High Temperature Electronic Aspect of Venus Landers

To explore the planet Venus has been always an interesting subject for mankind. Improving and understanding of Venus is very important. It is second planet from the Sun, It orbits sun at 224.7 days earth. Atmosphere of this planet is very challenging for any kind of surface exploration, atmospheric composition of Venus is approximately 96.5% CO2, 3.5% Nitrogen and traces of SO2,HCL and HF, however atmosphere in Venus is thicker than Earth so nitrogen contents is almost four times than that of Earth. Our main goal is to design a probe that will survive on the surface of Venus, so we would focus on the condition of troposphere that stretches from the surface to 65 km radius of the planet. Air density at surface is 67Kg/m3, temperature at surface is 467°C and pressure at surface is 90 Earth atms.

High Temperature Electronics play a vital role in missions to Venus. These technologies are still being developed for Venus planetary exploration applications. Due to lack of electronics that collect and transmit the data in Venus’s +450°C, almost all of the proposed missions were for very limited Duration to explore this planetary environment.

In the past, few landers have been sent to the surface of Venus but due to the lack of high temperature electronic components and old technology, they didn’t survive more than almost 2 hours.

Main Problem

• Atmospheric conditions at Venus are very Extreme so prolonged operational Time for High Temperature Electronic circuit is difficult.
• Due to very High Temperature (About 500°C) and Pressure (About 90 atmospheres ) at Venus the Survivability of the active device, passive device and the packaging materials is a challenge.

State of the Art

Many Agencies have deployed some spacecraft to Venus, more than 23 so far, such as Landers, Balloons, and Probes. Here we have a quick look at the pervious Venus landers:

As can be seen, in all cases the maximum survival time on the surface was 2 hours and 7 minutes. Beside old mechanical technology, no high temperature batteries/electronics were used for these landers.

Thanks to the modern technology, instead of a heavy and big lander, we can have a small

Rover. Due to the hellish situation of Venus, for a long time surface exploring, the rover contains some special components like:

• Pressure vessel integrated with advanced thermal control.
• High temperature electronics with Low power.
• Some components e.g. sensor and/or telecomm at 480°C.
• Rapid sample collector and analyzer system at 480°C.
• High temperature energy storage.
• Cooling system.

One of the most important parts of a Venus rover which directly connected to high temperature electronics is the pressure vessel with advanced thermal control. It protects the brain system of rover from harsh environment of Venus’s surface (pressure, heat and oxidation). Advanced thermal control system which fits inside the pressure vessel serves two functions.

The first is to minimize the heat transfer from the environment to the electronic devices and the second is to match the heat generated by the internal electrical components such as power system, transmitter, and instruments.

Some of Technologies have been Designed and Operated

• Solid state Material Devices Use which can operate at High temperature such as Silicon and Silicon Carbide.
• Thermionic Vacuum devices are inherently high temperature devices, as they are designed to run between 600°C to nearly 1000°C. These of the 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.

Recent generation of semiconductors, including the silicon carbide (SiC), diamond, and gallium nitride, has enabled the short-term demonstrations of electrical Devices at temperatures from 550°C to 650°C. Until now these devices have given only very few hours durability for operation at these high temperatures. For a long-duration the stability of these devices is very important. NASA Grc developed the SiC-based technology transistor for continuous operation at 500°C for over than 3000 hours.

In this essay, I am going to focus on high temperature electronic aspect of Venus Landers and take a quick look at NASA GRC’s test results on high temperature components and finding the best materials and devices.

Solution

I am going to explain the solution in detail. Generally, high temperature electronic components for a Venus lander divided into three categories: Active devices, passive devices, packaging materials and high temperature pressure sensors. We are going to take a look at these categories and their subsets.

Active Devices

Solid state (SOI Devices): By using an active thermal control which contains a powerful cooling system, we can keep the temperature of inside of pressure vessel ~300°C (theoretically). In that case, Low power SOI-based electronics which operating at 300°C, can be considered for use inside the thermally controlled Venus Lander. These electronics are currently used in oil drilling equipment. Such electronics would help to relieve thermal control load and dramatically increase mission survivability and lifetime and also leakage current may be managed.

Solid state (Wide Bandgap Devices)

By using passive thermal controls, wide bandgap semiconductors have to be considered for temperature above 300°C. Taking a look at table above, it can be seen that Gallium Nitride, Silicon Carbide, Diamond and Thermionic Vacuum Devices are all be able to operate at temperature of planet Venus (~500°C). But considering facts such as cost, lifetime and range of technology, Silicon Carbides (SiC devices) are the best choice for this task.

However on paper, maximum operating temperature for SiC is 600°C, but problems with diffusion and oxidation of metal contact layers dramatically reduce both operation temperature and lifetime. In that case, proper metal layers must be chosen to reduce diffusion and oxidation. Also other facts such as integration and metallization process must be considered.

Recently scientists at NASA Glenn team invented new type of SIC device. 4H-SiC based JFET integrated circuits (24 transistors, with 2 levels of the metal interconnect) and ceramic packaging for over 1000 Hours consistent function at 500°C for testing in Earth’s atmosphere. This is a one big step forward for Venus landers because this SIC device doesn’t need cooling or thermal system or even a pressure vessel. The current, voltage and some key parameter characteristics are very good as shown below.

Steps of Venus surface test on 4H-SiC JFET.

a) The complete assembly showing SIC Ring Oscillator chip before heated testing.

b) Complete assembly prior to the heat testing show the mesh screen cap that permits chip

immersion in simulated Venus atmosphere during the test.

c) And after the 1000 hours of s Venus surface conditions testing the following mesh screen cap removal.

d) During the electrical testing conducted with the chip is disconnected from the short circuited feed through by the removal of nickel alloy wires.

Thermionic Vacuum Tubes

It is inherently high temperature device which controls the electric current between the electrodes in evacuated reservoir. Vacuum tube relies on thermionic release of electrons from the hot filament. They are among one of first and oldest high temperature electronic devices. Although by inventing transistors in 1950s these tubes almost faded from electronic industries, they are still useful for some cases related to microwave, high-frequency amplifiers and especially for pressure sensor pre-amplifier for Venus rovers. The figures below show vacuum tubes and a concept of inner components.

For Venus rovers, using TVTs can be considered because the cathode is designed to run at 700 to 900°C. Yet, these tubes still need to be optimized for harsh environment of Venus because there are some challenges ahead of using them for Venus rovers. Challenges related to packaging, high level of integration, lifetime, power supply, size and weight. The graphs below indicate the Influence of temperature on thermal vacuum tubes.

Recent tubes use the cathode of filament. This is a direct heated tube. The figure and concept below show vacuum tube including the heater and cathode and grid and anode.

As mentioned before, TVT devices can be used for high temperature pressure sensor amplifier in a Venus rover. The thermionic vacuum tube pre-amplifier was evaluated at room

temperature and 500°C. The results are shown below:

Passive Devices

Resistors: For a Venus rover, resistors are essential needs to enable a high temperature wireless system. High temperature resistors are one of the big challenges for Venus missions and this technology is still under development. Currently H.T. resistors are able to operate at +500°C but with limited lifetime. There are some factors for choosing the best resistor for Venus rover such as:

• Noise
• Thermalstress
• Interdiffusion
• Oxidation

By looking at table, we can see that Ruthenium Silver and Ruthenium Oxide resistors have highest maximum operating temperature. Also Ruthenium Oxide has superior stability, low thermal stress and low noise. But the main problem with these two resistors is that ruthenium is very rare and hard to find. In that case the cost of these resistors would be high. So technically we should look for another type of high temperature resistors.

On the other hand we have Thin-Film and Thick-Film resistors. The noticeable advantage of these resistors is that they rely on ceramic substrate and they don’t need mechanical attachment anymore. Recently a new type of thin film (NiCr) nickel chromium resistors have been introduced by the scientists. The resistors NiCr are stable within nearly 10% to the 300°C. They have high rate of stability and low noise (not good as ruthenium resistors). But the most advantage is the lower price. In my opinion, and considering the cost, stability and maximum temperature factors, NiCr thin film resistors are the best choice for a long time surface mission on Venus. Capacitors: Beside resistors, capacitors are also key elements of high temperature wireless system for Venus rovers. Current capacitors are not ready for a long time surface mission on Venus. This technology is still developing. Innovations on all fronts, including materials, device designs, and packaging are being pursued. Before making a capacitor for extreme harsh environment, we should consider some parameters such as capacitance,, leakage current, equivalent series resistance, voltage rating, dissipation factor, dielectric absorption, and volumetric or weight efficiency. Here we are going to take a look at some of famous high temperature capacitors.

X7R: Capacitance is a strong function of temperature. High current leakage at the elevated temperature.

NP0: Stable up to nearly 500°C with zero coefficient of capacitance. Dissipation problem at high temperatures.

Piezoelectric: Composition selected to peak in capacitance and dissipation factor for specified temperatures. Difficult to implement.

Diamond: Theoretically functional to almost over 500°C with stable, high capacitance. Still under development to achieve uniform diamond film and stable metal contacts.

Air Gap/Parallel Plate: Low capacitance, but stable over entire range of temperature. Very large area capacitors would be required.

All mentioned capacitors have limited life time for harsh environment of Venus but recently, a new design for high temperature capacitors has been invented by scientists. It is known as MIM (Metal Insulator and Metal) capacitors.

MIM capacitor is composed of the two parallel plates with dielectric layer between plates. Microstrip line is connected to the each plate.

I believe MIM capacitors which based on SiC technology are the best choice for Venus rovers. Yet these capacitors still need optimization due to limited life time. Expectation is by +2020, scientist can make MIM capacitors with long lifetime.

Oscillators: Oscillator is One of important component of the wireless sensor system for signal generation, which is modulated by sensor and the data will transmit to the cooler environments.

At this moment, the Glenn team in NASA is working on high temperature oscillators so that SiC devices advancement can be achieved, as well as for improvement of passive devices. The figure below shows a prototype of H.T. oscillators that has been tested by Glenn team in condition similar to surface of Venus.

Oscillator with SIC MESFET and capacitors with ceramic Chip, spiral inductor, and interconnects.

Packaging Materials

For harsh environments, packaging is require for operation of Sensors and electronics technologies beyond those for regular electronics and sensors. For Venus missions, sensors and electronics must to operate at the temperatures of about 500°C and the above. For that case, the packaging materials and basic components, such as substrate, metallization material(s), electrical interconnections (such as wire-bonds), and die attach must be operable and reliable in high temperature (500°C) and chemically reactive (especially oxidizing and reducing) environments.

These packaging components might also experience high dynamic pressure and high acceleration, depending on the application. These harsh operation environments are far beyond those which commercially available packaging technologies can withstand; therefore, development of high temperature and also harsh environment packaging technologies is necessary to implement high temperature sensors and microelectronics in Venus missions.

Interconnect: The main concern with the selection of interconnects material combinations involves Inter diffusion of the pad and wire metals. The formation of brittle intermetallic phases and voids, due to diffusion at higher temperatures, decrease the strength and conductivity. Such these problems can be reduced via the use of a mono-metallic interface. Al-Al pads with melting temperature of Aluminum and Au-Au with melting temperature Gold are the best choices in order to use for harsh environment of Venus’s surface. But beside temperature, oxidation and stresses must be considered. In that case Au-Au seems to be better choice since it has better stability and reliability.

Die Attach: The main purposes of a die attach material is to secure a die the substrate, to ensure electrical connection to the backside of the die, and to ensure that the die does not fracture following power and temperature cycles. Exfoliation stresses at the edge of the die can cause horizontal crack propagation and die lifting. Strong die attach materials concentrate thermal stresses in the die, which can cause die fracture and lifetime

The researchers of Glenn team of NASA had done some tests on some new die attach materials in a situation similar to surface of Venus. The following texts are some parts of results: “Ceramic substrates and precious metal thick-film metallization have been proposed for packaging of harsh environment electronics and sensors, based on their excellent stability at high temperatures and in chemically reactive environments. As a packaging substrate material, aluminum oxide has acceptable variation of 0.5 inch AlN (Aluminum Nitride) and Al2O3 (Aluminum Oxide) high temperature chip-level packages. AlN PCB designed for AlN packages.

Dielectric constant and dielectric loss in the temperature range from 25 to 500°C for a wide frequency range. AlN was proposed to package high temperature SiC MEMS and power devices because it possesses a low thermal expansion coefficient and high thermal conductivity. Recently, ceramic (aluminum nitride and aluminum oxide) substrates and gold (Au) thick-film metallization based chip-level electronic packages and printed circuit boards have been designed and fabricated for testing high-temperature devices. The electrical interconnection system of this advanced packaging system, including the thick-film metallization and wire bonds, has been successfully tested at 500°C in an oxidizing environment for over 5000 hours with DC electrical bias. Electrically conductive die-attach materials with low curing temperature are being developed for packaging of SiC devices.

An 96% aluminum oxide based packaging material system was successfully used to facilitate the test, previously described above, of an in-house-fabricated SiC MESFET under electrical bias in a 500°C air ambient for more than 2000 hrs. The packaging components continued to successfully operate without observable electrical degradation for the full duration of the 500°C test that exceeded 2000 hours in duration. Further, the demonstration of a functional 500°C amplifier, discussed above, highlights the most recent progress in printed circuit board level packaging and passive devices for 500°C and is a significant step towards 500°C and Venus missions. “

High Temperature Pressure Sensor

As noted, high temperature thermal vacuum tubes can be used for high temperature pressure sensors of Venus rovers. But due to the limitations such as lifetime, these devices are not good as SiC for high temperature pressure sensors.

Customary pressure sensors are temperature limited while SiC-based pressure sensors have a much wider temperature range and have the added benefit that high temperature SiC electronics can be integrated with the sensor. By expanding SiC technology, progress has been made in both SiC pressure sensor micromachining and packaging. The resulting sensors have shown the capability to withstand high temperatures with improved reliability and operation up to 600°C. These temperature ranges are more than adequate for Venus missions. Furthermore, high temperature operation (600°C) of a SiC pressure sensor and anemometer has been previously demonstrated as separate discrete sensing devices.

Conclusion

High temperature electronics play a significant role in Venus missions. Although this technology is still under developing but with help of new generation of SiC semiconductors, designing a Venus rover for a long time surface exploring is closer to reality than before.

During recent years a significant progress has been achieved in both high temperature active devices and electronic packaging. Designing a Venus rover requires more than high temperature electronic. The mechanical part of this task is still a challenge.

NASA (GRC) is now leading the development of sensors and Electronics which is capable in harsh environments (500°) for prolonged time stable operation . This includes the recent development of SiC JFET technology.

For building electrical devices, 4H-SiC JFET with long life time and superior stability is the best semiconductor for extended Venus operations. Working on Venus projects has helped us to improve our knowledge about high temperature electronics in other aspects, such as oil drilling. Sending a rover to surface of Venus for a long time exploration is not yet possible but, it is predicted that within few years (perhaps +2020) we can achieve this goal.