Can you get a Passive ASIC?

Question

In super small low power devices, where space is a big premium, complicated passive networks can take up a lot of space. Is it possible to get a passive or mostly passive ASIC fabricated? If it is possible, how much would it cost? Obviously it would never approach the tiny cost of discrete passives, but when you really need that space it might be worth it.

In terms of costing for ASIC manufacture, this question was really helpful, but it's talking about more traditional ASIC's where the main thing is the active components.

I'm also wondering, maybe ASIC is the wrong word? Is there another word for this kind of thing?

Answer 1

Not an expert answer, but some disconnected relevant facts:

One of the reasons that ICs often require external passives is that standard silicon ICs are not a good substrate for making capacitors and inductors of significant value on-chip. Thus, a conventional IC, "AS" or not, is not a space-efficient way to put large passives on a board (other than high-value resistors). It might still work out to be an improvement if it's not the value but the number of parts that is a problem (i.e. packages and interconnects account for most of the area), but this is why, for example, your microcontroller always has external decoupling capacitors.

Resistor networks and capacitor networks are an existing common form of passives-in-a-single-package. I haven't heard of any mixed passive types in a single package, though — I expect it would be difficult in the same way as putting passives in a normal silicon IC because they have very different materials and manufacturing techniques.

Resistor networks can be used:

• To save space on the board; they were much more common and relevant in the through-hole days, I believe, where even if you placed the resistors vertically you would need an entire hole for the other pin, and a network would let you have \$N\$ resistors with only \$N+1\$ pins).
• As a high-precision voltage divider or other configuration of matched resistors (this is often seen in high-end measuring instruments). Multiple resistors made using the same process and on the same substrate can have tighter tolerance, more temperature-stable ratios of resistances than independently manufactured resistors.

Hybrid integrated circuits, or hybrid modules, are sort of an intermediate technology between PCBs and single-semiconductor-die ICs, which can include passives either formed on the board itself or soldered on — but they are not noteworthily compact nor cheap. The value is in having a precise, pre-built, sealed module that you can plug/solder into a larger board.

Answer 2

ASIC means Application Specific Integrated Circuit. If that integrated circuit would contain only passive components then we could still call it an ASIC.

Making a silicon die with only passive components is very well possible but there are severe limitations. On an IC capacitors are only practical up to a few nF. Inductors can only be a few nH. Resistors can be made in almost any value.

These passive components (except for the inductors) might have parasitic diodes to the substrate so it is possible that they're not 100% passive.

Designing and producing an ASIC is never cheap. Depending on the manufacturing process, making the masks (needed to define the patterns to make the components) can be anywhere between $ 10000 to $1000000 (yes, on million US dollars).

The actual cost of producing one IC once you have the masks is lower, it can be a few cents each up to tens of dollars depending on the size (smaller size cheaper). But with a high mask cost you'd need to make many of these chips (and use/sell them) for this to be cost effective. The mask cost is shared between all the ICs you produce so the more the better.

I worked at Philips Semiconductors in the past and they had a special process for passives only. The idea was that these ICs would provide the supply decoupling capacitors and RF matching components for a separate, small IC with the active components and made in a more expensive process technology. This smaller IC would then be mounted on top of the die with the passive components.

This was a bit complicated to produce so it was only used for some niche products as far as I know. It is certainly not mainstream technology.

Answer 3

You don't have to have actives on an ASIC (although I don't know if anyone has ever done this). There are manufacturers who will do this kind of thing, it is expensive and you need large quantities.

There are better ways that would still be more costly than a PCB with components but cheaper than an ASIC. Components can be built inside the PCB.

Source: EDN

Or you can get integrated packages:

These last two would probably be cheaper than an integrated circuit where the passives are built into silicon, however embedding components in these new processes will be much more expensive than using a PCB with micro vias and stacking PCB's

Answer 4

Yes, definitely. Here at Keysight, for example, we do create some of our own basic strip-line filters for probes and other equipment. Building passive components into chips is standard.

Other clarifications. ASIC vs. IC - If you're designing a chip for a specific purpose, then the "AS" is valid. It's a potato/potahto situation.

Digital vs. Analog? - Both are common. Digital ASICs generally do processing and computations, analog ASICs generally massage a signal into a more usable form. We talk about this one on the EEs Talk Tech electrical engineering podcast here:

https://eestalktech.com/2017/07/13/fast-adc/