VIKRAM3201: Space grade versus regular processors

Chandrayaan 3 was a landmark moment for Indian Space Research Organization (ISRO). It showcased India's space prowess and demonstrated that space missions can be economical. In 2020, the mission was estimated around 75 million USD. For comparison, Avengers Endgame spent around 200 million USD in just marketing. A moon mission cost lesser than a big budget superhero movie. As a result of this success, the spotlight has fallen on government laboratories. Keeping in line with the theme of this blog, we shall talk about semiconductor devices used in this mission; the one that has gained the most notoriety is the VIKRAM3201 32 bit processor used in the launch vehicle.

Figure 1: VIKRAM3201 processor (Reference: SCL)

Figure 2: Chandrayaan 3 Lander (Reference: Scientific American)

This processor is a triumph of India's semiconductor capabilities. Designed by Vikram Sarabhai Space Centre (VSSC), Thumba, and fabricated in Semiconductor Laboratory (SCL), Mohali, it is a fully indigenous processor. A detailed description of this processor can be found in its datasheet. Anybody familiar with electronics would immediately notice that the datasheet is a bit low on information. Compared to a cheap consumer grade processor like the STM32F407, you can see the difference in the density of information. Well that is because ISRO would like to keep their technology confidential. 

But is that necessary? Let us compare some metrics: a) power consumption of STM32F407 at full speed will be around 130 milliwatts (238 microampere/MHz * 168 MHz * 3.3 V) at full operation, whereas a vague ‘less than 500 mW’ is given in VIKRAM3201's datasheet; b) The technology node of STM32F407 is 90 nm compared to VIKRAM's 180 nm node, this directly contributes to decreased efficiency and lower transistor density; c) operating frequency of STM32F407 is 168 MHz and VIKRAM sits at 100 MHz. Looking at just these three metrics makes us realize that VIKRAM is an inferior processor in terms of raw computing power. Pretty odd as processors developed recently should be more powerful (remember STM32F407 was released in 2011, while VIKRAM was utilized in 2023). Seems like a slam dunk for STMicroelectronics, they developed a better processor a decade before Chandrayaan 3 was launched.

This post would be pretty short if my message was that VIKRAM is a bad device. There is a critical aspect that we have missed this entire time: the operating environment. Space is the harshest environment for a processor. STM32F407 would stop functioning if it ever left the atmosphere. The margin of error is very low for a device and reliability is paramount. The way the device is manufactured is kept secret as no country wants any competitor to know how they managed to figure out space electronics.

What are the troubles a device faces in space? The most talked about problem is that an energetic particle like heavy ions, alpha particles and the like, penetrate the packaging and enters the device which may cause a bit to flip. Basically, a zero may be changed to one, causing wrong information to be processed and completely botching the operations happening after it. This is just one catastrophic failure that may occur, a lot of these effects completely destroy the device from inside making it completely useless. Let us go in order of occurrence:

Figure 3: Phenomenon occurrence by increasing altitude.

1)Launching the spacecraft causes tremendous amount of shock and vibration. The package may be damaged and cracks might form even before reaching stratosphere.

2)Atomic oxygen erodes materials on the device. Oxygen by its lonesome is very reactive, it will react with almost anything available. Ultraviolet radiation causes chemical bond breakage. It accelerates aging in protective layer like encapsulants and adhesives.

3) Entering space means entering a complete vacuum. Any gas particles trapped inside the device will try to free themselves, this phenomenon is called outgassing. It causes contamination and these particles may deposit themselves on sensors.

4) Heat is released by three mechanisms: conduction, convection, and radiation. Convection transfers heat through the bulk movement of fluids (liquid or gas). It allows heat to be removed efficiently and without fast changes. The absence of air leaves us with conduction and radiation. Heat can still be conducted away from a chip through direct contact with heat spreaders, heat pipes, or cooling systems, but it must ultimately be dissipated through thermal radiation into space. In direct solar radiation, the device will face a large temperature increase, and in shadow the temperature drops sharply, causing expansion and contraction cracks.

5) Exposure to ionizing radiation like X-rays and gamma rays happens immediately upon leaving the atmosphere. It slowly degrades the device causing oxide charge build-up, the oxide is used to control the conduction channel in a metal oxide semiconductor field effect transistor (MOSFET). Charge build-up causes the threshold voltage (minimum gate voltage required to switch the transistor on) to shift. A decrease in threshold voltage causes current to flow when the transistor is supposed to be off.

6) High energy particle strikes, as we discussed earlier, cause bit flips and other transient (short time period) events.

Figure 4: Cycle of space degradation (Photoshopped image of reference: Dongwhan Yu et.al 2025)

Extended exposure to such an environment accelerates existing degradation mechanisms such as a) bias temperature instability (BTI), defect formation at silicon/oxide interface due to charge build-up caused by continuous application of gate voltage; b) hot carrier injection (HCI), under high electric field, electrons can gain enough energy to penetrate the gate oxide and become trapped in it; c) time dependent dielectric breakdown (TDDB), gradual breakdown of the gate oxide under long-term electric field stress by defect accumulation; d) electromigration, the physical movement of atoms occurring due to prolonged current flow.

You can see the sheer amount of scenarios a processor has to face. Increasing operating speed and adding extra peripheral support cannot come at the cost of reliability. I do not know the specifics of how they manufacture space grade processors because if I did, I would be doing it myself. Instead let us use what we learned to find some solutions.

The most agreed upon idea is to use a more mature technology node like 180nm, 90nm, 65nm, and the like instead of cutting edge 7nm, 3nm, 2nm. These transistors possess lower electric fields and thicker gate oxides leading to slower degradation. A clever solution is instead of using the conventional planar geometry, we change it so that we can continue to use commercial fab tools prevalent for CMOS (lithography, deposition, ion implantation, etching, etc.), one such geometry is enclosed layout transistors (ELT). It proposes a closed circular/annular gate structures so that transistor has no exposed edge conduction path.

Figure 5: Planar transistor (left) and ELT (right) (Reference: Nicola et. al 2011)

Packaging of the device is as critical as the device itself. Hermetically sealed packages are the minimum standard. Hermetic sealing is done to ensure no particle can enter the package, helium used as its properties are favourable since it is very small, inert, monatomic gas with high diffusivity, allowing microscopic package leaks to be detected with high sensitivity. Controlling thermal expansion is done by choosing materials matching silicon in their coefficient of thermal expansion like ceramics, Kovar alloys, and aluminium nitride to reduce mechanical stress during repeated thermal cycling, helping prevent cracks, delamination, and interconnect fatigue. Radiation shielding to prevent high energy electromagnetic waves from entering the package is achieved by placing absorbing materials between the space environment and sensitive semiconductor devices. Materials such as aluminium (again) and hydrogen-rich polymers help slow energetic charged particles, while dense materials like tungsten or tantalum are used to damp X-rays and gamma rays.

These are some of the ways in which these devices are made space-ready. Only a handful of countries have a space program like India's and fewer still can produce space-grade electronics. India has entered an exclusive club with this achievement. The only way to go from here is up.

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