IC Materials For Extreme Conditions

The number of materials being researched for chips used in extreme environments, such as landing on the planet Venus, is growing.

While GaN has captured much of the attention for power conversion circuits, it’s just one of several applications for semiconductors in extreme environments. The high voltage, high temperature, and caustic atmospheres found in many industrial and aerospace environments can subject devices to conditions far beyond the operating envelope of silicon.

While appropriate packaging often can protect components from such harsh conditions, doing so adds weight and system complexity. Materials like SiC and diamond that can tolerate extreme conditions may offer benefits well beyond their electronic figures of merit.

On Venus, surface temperatures are in the neighborhood of 500°C, with pressure comparable to an Earth ocean depth of about 900 meters. The carbon dioxide atmosphere seethes with sulfuric acid clouds. Moreover, as with all space destinations, every gram of material needed to protect components limits the overall mission capabilities.

Phil Neudeck, lead semiconductor electronic device engineer in NASA’s Silicon Carbide and Sensors Research Group, said NASA’s original Venus lander mission design, proposed in 2009, called for a 686-kilogram sealed pod with an operational lifetime of just 5 hours. In work presented at December’s virtual IEEE Electron Device Meeting, he said a revised design with electronics that can tolerate the environment weighs just 20 kg — most of that battery. And more importantly, NASA is now able to contemplate a 60-day mission plan.

Diamonds are everyone’s best friend
Using diamond in semiconductors adds a whole different set of benefits for high temperature environments. Dopants in diamond are incompletely ionized at room temperature, and carrier concentration increases at elevated temperatures. This behavior, the opposite of that seen in GaN, makes operating temperature hugely important for device performance.

Mobility decreases with temperature, though, so electrical conductivity in diamond peaks at around 150°C to 200°C. Drive current also increases with temperature, but so does leakage current.

Cristian Herrera-Rodriguez of Michigan State University, in work presented at December’s virtual Materials Research Society meeting, reported that high temperatures reduced the On/Off current ratio of their MESFET devices from 1 x 10⁸ to only about 1,000. These devices were made from intrinsic CVD diamond on commercial diamond substrates, followed by p- and p+ doped layers. After mesa and source/drain etching, they made Ti/Au ohmic contacts for the source and drain, plus a Mo/Au Schottky contact for the gate. Mesa etching and ohmic contact formation are both important process challenges.

Diamond also offers a wide bandgap (5.45eV), a high breakdown field, and excellent thermal conductivity. Yasuo Koide, director and group leader in Japan’s National Institute for Materials Science, explained in work presented at MRS that a hydrogen-terminated diamond surface gives good conductivity with a 2-dimensional hole gas, while the oxygen-terminated surface is not conductive.

In research for NASA, researcher Robert Nemanich and colleagues at Arizona State University discussed n-i-p Schottky diodes on boron-doped diamond. In these devices, which depend on injection mode transport, current density was described by the Mott-Gurney expression:

Current density (J) flowing through a thin slab depends on the permittivity of the slab (ϵ), the carrier mobility (μ), and the square of the applied voltage (V_a). This expression implies non-Ohmic behavior in intrinsic semiconductors, but is only strictly applicable in the absence of defects.

Though diamond MESFETs are very thermally stable, the use of a metal-semiconductor junction limits the available gate forward bias and the maximum drain current. To improve the current capability, Herrera’s group also created similar MOSFETs with Al₂O₃ gate dielectrics. These devices were normally off at room temperature, probably due to trapped interfacial charge, and turned on as temperature increased. Unfortunately, the Al₂O₃ deteriorated at elevated temperatures, reducing the breakdown field.

Koide’s group demonstrated a new metal-insulator-metal transistor (MIMSFET) on diamond, which he described as a combination of a MOSFET and a MESFET. It’s a normally “off” device, thermally stable to 350°C, with low leakage current and a high on/off ratio. Starting from that concept, they also built a MOSFET with a nanolaminate TiO₂/ Al₂O₃ gate. The gate capacitor offered a k value of 68.7, giving a very large maximum current.

Because of the lower activation energy of boron (0.34 eV) in diamond relative to phosphorous (0.58 eV), only p-type diamond devices are feasible at this time. Inversion MOSFETs and complementary logic both need both n-type and p-type doping. Surface quality is critical because diamond depends on surface conduction, but it’s hard to get a flat, smooth surface in phosphorous-doped diamond. In work presented at MRS, Satoshi Yamasaki, professor at Kanazawa University, and his colleagues used H₂O annealing to get a flat OH-terminated surface, which improved both drain current and channel mobility. They believe OH-termination reduced the number of interface trapping states.

Reliance on surface conduction makes diamond devices vulnerable to surface trapping and non-uniform heat generation, too. A fin-based vertical transistor based on bulk conduction would help mitigate these issues while still exploiting the superior material properties of diamond.

At MRS, MIT researcher John Niroula and colleagues applied models developed for GaN finFETs to diamond-based devices. Besides the incomplete ionization of boron, the model needed to account for a metal-insulator transition as a function of doping, and for the importance of hopping conduction. The modeled devices reached a linear power density of 7.36 MW/cm2, about 2.7x higher than a comparable GaN device. Unfortunately, achieving bulk conduction in diamond remains challenging, as ion implantation tends to graphitize the structure.

Silicon carbide electronics mature
While development of diamond devices for extreme environments is in the early stages, SiC integration is approaching a viable technology platform. Neudeck and colleagues demonstrated stable operation over a 1,000°C temperature span in NASA’s “Venus cell,” without changing the input signal or supply voltages. Their device is based on SiC JFETs, with TaSi₂ metallization completely buried under SiN. Burying the interconnects protects them from oxidation and exposure to the caustic environment. Similarly, JFETs, rather than MOSFETs, were used because gate oxide stability is a problem for MOSFETs under these conditions.

“There’s nothing more stable than an epitaxial SiC p-n junction,” Neudeck said.

In fact, both SiC and diamond are stable under extreme conditions. The more difficult challenge has been making stable interconnects and stable contacts, and protecting them from cracking and oxidation. The two main failure mechanisms in high temperature SiC devices were migration of gold and atmospheric oxygen toward the contact interface, and inter-metallic mixing between the diffusion barrier layer and the contact metallization. Robert Okojie, a research electronics engineer at NASA, and his colleagues used a Ti/TaSi₂/Ti/Pt stack with a TiPt diffusion barrier to contact n-type SiC. Devices were reliable for more than 100 hours at 800°C. Though diffusion did occur, device characteristics stabilized after this burn-in period.

Further reducing feature sizes from NASA’s Gen 10 (~200 transistors) process has been challenging, though, with interconnect yield and durability issues increasing at higher densities. Clearly substantial process optimization work remains. On the other hand, Neudeck said, advances in interconnects are portable across device technologies and should be applicable to both SiC and diamond electronics.

Bringing Venusian electronics to Earth
While planetary exploration is a tiny, specialized market niche, engine design, industrial chemistry, and power plants are just a few of the earthbound applications with similarly demanding requirements. First Venus, but then, perhaps, a jet engine near you.