Introduction which rapidly degrade component lifetime. As one

Introduction Probes are regularly sent to other planets in our solar system to study about a planet’s atmosphere and its composition. A typical planetary probe can typically consist of the following electronic instruments: ? Temperature and Pressure instruments ? Optical Instruments ? Different types of Spectrometers ? Maybe some acoustic instruments like microphone etc. ? Communication instruments ? Inertial Measurement Units ? Computer Apart from that we would need some sort of electrical power supply for such electronic components as well. The typical electrical composition of the instruments above is: ? Thermocouples ? Capacitive, Magnetic or potentiometric pressure sensors. ? Microcontrollers and other pcbs containing smds etc which are low voltage and very sensitive to radiation and other environmental factors. Also the complexity cannot be very high because for components operating at Venus conditions, technology is not very advanced to support such complexity. Prepping a probe for landing and collecting data on Venus is an especially hard case. Venus has been aptly described as a realistic version of hell as any known to man. The hazardous conditions facing our probe on the planet’s surface are: ? Temps reaching 450 C. ? Pressure 90 times at the surface of Earth. ? Very high carbon dioxide environment with sulphuric acid rains. This leads to chemical reactions like oxidation and diffusion which rapidly degrade component lifetime. As one can see, designing electronic Figure 1. Earth and Venus High Temperature Electronics for Venus Probe 4 components for such a mission require high degree of importance given tohigh temperature tolerance and ability to work in extremely corrosive environments. We also need to present some sort of novel cooling solution and heat removal as well otherwise our components won’t long a very long time. 1.1 History of Venus Planetary Probes Probes have been sent to Venus’ surface in the past. The Soviet probe, Venera 7 was the first probe to land on Venus. Unfortunately, it was put out of operation within an hour by Venus’ high temperature. In 1982, Venera 13 transmitted the first color pictures from Venus’ surface. Uptil now, Venera 13 has survived the longest on Venus’ surface for approximately 2 hours. One can appreciate the difficulty involved in designing a probe for this planets with respect to this point alone. Another way is to actually send a probe to the atmosphere of Venus instead of planet surface. If this is taken into account then the Vega 1 balloon in 1984 survived for 47 hours at a height of 53 Km from the planet’s surface. But for my report, I will just be talking about the probe landing on the surface and look into selecting and implementing novel design ideas for high temperature electronics for a´such a mission. From this point forward, I will be utilizing the following approach for writing rest of my report. ? Divide task into looking at all the electronic component technology into parts for every component. E.g. Active devices (solid state and Vacuum technology of old) and Passive devices (Capacitor resistor etc.). Look into the component in both these categories separately and find out the best suitable component for the task at hand and the reason as to why I am considering them ? Select the best soldering or other connection technology to be utilized and the best components for such job. ? Look for adequate heat and corrosion shielding technology for these components. ? Look for heat removal technology. ? Try to add my own twist into the mix and see if I can maybe provide a little better solution. ? Look into proposed far-fetched technologies whether material science improvements or something completely new for future missions. High Temperature Electronics for Venus Probe 5 2. Active Devices I will be dividing this portion into two further sub-divisions i.e. solid state devices and active thermionic (vacuum devices) 2.1 Solid State Devices Following is a table showing temperature ranges for different solid state materials For our use, Silicon on chip especially with SiMOX technology and materials further down the list can be utilized. We need to use wide band gap devices, so that they are able to accommodate higher charge concentration for higher temperatures. Gallium Arsenide and Gallium Nitride are actually better than Silicon based devices for high temperature but it is difficult to build integrated circuits with small structures. Although research is being taken place to improve Gallium based devices, industry as a whole is not moving in this direction so I will avoid it. The best bet with respect to active devices using solid state material for future Venus missions lies with Silicon carbide. The problem with Silicon Carbide at present is first most integration, just like all non-bulk Silicon solutions, for now. But the lack of integration circuits is not such a big hamper for Venus probe mission. Commercially available devices include Figure 4. Characteristics of Silicon / SiC/ GaN Figure 2 Temperature ranges Solid state materials 1 Figure 3. Intrinsic temperature compared 2 High Temperature Electronics for Venus Probe 6 junction field effect transistors (JFET)s, MOSFETs and BJTs. 2.2 Vacuum Tube Superseded by Solid state devices from 1950’s onwards, the Vacuum tube is being the subject of renewed interest. The major disadvantage of Vacuum tube is its large size and thus inability to be implemented as huge integrated chips. But a lot of miniaturization has taken place for Vacuum tubes. Thermionic Vacuum devices are inherently high temperature devices, as they are designed to run between 600°C to 1000°C. These devices have been demonstrated for operation within a 500°C environment but further optimization is still required. Challenges for the development of this technology include integration and power requirements. For me between SiC and Vacuum tube, I would choose the material depending on the complexity of my application. Vacuum tubes, I think, should be preferred if possible because compared to SiC, we have been using these devices for a much longer time and thus have better understanding and better data and results for it. They are much more reliable high temperature active devices. Figure 5 Vacuum tubes Modern designs 3 Figure 6. Gain vs Temp for Vacuum Tubes 4 High Temperature Electronics for Venus Probe 7 3. Passive Devices With regards to passive devices, I would be discussing the three most important ones for the space probe mission. 3.1 Resistors For resistors, we can utilize two types, thin film resistors and thick film resistors. The very high temperatures lead to following problems for resistors: ? Noise ? Thermal stress ? Oxidation Most of these are mechanical failures for resistors and we can eliminate those using ceramic based substrates with thin or thick film resistors deposited on them. Based on this data, for our application I would choose Tantalum Nitride as thin film resistor because of its super low temperature coefficient and also because in modern time, we like to avoid working with lead so I would choose this over tin oxide. For thick film resistors, we can choose Ruthenium silver. The thin film resistors have superior high temperature performance and are more stable. It therefore competes with other technologies that feature high precision, such as wirewound or bulk metal foil. But on the other hand thin film resistors are more expensive but for a very expensive project like space probe where millions are being shelled out anyway, it’s better to use thin film resistor and add more reliability to our project. Figure 7. Max operating temps for different material resistors High Temperature Electronics for Venus Probe 8 Figure 9. Spiral Inductor 6 3.2 Capacitors Capacitors are hard to develop components for reliable high temperature operation. Important parameters which are affected especially by temperature include capacitance, dissipation factor, leakage current, voltage rating, and dielectric absorption. After having considered many different type of capacitors like Piezoelectric, Airgap/Parallel Plate, Diamond, X74 etc. I saw that all of these had various short comings like capacitance being a strong function of temperature or for our specified temperature range, dissipation factor peaking. Diamond capacitors 5 I think are best bet. They are still in development stage but display stable, high capacitance in 500 Celsius range. Since we have limited area of our probe, I would have suggested parallel capacitors for their stable capacitance but we would have required a lot of them because of their large area. 3.3 Inductors For our temperature range, iron and cobalt core inductors are the safe bet with high cure temperatures. For on-chip devices, Spiral inductors fabricated on high purity semi insulating 4H SiC substrate have also shown promise. Inductors are expected to be fully stable through 500°C with a redesign of the associated air bridges. Figure 10. Typical Inductors Figure 8. Typical capacitance vs temp graph High Temperature Electronics for Venus Probe 9 Figure 12. Die Attach materials 4. Electrical Interconnect technology Due to the extreme temperatures being utilized, normal soldering will not be much effective and we will need to utilize brazing process. The primary problem that can arise is the selection of interconnect material combinations involves inter-diffusion of the pad and wire metals. At high temperatures, due to diffusion and reduced strength, conductivity, this leads to brittle voids. We need to use mono-metallic interface for minimization of such problems. I would recommend Aluminum-Aluminum interconnect using brazing technology for connecting wires. For wires themselves, I would recommend copper wires as their melting point (1085 celsius) is quite high and they have very good thermal conductivity as well (400 W/(m·K)) The other important thing to look at is the attachment material for the components. The function of a die attach material is to secure a die to the substrate, to ensure electrical connectivity to the backside of the die, and to ensure that the die does not fracture following power and temperature cycles. Thus this is a very critical and important part of our electronic assembly which can make or break the entire system irrelevant of whether we had chosen other components optimized for our environment and for the task at hand. Stiff die attach materials concentrate thermal stresses in the die, which can cause die fracture and fatigue. I would recommend using 72 Ag/28Cu die attach material for our Venus probe. Finally, we have to select a substrate material itself. As this is the base on which all of our design be built, we need certain properties in our substrate material, namely ? temperature resistance (min. 150°C permanent) ? electrical insulation ? mechanical stability (strength, elasticity) ? chemical resistance (moisture, solvents, corrosion) ? minimum cost Figure 11. Interconnects with max temps High Temperature Electronics for Venus Probe 10 By valuing in cost and production ease, I would recommend DBC (Direct bonded Copper) inorganic substrate with an eutectic reaction for our thin film design. It provides us with the high temperature performance we are looking for. 5. Heat, Acid and Pressure shielding technology The components of the Venus probe need to be shielded from the extreme heat, pressure and sulphuric environment of the planet. Although we have designed the different materials above which should work under Venus conditions, they won’t last very long without any type of shielding. When the soviets sent the first Venera spacecrafts they had no knowledge of the extreme pressures and their spacecraft buckled. In later models they decided to include a type of spherical pressure vessel with thermal controls as shielding. These vessels were aluminum or titanium shields which are also inert to sulphuric acid. But they are quite bulky and add a lot to the weight and thus cost of the mission. These vessels are tested for Venus pressure i.e. around 100atm and extreme heat. Of all the promising materials for making such a shell, I believe silicon carbide titanium matrix composite shows the best promise. SiC/Ti matrix composite has superior strength/density performance compared to other materials. It is creep resistant at 500ºC. It is suitable for fabricating a monolithic shell configuration. As seen from the graph, Beryllium is also a good candidate with highest modulus per unit mass of all candidates even at 500 Celsius but It is limited by yield and creep. Figure 13. Pioneer Venus Large Probe interior layout 7 High Temperature Electronics for Venus Probe 11 One of the main areas of research for Venus probes is the development of effective thermal insulation solutions capable of protecting spacecraft electronics for extended periods of time from the extreme heat associated with the lower atmosphere of Venus. One of the unique materials that might satisfy our conditions is an aerogel composite being developed by Aspen Aerogels Inc. which supposedly is stable at 2000 Celsius under inert conditions and can provide chemical protection as well against sulphuric acid. The use of thermal mass to maintain an operable temperature range is likely impractical and active refrigeration may be required to keep components at a temperature below ambient. Some sort of radioisotope power converter will need to be utilized for cooling power. Other approaches like solar power (sun light cannot reach surface) or chemical storage (only short term solution) do not approach merit for long lasting heat removal or cooling solution. 6. Summarization After going through all the stuff above, I am presenting a table of my summarized resul