High Temperature Electronics

F. Patrick McCluskey, Thomas Podlesak, Richard Grzybowski

• Publisher: CRC Press
• Year: 2018
• eBook ISBN: 9781351440806
• Topic: Technology & Engineering
• Subtopic: Biomedical Science
• Index: Technology & Engineering

The development of electronics that can operate at high temperatures has been identified as a critical technology for the next century. Increasingly, engineers will be called upon to design avionics, automotive, and geophysical electronic systems requiring components and packaging reliable to 200 °C and beyond. Until now, however, they have had no single resource on high temperature electronics to assist them. Such a resource is critically needed, since the design and manufacture of electronic components have now made it possible to design electronic systems that will operate reliably above the traditional temperature limit of 125 °C. However, successful system development efforts hinge on a firm understanding of the fundamentals of semiconductor physics and device processing, materials selection, package design, and thermal management, together with a knowledge of the intended application environments. High Temperature Electronics brings together this essential information and presents it for the first time in a unified way. Packaging and device engineers and technologists will find this book required reading for its coverage of the techniques and tradeoffs involved in materials selection, design, and thermal management and for its presentation of best design practices using actual fielded systems as examples. In addition, professors and students will find this book suitable for graduate-level courses because of its detailed level of explanation and its coverage of fundamental scientific concepts. Experts from the field of high temperature electronics have contributed to nine chapters covering topics ranging from semiconductor device selection to testing and final assembly.

Chapter 1

OVERVIEW OF HIGH TEMPERATURE ELECTRONICS

1.1 What is High Temperature Electronics?

Commercial and military aircraft that can fly at greater than twice the speed of sound with improved safety, reliability, and maintainability, automobiles with longer lifetimes and greater fuel economy, chemical processes with ultra-precise control and minimal waste — these are just a few of the future products which are possible with the use of elevated temperature electronics. The ability to use electronic systems at elevated temperatures will not only make new products possible but it will decrease the cost and increase the reliability of current products by removing the need for large, heavy, complex cooling systems and the cabling and interconnections required for remote placement of the electronics.

However, there are many technical challenges involved in developing electronic systems that will operate at elevated temperatures. These range from proper IC design, the appropriate use of passives, and the development of robust packaging structures to the use of the latest thermal management techniques. Before attempting to describe all these issues of elevated temperature electronic design, however, we need to define elevated temperature electronics and put it in a historical context.

The 1960s saw the gradual displacement of the vacuum tube from its predominance in electronic products. Tubes were slowly replaced by new devices, first referred to as “solid-state,” but later called semiconductors. Pundits of the time extolled the virtues of these devices, which eliminated the negative features of the old-fashioned vacuum tube, such as high interelectrode voltage drop and the problematic need for tubes to be heated to high temperatures in order to function. The latter shortcoming necessitated stocking filaments and expending power for heating. The reference to electronic equipment as “heaters” was as literal as it was figurative. One of the last remaining tube manufacturers in the U.S. has no heating system. The heat from the tubes undergoing qualification testing is more than sufficient to heat a large factory.

Semiconductors were to change all that. It is ironic, however, that the early semiconductors, particularly those with higher power ratings, generated considerable heat, even without a heater circuit, due to their own operating characteristics. Worse, this heat tended to destroy the devices. The vacuum tube functioned, by and large, quite well at higher temperatures; not so the semiconductor.

Various methods were used to alleviate this problem. Heat sinks were added to absorb the excess heat generated, but greater power ratings required ever larger heat removal systems. In effect, the inability of these devices to function at elevated temperatures has restricted them to relatively benign operating environments. Although this is not an issue in most cases, it can seriously limit the ability to operate electronic systems in some applications where there is a critical need (for example, sensors in the combustion chamber of a jet engine).

What is needed is a new class of electronic components capable of operating at higher-than-normal temperatures and the associated packaging to support them. These components include both semiconductors and passive components. In the past, associated components and packaging were not issues, since if the semiconductors could not survive, it made no sense to develop related components that could. Today, however, their development is a rapidly growing field of technology. This work will examine all aspects of making high temperature electronics a reality, from the basic issues of device physics and material science to manufacturing and relevant applications.

Currently, there is no agreement as to just what is “high temperature electronics,” or, better stated, what constitutes high temperature? Different experts will cite ranges to define the realm of high temperature electronics. These definitions are particularly troublesome when they imply that devices operating at temperatures lower than one expert’s range are not high temperature devices, while another’s range, lower than the first, places them in the high temperature electronics category.

For example, the sensor in the combustion chamber of a jet engine may experience temperatures of 300°C or higher. This is definitely a high temperature environment. However, so is the case of power semiconductors in the electric propulsion system of a military ground vehicle, such as a main battle tank, even though these devices may operate at temperatures no greater than 200°C, with the engine lubricant oil being used as a coolant.

The best definition for “high temperature electronics” is electronics operating at temperatures in excess of those normally encountered by conventional, silicon-based semiconductors or their auxiliary components. These temperature limits have been set by convention at 70 to 85°C, with some special military systems being operated at a maximum temperature of 125 °C. Even this definition has its shortcomings. For the lower levels of the elevated temperature regime, 125 to 200°C, it is often possible to use “conventional” silicon semiconductors by modifying the device design or packaging, or by appropriately derating various device properties. This is a completely acceptable procedure in some cases, especially in commercial applications where there is little additional burden. For high performance systems, such as those used by the military, however, this is unacceptable and a new approach is necessary. Similarly for passive components, the temperature limitations may be related to the choice of a packaging material or manufacturing process or they may be more fundamental requiring a totally new design.

It should be apparent that the realm of “high temperature electronics” covers a wide range of applications and must satisfy different criteria. The main objective of this book is to explore these criteria and the issues in each case. The first task is to describe briefly the applications of high temperature electronics, and how each will affect the realization of the technology.

1.2 Applications

High temperature electronics are essential for harsh environments in which the use of conventional electronics is impractical, such as under the hood of an automobile or in exposed areas of aircraft. Conventional applications in factories, consumer electronics, and laboratories, by and large, are well served by conventional electronics, a multi-billion dollar industry, with considerable investment in manufacturing and service facilities, which would need to be duplicated or modified to make a transition to high temperature devices. This transition, in most cases, is unwarranted.

Commercial and military ground and air vehicles stand the most to gain from high temperature electronics. Marine vessels are not expected to be a large consumer of high temperature devices. Conventional devices should be more than adequate, even when substantial active cooling is required. This is because once one gets past the corrosion problem, a ship is literally floating in one of the best coolants.

Air and ground vehicles, on the other hand, are ideal candidates for utilization of high temperature electronics. They have both regions of high temperature and limitations on their ability to provide cooling. This second point has been the impetus, to a great extent, the development of high temperature electronics.

Air and ground vehicles have severe limitations on the amount of weight and volume they can devote to auxiliary systems, such as electronics cooling. Although the devices producing the reject heat are, generally speaking, small and lightweight, and it is generally easy to remove the heat from the device generating it, disposing of that heat is a somewhat daunting enterprise. The only available medium for accepting the rejected heat is often ambient air. If natural or forced convection air cooling is not practical due to temperature differential, gas volume or ambient dust conditions, liquid cooling, either single or two phase, is necessary. Liquid to air heat exchangers are notoriously large, making severe demands on allowable weight and volume from the vehicles. Such systems also require pumps or compressors, coolant piping, and controls, which make additional demands on weight and volume.

A common misconception is that high temperature electronics do not require cooling. Although adiabatic operation is possible, to a limited extent, continuous operation will require cooling, and more complex cooling than has been required in the past [Mahefky 1994]. High temperature devices do not require as much cooling, or coolants at low temperatures, but require cooling just the same. This apparent advantage quickly becomes a disadvantage, when the coolant and the apparatus needed to handle the coolant are forced to operate at a temperature higher than standard, forcing the use of new materials, also capable of high temperature operation.

Since air and ground vehicles are largely propelled by heat engines, there are hot zones on the vehicle. To operate close to these hot zones is very advantageous, both from the standpoint of measuring operating parameters, and from the standpoint of mounting electronic control elements in critical locations, such as on the engine. This situation is encountered not only in military applications but commercial ones as well. It is often the case that the environment under the hood of the standard automobile requires components built to more stringent requirements than those of military specification components.

This ability to operate in demanding places leads to the second major application of high temperature electronics, that of hostile environments. There are places where it would be quite advantageous to make measurements, even though these locations exhibit quite high temperatures. Beside the previously mentioned combustion processes, there are other such places of high temperature on the earth or, more properly, under the earth. Deep wells for the exploitation of petroleum and other minerals, and geothermal application create a small but vital niche market for high temperature electronics. It will be shown how data must be gathered from deep in the earth, reliably and accurately, in locations where temperatures will reach several hundred degrees Celsius.

1.3 Technical Challenges

As has been shown above, there is a need to produce high temperature electronic components to fill the demands of a number of electronic systems with significant potential for technological and market growth. However, the establishment of a high temperature electronic industry, comparable to the present conventional electronic industry, will require answering many technical challenges.

Comparison to the conventional electronic industry is a very apt one. The principal problems of the current high temperature electronics industry closely parallel the conventional electronics industry in the 1960s. Similar problems presented themselves and these challenges were met. A similar strategy will need to employed to establish a high temperature electronics industry.

The first challenge could not be more basic. The semiconductor materials required to produce high temperature semiconductor components are in short supply or not of sufficient purity to produce high quality devices. While conventional silicon devices can be used to 200°C, higher temperatures to 300°C require the use of silicon-on-insulator (SOI) technology. This technology extends the useful range of silicon by isolating devices on the IC dielectrically rather than by means of reverse biased junctions. As a result, the structure is immune to the problems of leakage and latch-up at these junctions. However, these wafers are difficult to make and only a handful of companies manufacture them. Above 300°C, even SOI technology is not sufficient to stop leakage from rendering the silicon devices unusable. At these temperatures, a wide bandgap semiconductor is needed. The most commonly used material, silicon carbide, when it is formed into wafers from which components are made, is beset with a problem known as micropipes. Extremely small voids, microns or fractions of microns in diameter, form throughout the wafer structure and disrupt the ability of the material to withstand voltage. This is a critical shortcoming for components that perform switching functions, such as diodes, transistors, and thyristors. The problem is most serious for components designed to handle high power, since they tend to be not only high voltage devices but high current devices. This requires relatively large semiconductor devices, increasing the probability of a micropipe being present, thereby rendering the device useless. Progress is being made to reduce these defects allowing for the growth of better material.

The technical challenge of producing satisfactory semiconductor devices does not end with the production of a satisfactory semiconductor wafer. The next step, SiC device processing, has developed into a sometimes daunting task, definitely different from that of processing conventional silicon, as silicon carbide is most commonly used as an abrasive. New techniques are being developed to form semiconductor structures in such a material.

After processing, devices must be placed in a package in order to be used in a circuit. Conventional techniques are often not suitable for this application. At elevated temperatures, common plastic encapsulants begin to decompose. New die attach materials and solders must be developed which can withstand the temperatures without melting or decomposing as well. Packages are also susceptible to intermetallic growth at the wirebonds at these temperatures. For standard wire and metallization materials, this can result in voiding, reduced bond strength, increased resistance, and eventually bond failure. Metallizations must also be tailored for high temperature use so as not to fail by electromigration. The use of new materials and fabrication techniques are currently being investigated for each of these packaging elements. Research is also being conducted on high temperature resins and inks to improve the mechanical, dielectric, and adhesion strength of organic printed wiring boards at elevated temperatures. High temperature solders that are lead-free and have good fatigue resistance at elevated temperatures are also being developed.

Finally, new materials and designs are needed for passive components. One of the most difficult technological barriers to the development of high temperature electronic systems is the need to create compact, thermally stable, high energy density capacitors. Both the capacitance and the dissipation factor of any capacitive component will often change significantly with increasing temperature. However, this is especially true for the large devices of several microfarads or more which will be required for power conditioning associated with electric motors and power switching applications.

Fundamental properties of traditional ceramic dielectric materials dictate that stability of capacitance with respect to temperature and voltage must be sacrificed to achieve large values of the dielectric constant. Temperature compensating capacitors, such as COG or NPO, are highly stable with respect to temperature with a predictable temperature coefficient of capacitance, and few adverse effects of aging. However, they are usually made from mixtures of titanates with relatively low dielectric constants. General purpose ceramic capacitors, such as X7R, are made of barium titanate, and are ferro-electric with large dielectric constants, but as such they exhibit wide variations in capacitance with increases in temperature, particularly as the dielectric constant is increased. In addition, the leakage currents become unacceptably high at elevated temperatures making it more difficult for the capacitor to hold a charge. Capacitor manufacturers are actively involved in searching for materials that will provide increased capacitance and increased temperature stability. Glasses, glass-ceramics, and even high temperature polymers such as acrylates, polyimides, and fluoropolymers are being investigated as potential dielectrics.

1.4 Summary

There is a definite need for high temperature electronics, driven by the requirements of systems operating under harsh conditions. To achieve a viable industry capable of producing components to meet these needs, technical challenges largely in the area of materials and the integration of these materials into a device structure must be concurrently addressed. The technical challenges, though substantial, may be overshadowed by the perception of a limited market for such high temperature devices. Here again, lessons may be drawn from the experience of the conventional electronics market. The size of the electronics industry, in its current form, could not have been foreseen at the beginning of the development of modern semiconductors. What led to the development of the current large scale commercial industry was the creation of low volume, high performance products for the military, government, and private sectors. Now there is considerable support for high temperature electronics in the various military and civilian agencies of the U.S. government. These applications will be extensively discussed in this work. The consensus of experts in the field is that a military market is developing which is leading to a civilian market, a civilian market that will challenge, in some instances, the current low temperature electronic devices and thus penetrate into applications where conventional devices are currently used.

The balance of this work will address the critical issues, from the applications for high temperature electronics, the production of satisfactory materials, and the fabrication of components, both active and passive to the issue of thermal management at higher temperature. The work will conclude with a discussion of accelerated testing of these components. In a sense, there is no limit in sight for the operating temperature of semiconductors, and maximum high temperature for high temperature electronics has yet to be defined.

Chapter 2

SELECTION AND USE OF SILICON DEVICES AT HIGH TEMPERATURES

2.1 Basic Principles of Semiconductor Junctions

This section reviews important electrical properties of semiconductor materials and pn junctions with special emphasis on the temperature dependencies of these properties. We assume that the reader has some familiarity with elementary semiconductor device physics, but that it will be useful to focus...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Chapter 1 Overview of High Temperature Electronics
• Chapter 2 Selection and Use of Silicon Devices at High Temperatures
• Chapter 3 Wide Bandgap Semiconductors
• Chapter 4 Passive Device Selection and Use at High Temperature
• Chapter 5 First-level Packaging Considerations for the Use of Electronic Hardware at High Temperatures
• Chapter 6 Second and Third Level Packaging Considerations for the Use of Electronic Hardware at High Temperatures
• Chapter 7 Thermal Management for High Temperature Electronics
• Chapter 8 Applications
• Chapter 9 Accelerated Testing of Elevated Temperature Electronics
• References
• Index