Microcircuitry: Special considerations for military quality systems

Electronics Reliability ~ MicrominiatuHzation Pergamon Press 1962. Vol. 1, pp. 73-80. Printed in Great Britain

MICROCIRCUITRY: SPECIAL CONSIDERATIONS FOR MILITARY QUALITY SYSTEMS* G. J. SELVIN

Waltham Laboratories, Sylvania Electric Products Inc., Waltham, Massachusetts, U.S.A. Abstract--This paper presents design objectives and engineering solutions to the problems of building military quality electronic systems composed of semiconductor microcircuitry. The microcircuitry considered can be formed of mixtures of very small parts, integrated vacuum formed thin films and solid state or semiconductor circuits. A method is detailed, that we are developing, for providing true hermetic protection to each individual circuit stage, interconnexion of stages without the use of wires, plug-in capability of logical blocks (modules of circuitry), growth potential in operating temperature to over 500°C, and adaptability to reliable production automation. Examples of sealed circuit wafers, modules and plug-in interconnected modules are illustrated. Data is provided on module thermal dissipation limits, and ranges of suitable electronic component values. INTRODUCTION TrmR~ appear to be as many different approaches to the microminiaturization of semiconductor electronic circuits as there are organizations active in the field. T h e lines of attack range from present state of the art techniques of fabricating and assembling individual very small electronic parts to long-range and more radical application of multi-layer vacuum-deposited thin films and solid state material and device technology. We find that each organization claims that their method is the best way. Our studies indicate that there is no one best way, and that future microelectronic circuits will be comprised of mixtures of all of the technologies, using each where it does the job best. This conclusion stems from our conviction that the electronic system requirements come first, that the system requirements imposed upon subsystems and circuits come second, and that the method of forming those circuits comes third. We must avoid compromising electronic system performance just to use one exotic microcircuit construction technique throughout. As a result of our studies, we established requirements for protecting and interconnecting microcircuitry that would accept mixtures of any and all techniques, and would be prepared now

for more advanced thin film and solid state developments as they become available. This will provide the widest possible system.design latitude. Research work and development work which we are performing in thin film and solid state circuitry has confirmed that our development objectives are reasonable.

PROGRAMME OBJECTIVES Microminiature Module Programme: T h e following are the objectives of the Sylvania 1. Each functioning stage of circuitry should be formed on one wafer surface area. 2. Each circuit stage should be hermetically sealed from its neighbours and from the ambient, to transistor seal quality standards. (No plastics or potting). 3. Circuit stage-to-stage interconnexion must be performed without the use of wires. A total logical modular assembly should be capable of being plugged into an interconnexion system. 4. Construction and seal materials must be capable of direct extension to 400-500°C continuous operation. 5. Maximum reliability must be achieved through the proper application of basic inert material characteristics, and reduction in the number of joints and connexions.

* Presented at the AGARD Conference on Microminiaturization, Oslo, 24-26 July 1961. 73

G. J. S E L V I N

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6. The construction, assembly and protection techniques must be capable of economical, automated mass production. Price target: lower than conventional present-day standard parts and printed circuit board techniques. 7. Immediate parts density objective: 600,000 to 1 million equivalent parts per cubic foot. Long-range objective: more than 300 circuit stages per cubic inch. DISCUSSION OF OBJECTIVES

A brief examination of the thinking behind each of the objectives noted above is contained in the following paragraphs:

1. Each functioning stage of circuitry should be formed on one wafer area Electronic functions should be considered as active signal processing stages (e.g. one stage of an amplifier, one NoRgate, a detector stage, etc.) rather than as discrete electronic parts• The size of the electronic circuit wafer shown in Fig. 1 was determined by an analysis of circuit stages from 0'486"sq.

I

1

-4~-------- 0 -4 4 0"

0"020~ Seol bon

FlO.1. Ceramicmicrocircuitwaferdimensions.96% Alumina,thickness0.010in. many types of equipments and their component requirements, followed by a determination of the surface area required for each in order to create a circuit stage on a wafer. Future advancements in thin film and molecular circuitry will permit

multiple circuit stages on the same surface area. But this must follow as production yield, economics and reliability permit. The one-circuit-stageon-a-wafer concept permits fabrication, performance testing and adjustment, and protection on the circuit level rather than the single component level• We see no reason for compounding individual parts tolerances and protections when circuit performance is the criterion.

2. Each circuit stage should be hermetically sealed from its neighbours and from the ambient, to transistor seal quality standards The hermetic seal standard which our studies indicate as essential is that of present-day high quality transistor container leak tightness. Since a functioning stage of circuitry has been built on a wafer, there appear to be economic as well as reliability advantages to providing hermetic protection at one time to the entire circuit, rather than different standards of protection quality to different individual parts• By avoiding the use of resinous or plastic materials, we avoid the multiple problems that plague us when we "pot", including stressing of thin vacuum-deposited films, chemical interactions, electrical leakage and dielectric change effects, and eventual moisture and contamination penetration. In addition, most plastic materials are limited by their sensitivity to operating and process temperatures. 3. Circuit stage-to-stage interconneMon must be performed without the use of wires. A total logical modular assembly should be capable of being plugged into an interconnexion system With increasing size reduction, the problems of tieing microcircuit stages together by traditional wiring means become increasingly difficult and impractical. Our studies indicated that high interconnexion density with wires did not appear to offer a significant probability of economical production or high reliability• Hence the requirement for no stage-to-stage wires• Because of the many factors involved in maintaining electronic equipment, and building standardized blocks of circuitry usable in many equipments, it was established that each modular block of circuitry should have a standard plan form factor, variable height, and should be capable of

MICROCIRCUITRY: SPECIAL CONSIDERATIONS

being plugged into some intermodule connexion medium. This would permit maintenance by semiskilled personnel, and would permit logical (trial and error) maintenance in the field if necessary. Standard test points must also be available.

4. Construction and seal materials must be capable of direct extension to 400-500°C continuous operation Our rapidly increasing knowledge of semiconductor materials is leading toward advanced semiconductor elements capable of high temperature operation. Developments in gallium arsenide, silicon carbide, and in the tunnel diode confirm our belief that operating temperatures in the 400-500°C range appears feasible in the future. This goal was therefore established for our development efforts from the beginning. We cannot meet this today. However, by working toward this goal, we are aware of where additional development work is necessary to permit us to meet it on an overall basis. At this point the temperature limitation is still the semiconductor device. In addition, many of the processing temperatures utilized in the creation of microcircuits dictate the use of wafer and interconnexion and protection materials that can withstand these high processing temperatures today. We did not want to compromise our electronics and our electronic capability by the use of low temperature materials, particularly when our studies indicate that compromise is unnecessary. 5. Maximum reliability must be achieved through the use of basic material applications and reduction in number of joints and connexions Reliability is best achieved by detailed technical examination of each of the materials and constituents that form our electronic circuits. With increased basic scientific knowledge and understanding of materials, we find that it is possible to employ highest reliability and highest stability basic materials in our electronic circuit fabrication. Avoidance of the use of complex organics and stressing the use of basic ceramics, glasses and metals provides the first step in achieving basic, reliable electronics. Resistance to nuclear radiation damage is an added "fringe benefit". This helps to make "fixes" unnecessary in the future. A dis-

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cussion of the reliability improvement to be achieved through a reduction in the number of joints and connexions is almost unnecessary. Failure analysis of marly of our military electronic systems has indicated that there is almost a linear improvement in reliability with joint reduction.

6. The construction, assembly and protection techniques must be capable of economical, automated mass production. Price target: lower than conventional present-day printed circuit techniques No scheme of fabricating, protecting and interconnecting microelectronic circuits is worthy of recognition unless it produces ultra-small electronics with superior reliability, at prices which will ultimately be below those of our conventional electronic equipment today. Because the techniques and materials used in forming electronic circuit stages will change as time goes on, we can expect a high obsolescence rate in equipment used to produce the electronic materials and circuits. But the method of protecting and interconnecting should not be obsoleted. In finis way we can justify an investment in automated assembly equipment at a very early date. Quality automated assembly, with the resultant reduction in hand labour, should offer both reduced prices and higher reliability. Preliminary cost studies performed on the Sylvania microminiature module and circuit construction techniques indicate that with sufficient volume, our circuit fabrication prices can indeed be lower than present-day conventional parts assembly prices. 7. Immediate parts density objective: 600,000 to 1 million parts per cubic foot. Long-range objective: more than 300 circuit stages per cubic inch The figure of 600,000 parts per cubic foot was reached on the basis of space and volume analysis of our current microcircuit technology. The density figure is not based upon the multiplied parts density or volumetric efficiency of a single component or single circuit wafer. Rather, the parts objective relates to major portions of a full system. However, this is the numbers game. After all, if we build a solid state or thin film flip-flop, who counts parts when they are not there. We look forward to a new standard of comparison which might well be circuit functions per cubic inch.

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G. J. S E L V I N

rim on the wafer. This is done at temperatures in the order of 600°C without exceeding 150°C in the circuit zone of the wafer, and below 100°C in the wafer center. Obviously, high temperature seals are not created using furnace techniques when we must deal with such temperature sensitive devices as 1. The wafer element. This is a nominal 0.5 in ~ germanium transistors. By the use of high intensity by 0.010 in. thick, on which the circuit stage is local heating in the hat rim zone only (and cooling formed. There is sufficient working surface on one the wafer centre) we create the hermetic seal wafer to permit immediate application of a concapable of withstanding temperatures of at least servative minimum of five electronic parts. (We 200°C continuously. This is a true glass-to-ceramic presently work to a conservative average of seven seal. Dry air and inert gasses can be under the parts per wafer, and occasionally achieve more hat if the seal is made in the proper atmosphere. than twelve). It should be noted that sufficient In the future, as semiconductor materials can surface area has been provided that the fabrication withstand higher temperatures for short periods techniques are not limited by too small a physical of time, it will become easier to create the necessary s,ze for efficient processing. The positive prothermal gradient between the hermetic seal zone truding tabs on the edge of the wafer contain and the balance of the circuitry. In fact, the higher fired-on conductive material coming from the the temperature at which the seal can be made, circuit area of the wafer to the tabs for connexion the broader the suitable choice of suitable glassy into the intercormexion boards illustrated. The materials and the easier the process becomes. interconnexion procedure will be described later. When the required circuit wafers have been Wafer material is presently standardized as 96 + formed and protected, and each hermetically per cent alumina in two surface finishes: ground sealed from its neighbours and from the atmoto a 10 pin. smooth surface, or with a thin (under sphere, no encapsulant has been employed, and 0.001 in.) smooth glazed finish on it. Each will no foreign material othe~ than a dry inert gas withstand processing temperatures of over 750°C. contacts the electronic materials of circuit conNote that around the perimeter of each circuit struction. wafer is a black ring. This material is a glass Fig. 3 illustrates circuit wafers with a glass hat, material that has been fired to the edge of the a ceramic hat and a metal hat fused in place. wafer at over 600°C prior to circuit formation. When each wafer has been processed, and trans3. Interconnexion boards. These are employed to formed into a tested and adjusted circuit stage, the interconnect specific circuit wafers to form a next step in our process is to provide hermetic module. The board is presently formed of Coming protection from the atmosphere and from each Fotoceram, and is also being procured in 9 6 + per neighbouring circuit. cent alumina. The slots in the board are formed 2. "Hat." A "hat" is fused to the wafer rim to during the fabrication process and are spaced on a provide the circuit protection. Hats are presently standard pattern. The length of the board varies, being formed of glass which is 0.006 in. thick, depending upon the number of circuit wafers alumina ceramic of 0"008 in. thick, and glazed within a module and the height of each individual metal, also in the order of 0.008 in. thick. The circuit wafer with its fused-on hat, which defines hat can be of any modular increment of height the number of slots skipped and used. Inter(0-010, 0.030, 0.050 in. high, etc.). The height of connexion wiring from circuit wafer tab to circuit the hat selected and the material from which it is wafer tab (board-slot-to-board-slot) is screened made is a function of the particular circuitry being and high temperature fired on the interconnexion protected. Our goal is that any type hat can be board surface. The wafers are then assembled such fused over any circuit. This process is under that their slots fit through the four boards at the advanced development in our laboratory now. proper points and the conductors on the wafer Only the rim of the hat is fused to the glazed tabs are then soldered to the conductors on the THE SOLUTION TO THE ENGINEERING

OBJECTIVES Fig. 2 is an artist's exploded view of the Sylvania Microminiature Module. From the illustration one can see the three basic parts which form the module.

FIG.

FIG.

2. Sylvania

3. Hermetic

module-artist’s

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circuit

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FIG.

FIG.

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5. Modules

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MICROCIRCUITRY: SPECIAL CONSIDERATIONS

interconnexion boards. In this manner we interconnect circuit stages without the use of wires. \Ve anticipate a very high degree of reliability, based upon the reduction in the number of joints, ioint ruggedness and absence of small individual wires. After the complete microminiature module has been assembled and retested, a very thin coat of plastic (in later higher temperature developments we anticipate the use of glass) is applied over the entire assembly with the exception of the ends of the interconnexion boards. This protective film is in the order of 0"002-0"003 in. thick, and its only function is to protect the conductors on the interconnexion boards from atmospheric corrosion. The number of circuit wafers in a module should be dictated by the system requirements and the requirements for defining logical blocks for common replacement in maintenance. On this basis, from as few as two to as many as twelve individual circuit stages can be placed in one module. MODULE INTERCONNEXION It is our conviction as a result of the st udies previously described, that each microminiature module should be capable of being removed with,ut disturbing any other. Module-to-module direct connexion should be avoided except in special cases, since it tends to complicate trouble ~hooting and maintenance and also unnecessarily increases the module throwaway cost. Modules can be disassembled by removal of the interconnexion boards and individual circuit wafers replaced, but we believe this will be a home factory, operation rather than a field maintenance operation for many years to come. The Sylvania method of interconnecting microminiature modules is represented in Fig. 5. The process has a visual similarity to conventional printed circuit board electronics. However, there are some significant differences. The board is a molded alumina ceramic, in which connector finger holes are formed as an integral part of the board fabrication operation. One or both faces of the intermodule connexion board receives conductive material screened and high temperature fired in place to interconnect the various slot patterns in the board. Then small pre-formed metal connector fingers stamped from high nickel

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alloy are inserted into the board slots and furnace fused to the wiring at approximately 610°C. In this manner we achieve a printed circuit interconnexion te.chnique without the use of connector bodies. We also avoid the use of plastic materials or adhesives which might be susceptible to later peel or delamination. We have avoided processing in the form of machining or hole drilling in forming the board (it is a one-step die-formed piece). One or more edges of the ceramic intermodule connexion board can be used to plug-in to a conventional printed circuit board connector. Where highest connexion reliability is required (for example, in missile and high shock and high vibration equipment) spring-type friction connectors are not desirable. In these instances the same intermodule connexion board and wiring technique and spring connector fingers are used. Only one step is added. The spring connector fingers are pre-tinned with a solder alloy. By application of heat to the spring connector fingers where they come through tl'te reverse side of the intermodule board, we can cause heat to flow up through the metallic spring connector fingers to the module interconnexion board. In this way we can solder the microminiature module to the spring connector fingers and not rely upon friction contact. No change in construction or materials is required. The operation can be performed in the field without special tools. Because the board and wiring and fingers are capable of withstanding high temperatures, they will not loosen or be deteriorated by the application of conventional soldering temperatures for extended periods of time. A special soldering iron tip has been fabricated which contacts all twelve of the connector spring finger bottoms simultaneously. R A N G E OF CIRCUITS CONSTRUCTED

Fig. 6 is an illustration of five different circuit wafers before opaque "hat" seal application. Our objective is to show a part of the range of devices and circuits that can be accommodated and that have already been formed and tested in our laboratories. The number opposite each circuit wafer corresponds to the following description: 1.20 Mc/s divider wafer Two wafers constitute a 20 Mc/s flip-flop. Illustrated are four metal film resistors

G. J. S E L V I N

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(500-24 k~), two ceramic capacitors (240 pF), a silicon mesa transistor and a silicon diode, for a total of eight parts. 2. 150 Mc/s oscillator amplifier wafer Illustrated are six parts: A flat-precision etched spiral 4 ~H inductor, a 15 pF finger type capacitor etched in place, three ceramic capacitors, and one 47 f~ metal film resistor. 3.60 Mc/s wide band IF amplifier wafer The particular wafer contains eight parts: One diode, two variable capacitance diodes, two dual two-layer ceramic capacitors (500, 500, 24, 24 pF), and one 47 kf~ metal film resistor.

germanium transistors distributed through the module. The second was a 150°C wafer centre temperature against which we could monitor maxim u m allowable power dissipation when all silicon semiconductor circuitry was employed. Fig. 7

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2"0

"o 1"6 ~

4. Audio frequency amplifier wafer Six parts are in place: One transistor, one 100 pF capacitor, three metal film resistors (1.5-47 kf~), and one 4"7 ~F 6 V tantalum slug capacitor.

5. Digital circuit wafer This wafer contains a solid state 200 kc/s flipflop and a thin film power pulse amplifier containing a silicon transistor and three metal film resistors. HEAT

DISSIPATION

Because of limited time and manpower, we have not secured power dissipation limits on the module in all conceivable power dissipation configurations. However, to determine a worst case condition we made the following test set-up: A six-wafer module was constructed. On each wafer we vacuum deposited a metal film resistor for power dissipation, and two parallel thin film thermistors, also by vacuum deposition. The thermistors on each wafer were calibrated, and then all six wafers were assembled into a module. The module was placed in a constant temperature oven, supported on a 0.010 in. thick ceramic intermodule board. No other heat sinking was used during thermal dissipation tests. An equal amount of power was put through the metal film resistor on each of the wafers simultaneously. This provided the best simulation for a uniform thermal distribution throughout the module. Two wafer temperature limits were established. The first temperature limit of 80°C was established to determine what the allowable power dissipation per module would be if there were

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FIG. 7. Six-wafer module power dissipation limits (uniform dissipation per wafer). illustrates the decreasing power dissipation permitted within the module as the ambient (oven) temperature was raised. It is significant that when we utilize all silicon semiconductor circuitry we can dissipate at least 0.2 W at an ambient temperature of 100°C. The test module is the equivalent of six full active circuit stages compressed into a volume of less than 0.1 inL As one might anticipate, very few circuits utilize uniform power dissipation throughout the signal processing train. Tests will be performed in which the circuit wafer which contains a high power dissipating element is placed at the very top of the module, where the thermal generating element is separated from the ambient environment by only 0"010 in. of alumina ceramic. We anticipate significantly higher allowable power dissipation in the module if the hot spot is placed at the top wafer. In addition, metallic fins can be brazed to the ambient side of the top wafer to increase the thermal dissipation. In short, the thermal dissipation tests plotted in Fig. 7 are a conservative or "worst case" limitation.

MICROCIRCUITRY: SPECIAL CONSIDERATIONS PRESENT COMPONENT VALUE LIMITATIONS As one might anticipate, we cannot make available to the system and transistor circuit designer the tremendously broad range of electronic component values that are presently available in standard parts form. However, it has been our experience that the circuit designer can perform well within the parts value limitations which are available today when one utilizes a combination of solid state, thin film, and discrete part microcircuitry married into a single functioning unit. Table 1 is an abbreviated list of the range of circuit

Table 1. Abbreviated list of available component values Resistors Capacitors Ceramic Tantalum slug

1 f2 to 1 Mr2--0.1 W max. dissipation preferred (High stability metal film) (2 W max.)

0"001 pF to 0'01 ~zF--Dielectrie bodies from NPO to K 10,000 Up to 200 ~tF-V--6 V min., 35 V max.

Etched finger Inductors Flat spiral Wound ferrite Transistors Diodes

Up to 15 pF--High temp. stable Up to 2"5 F.H--Q to 85 at 60 Me/s (High temp. stable) Up to 1 mH (Ex: 20 ~tH--150 Mc/s - - O 100) Silicon mesa or planar structures preferred Almost any device can be micromounted

parts functions that are achievable today, and that we have used in our circuitry to demonstrate feasibility. As one might anticipate, this list is continuing to expand. In addition, continuing results of research and development are permitting us to build more effective circuit values in less space with more stable materials, which should offer higher reliability. At this date, inductance values above the millihenry range are our chief stumbling block. However, since these occur infrequently in electronic systems, and then generally in small quantities toward the final output of the system, there is no reason why standard parts cannot be employed in conjunction with microminiature modules to perform the system function. Fig. 8 is a photograph of a standard breadboard experimental model of a major portion of a c o m -

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munications receiver being built by Sylvania for the United States Air Force Communications Laboratory, Dayton, ()hio. Visible in the photograph are a five-stage 60 Mc/s I F amplifier plus emitter follower output stage, AGC and audio amplifier section, a 60 Mc/s low noise I F preamplifier, and a wide band video amplifier. Also visible in the photograph (rear right) is the eight microminiature module assembly which is the performance equivalent of this communications receiver, and also contains no parts compromises in the sense that every circuit part present in the standard receiver breadboard is present in the microminiature version. No circuit forming corners have been cut or short cuts taken to match the performance. The receiver circuitry is completely redundant, containing both parallel path failure systems, parts redundancy, and circuit stage redundancy. The basic standard-parts receiver design has operated for over 8000 hr without a single failure, and it is calculated to perform for over 30,000hr mean time between failure. The microminiaturized version, being an experimental model, will probably not match this low failure rate accomplishment. However, as we gain experience and practice, we see no reason why the microminiature version should not exceed the standard parts version in operating reliability. FUTURE RESEARCH DIRECTION It is our conviction that new microcircuit forming techniques will develop in two parallel directions: Thin film circuit techniques and circuits formed on single crystal semiconductor substrates. Both are compatible, and mixtures of both disciplines will be used together. Groups within Sylvania are pursuing both approaches, with my organization concentrating on thin film techniques. Specifically, we are attacking the problems of vacuum or gas atmosphere deposition of high dielectric metal oxides by means of high energy electron beam evaporation. We are also working on ultra-high vacuum electron beam evaporation of refractory metals and insulators to create higher temperature and power tolerance films. Success in these endeavours will broaden the types of microcircuits that can be built, increase the circuit density per wafer, and raise the allowable operating temperature limit. All three benefits are essential.

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G. J. S E L V I N CONCLUSION

This paper presents the reasoning behind Sylvania's establishment of a microminiature module construction, connexion and protection scheme that can accommodate mixtures of the various disciplines in forming microcircuitry. It is intended to be as obsolescence free and to have the greatest growth potential possible. It is our belief that it can form the foundation for a method

of standardizing the form factor and protection and connexion techniques without dictating to any circuit forming organization which materials or circuit formation techniques are employed. Acknozdedgement--The work and developments described in this paper (except for the receiver application in Fig, 8) are funded by Sylvania Electric Products Inc., a subsidiary of General Telephone and Electronics Corp.; a number of patents are pending on these developments.