to deter- mine if the photoresponse of each array was within the f 5%

range required by the system. Rest Its are given in Table 4 for the four arrays of a single sensor. The measurements were made by illuminating the diode arriry with white light through a diffuser. The intensity of the light was chosen to simulate

operating conditions, and the photocurrents were measured at

a reverse bias of 0.1 volt. The data in Table 4 show that two of the arrays met the uniformity slbecification, a third almost meets the requirement, whereas the fourth exceeds the specifi-

cations by about 3 percent. These data clearly indicate that the fabrication techniques being used will yield arrays within

the + 5% specification, and the intlividual arrays will have to

be pre-evaluated before they are in.zorporated into the sensor.

TABLE 4

Uniformity of Photoresponse for the Four Diode Arrays of a Completed 5 ensor

Minimum Milximum Array Photocurrent Pho:ocurrent Percent

No. at 0. 1 V at 0. I V Deviation

1 1.92 )rA 2.05 p.A *3.20/o

2 1.79 fl 2.00 pA *5.3%

3 1.98 @ 2. 15 pA l 4. 1%

4 1.76 pA 2. 10 pA l 8. 1%

BACHNER eta/.: ARRAY FOR VISIBLE EARTH-HORIZ3N SENSING

Another aspect of the photoresponse of these d’odes is the uniformity of response across an individual diode, Although

this property is of considerably less importance in the opera- tion of the station keeping system than the diode-to-diode uniformity, it is nevertheless of some interest. Fig. 4 shows the response of two adjacent diodes as a laser beam is scanned across the active area. The plateau can be seen to be quite flat,

and the curvature that does exist is attributed to d stortion in the optics of the laser scanning system rather than non- uniformity of photoresponse in the diodes.

STRESS TESTING

A. Immersion Testing

To investigate possible corrosion problems, an array of four

diode arrays mounted on an alumina substrate was subjected

to an immersion test according to Mil Std 20211, Method 104A, Test Condition “C”. In this test the device is cycled

between a saturated solution of sodium chloride in water at

O°C and tap water at 65’C. The duration of each cycle is 60

minutes, and the unit was cycled five times. After the com- pletion of immersion cycling, the unit was visually inspected

and electrically tested. The electrical tests showed no change

in any parameters after completion of the test, and visual inspection revealed no signs of corrosion or chemical attack.

B. High Temperature Life Tests

In order to investigate the effects of elevated temperature on the device characteristics, several sub-arrays were mounted on TO-8 headers, sealed and put on life test for 1000 hours at

2OO’C. It should be noted that this temperature is consider- ably in excess of maximum expected operating temperatures. In operation, the surface temperature of the silicon wafer is

not expected to exceed 120°C, and the epoxy bond is not expected to exceed 40°C. As a control for the Ti/Pd/Au metallized devices mounted with epoxy, three other types of diode metallization/mounting combinations were run simul- taneously. These were Ti/Pd/Au metallized diodes eutectically bonded to the TO-8 header, aluminum metallized diodes

epoxy bonded to the header, and aluminum metalli zed devices

eutectically bonded to the header. In all cases the back side

metallization was plain gold rather than the Ti/Pd/Au used for the final device. This was done as a matter of convenience and

because the tests were done early in the diode development

program. Test results are summarized in Figs. 7-9. The parameters

reported in the figures are the reverse breakdown voltage (V, )

measured at a leakage level of 10 PA, the leakage current (IL )

measured at a five volt bias, and the forward voltage (VF)

measured at a forward current of 100 mA, all measured in the absence of light. The data points are typical parameter values

for a diode on the type of sub-array indicated. The most

important conclusion from these data and also from subse- quent visual inspection is that the devices and the front con- tact metallization will survive at 200°C for 1000 hours; how-

ever, the epoxy begins to deteriorate after a few hundred hours leading to erratic leakage currents, increasing forward resistance, and ultimately to failure of the device by separation

b Failed at 300 t-tours

---a 65

r

Failed Between 500 and 1000 Hours

o To/Pd/Au-Epoxy a Ti/Pd/Au-Eutectal 0 Al-Epoxy 9 Al-Eutectic

t35+ q I - I = 0 200 400 600 600 IOC

TIME (hr)

191

Fig. 7. Breakdown voltage measured at 10 PA in the absence of light as a function of time at 2OO’C for photodiode arrays using different metallizations and mounting techniques. Data points are typical values for diodes on a particular array.

‘\ 2 5 ‘m 300 Fooled HO”rS 01

?I 4 4- :: o Tb/Pd/Au-Epoxy

* Ti/Pd/Au-Eutectx 3 3 Al-Epoxy

3- . Al-Eutectic

0 c

TIME (hr)

Fig. 8. Leakage current measured at 5 volts reverse bias in the absence of light as a function of time at 200°C for photodiode arrays using different metallization and mounting techniques. Data points are typical values of diodes on a particular array.

of the array from the header. In all cases where all the devices

on an array failed it was by this mechanism.

C. Extended Aging at 125’C

As was mentioned in the previous section, 200°C is an

extremely harsh environment for the epoxy used to mount the arrays to the ceramic substrate, and the failure of the epoxy

bond was not surprising. However, the maximum temperature that any portion of the sensor is expected to reach is 120°C,

and this temperature will occur only on the surface of the silicon. Therefore, an extended aging test at 125’C was

192 IEEE TRANSACTIONS ON PARTS, HYBRIDS, AND PAC:KAGING, SEPTEMBER 1974

Foiled ot

P 300 Hours

/

i

o Ti/Pd/Au-Epoxy o Ti/Pd/Au-Eutectlc LI Al -Epoxy IP Al -Eutectbc

Failed Between

/ 500 and 1000 Hours

A;, -

800 1000

TIME (hr)

Fig. 9. Forward voltage measured at 100 mA in the absence of light as a function of time at 200°C for photodiode arrays using different metallizations and mounting techniques. Data points are typical values for diodes on a particular array.

postulated as a good means of testing the reliability of the

hybrid device as it will be used in a space environment.

The results of this test through 350 hours are summarized

in Table 5. The parameters reported are the leakage current (IL) and the forward voltage (VF), both measured in the absence of light. The leakage current is expected to give an

indication of any degradation in the device itself, and the forward voltage will indicate if the epoxy bond is deteriorat-

ing. Data points represent the average value of the specific

parameter for the eight diodes on an array except for Array 2 where only seven diodes were operative. (On Array 2 a wire bond failed on one diode between 0 and 100 hours. Steps are being taken to preclude future wire bond failures. However,

the failure of a single diode on an array will only reduce the

accuracy of the entire device by a small amount.) Because of a shortage of good 8-diode arrays, arrays with initial leakage

TABLE 5

Leakage Current and Forward Voltage for the Photodiodes in a Complete Earth Horizon Sensor Aged at-125OC

Leakage Current (nA) Forward Voltage (Volts) Hours at 5 Volts at 100 mA Array

No. 0 100 200 1350 0 100 200 350 _

1 58 64 - 61 0. 98 0. 98 0.96 0. 98

2 55 58 - 59 0.96 0.96 0.96 0.97

3 61 65 - 61 0.94 0.94 0.95 0.95

4 56 59 - 58 0.93 0.93 0.94 0.93

current slightly above specification; were chosen for this test. In no case did the value of V,= for the diodes on a single array

vary from the average value by more than * 3%. Variations of leakage current between diodes OII an array were somewhat greater.

The data in Table 5 show quite conclusively that the earth horizon sensor using photodiod? arrays metallized with

Ti/Pd/Au for both top and bottom contacts and mounted on a

Ti/Pd/Au metallized alumina substrate with a gold-filled epoxy

will survive for extended times :t 125’C. This life test is

continuing, but no changes in ei.:her of the parameters are

expected. As a result of the data generated in this test, we are

very confident of the reliability oi the complete sensor in its

satellite environment.

SUf’ilWlAFY

A hybrid, integrated earth-horh on sensor operating princi-

pally in the visible portion of the spectrum has been built and

tested for use in the orientatiorr system of the LES 8/9

satellites being built by Lincoln Laboratory. The sensor consists of four silicon chips each contairing a linear array of eight

photodiodes. The diodes are planar, n+ on p, shallow junction devices which use a channel stop to prevent surface inversion between diodes and a metal shielll to eliminate nonuniform

photoresponse where the junction intersects the surface. En- vironmental constraints, primarily that the devices must

survive unpackaged in a poteni ially corrosive, prelaunch environment, dictate the use of a Ti/Pd/Au metallization for

both the top and bottom of the silil:on chip. The diode arrays are mounted 011 a 99.5% alumina substrate

which is also metallized with Ti/Pc /Au. Since the diodes must

be mounted with an alignment accL racy of + 0.001”. an epoxy mounting system was chosen. The epoxy was a gold-filled, single component epoxy which did not harden at room tem- perature and allowed alignment corrections after the device was initially placed on its mounting pad.

To test the validity of the dec’sions made to protect the sensor from corrosion, and to determine the projected reli- ability of the completed sensor uncler the operating conditions

to be encountered in space, a series of tests were performed on

individual arrays and completed .Issemblies. These were im-

mersion tests to establish corrosil)n resistance, and aging at

200°C and 125’C to determine thl? stability of the diodes and

the epoxy mounting technique. The data from these tests

indicate that the sensor is not subject to corrosion; and,

whereas the epoxy will not withstaild long times at 200°C, the

diodes will survive 1000 hours at 200°C and the entire hybrid will survive extended periotls at 125’C with no notice-

able change in critical parameters.

REFERENCES

111 John S. Fisher and Peter M. Hall,P:oceedings IEEE, vol. 59, 1418, 1971.

[21 Robert E. McMahon, Electronics, vol. 44,92, April 12, 1971.