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Design >> RF Design >> Spiral Inductor in CMOS vs. MEMs
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Message started by Marc Murphy on Apr 7^th, 2006, 3:17am

Title: Spiral Inductor in CMOS vs. MEMs
Post by Marc Murphy on Apr 7^th, 2006, 3:17am

Hello, is there any physical/technical reason why there can't be a spiral inductor using a standard CMOS process?

A guy I know claims that a perfectly rounded inductor, e.g. like a cinnamon roll, is only possible with a MEMs process, but I didn't buy it, and he couldn't give me a satisfactory answer. I'm not an RF guy, so maybe the spiral inductor exists, but I haven't seen it...only squared spirals and pseudo-circular ones so far.

Thanks,
Marc

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by ACWWong on Apr 7^th, 2006, 7:46am

I guess you are only limited by metal mask rules, so could create a much better "circular" structure (than e.g octagonal) by using the minimum design rules to create a manhattan effect on the metal spiral. Gievn modern CMOS process metalisation gives metal features of 100s of nm, then on a big inductor (diameter 100s of um) you should be able to get a very nice "spiral" but with the effort of designing and laying it out.

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by Marc Murphy on Apr 7^th, 2006, 11:36am

ACWWong wrote on Apr 7^th, 2006, 7:46am:

Right, but you are talking about the design rules. Let's forget them for the moment since they are written with standard CMOS stuff in mind. Is there any fundamental lithography or metal reason why you couldn't make a perfectly round inductor with CMOS?

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by ACWWong on Apr 8^th, 2006, 6:26am

ohhh.. i get you now...
hmm... i haven't ever seen circular metal features in CMOS, but with my limited knowledge of CMOS processing, i don't know of any phyiscal issue, more a costs/need/tooling problem...

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by Paulin on Apr 26^th, 2006, 11:26am

I just saw a real oval inductor on silicon using CMOS process and L/Q is pretty good.

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by Marc Murphy on Apr 26^th, 2006, 7:32pm

Paulin wrote on Apr 26^th, 2006, 11:26am:

I just saw a real oval inductor on silicon using CMOS process and L/Q is pretty good.

Do you have a document with this oval ind?

Cheers,
Marc

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by gitarrelieber on Apr 27^th, 2006, 12:33am

Hi Marc,

I think the only limitation is the design rule. You can approximate the round curve with tiny rectangualr cells. The resultant edges will contain small zig-zags. But you will not notice them if you look at the spiral inductor at distance. It is inefficient to manually draw the layout of the spiral inductor. Of course you will need a programming language like SKILL in cadence to generate it automatically. But as said, it is generally possible to realize the inductor in CMOS processes. It is only very time-consuming. This is also the reason why most of the designer are reluctant to do it. You can refer to the following paper for more detail. The inductor described there contains some extra features to reduce the proximity effect and skin effect. Nevertheless, those extra features have already been patented.

Werker. H, et.al.," A 10 GB/s SONET-Compliant CMOS Transreiver with Low Cross-Talk and Intrinsic Jitter, " IEEE Journal of Solid-State Circuits, Vol. 39, No. 12, December 2004.

Regards,
Jiawen
Analog Design

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by 337see733 on Apr 27^th, 2006, 8:31pm

Hmm...I thought i did draw and fabricate some circular spiral inductor last time..for CMOS RF 0.18um tech process ...and that was like 2-3 years back...ha ha..Anyway, there should be no design rule restriction on the shape of the spiral inductor(circular or octagonal)...except the metal-to-metal distance. Anyway, the circular shape are approximated with tiny triangular/manhattan shape...i guess if u can produce it in drawing, they can produce the mask, why not, right?

Rounded inductor always performed better compare to the rest. There are a lot have talked abt it. Oval one?! Hmmm... it is new to me....probably u gain some L but losing some Q...ultimately it depends on which operating frequency u r working at..

ha ha...all the best ya...

Title: Re: Spiral Inductor in CMOS vs. MEMs
Post by mikki33 on May 17^th, 2006, 7:38am

As I understand correctly, Jiawen has in mind Xignal's design:
http://www.us.design-reuse.com/articles/article7431.html
and an inductor layout: http://i.cmpnet.com/eet/news/04/March/SS1313_XIGNAL_1.gif
(One day I drunk beer with Heinz, Fred and Chris... It was good...)

Design rures are playing here hard. You can't use min zig-zag (rectangulars), becouse you may create "jogs", which in some processes are just DRC errors (TSMC for sure), additional consideretion may be the mask creation step, called "Optical Proximity Correction" (OPC). This will change the layout...