The Resolution Limit of Hard Drive Manufacturing
In lithography, pushing the limits of resolution is what we do. These efforts tend to get a lot of press. After all, the IC technology nodes are named after the smallest nominal dimensions printed with lithography (though the marketing folks who decide whether the next generation will be called the 16-nm or 14-nm node don’t care much about the opinions of lithographers). And the looming end of lithographic scaling has gotten all of us worried – regardless of your faith in EUV. Yes, resolution is the signature (though not the only) accomplishment of lithographers. That is why it is so important to carefully define what we mean by the term ‘resolution’ and understand why it is different for different tasks.
As I have said many times in papers, courses, and my textbook, the resolution one can achieve depends critically on the type of feature one is trying to print. In particular, the nature and limits of resolution are very different for dense patterns as compared to isolated patterns. For the last 10 years or so, the IC industry has been focused almost exclusively on pitch resolution – the smallest possible packing of dense lines and spaces. In optical lithography this resolution depends on the ratio of the wavelength (λ) to the imaging system numerical aperture (NA). For a single, standard lithographic patterning step there is a hard cut-off: the half-pitch will never drop below 0.25λ/NA (i.e., the pre-factor in this equation, called k1, has a lower limit of 0.25).
For 193-nm lithography, the NA has reached its maximum value of 1.35, so that the dense pattern resolution has bottomed out at a pitch of 80 nm. To go lower, one must use double patterning, or wait for Extreme Ultraviolet (EUV) lithography tools to drop the wavelength. Either way is costly, and the proper path past a 40-nm pitch is currently unknown.
But the resolution limit for an isolated feature is not so clear cut. While resolution still scales as λ/NA, there is no hard cut-off for k1. As k1 is lowered, lithography just gets harder. In particular, control of the feature width (called the critical dimension, CD) is harder as k1 goes lower. Thus, for isolated lines, resolution is all about CD control.
And that’s where lithography for hard drive read/write head manufacturing differs from IC manufacturing. When manufacturers like Seagate and Western Digital increase the areal density of their drives, you can bet there was a shrink in the feature size on some critical geometry of the read and write heads. And that feature is an isolated line printed with optical lithography.
So how small are the smallest isolated features printed at Seagate and Western Digital? While I don’t have the exact values, I do know they are on the same order as the smallest features obtained by IC lithography – when double patterning is used. In other words, today’s hard drive manufacturing requires 2x-nm lithography (isolated lines) using single patterning.
The CD control requirements for these critical features is about the same as for IC critical features: +/- 10% or so. Overlay is critical too, but maybe a bit relaxed compared to the standard 1/4 – 1/3 of feature size that is the rule of thumb in the IC world. But there are a few extra requirements that make read/write head litho challenging. The wafers are smaller than the standard 300mm diameter (it is a thick ceramic wafer, not silicon), with no plans for a change to 300 mm. On each wafer, tens of thousands of heads are made (the standard lot size is one to four wafers), so throughput is not quite as critical as for ICs. But this also means that none of the latest generation of lithography tools (such as 193 immersion) are available for this task (they are all 300-mm only tools). Not that these guys would buy an immersion tool anyway – hard disk manufacturing is extremely cost sensitive, so they make do with lower-NA 193 dry tools.
So let’s do the math. To print 2x-nm features with a moderate-NA 193 dry tool, the hard drive makers are doing single-pattern lithography with k1 below 0.1. This is remarkable! The IC lithographers have never attempted such a feat. How is it done? Of course, you use the strongest resolution enhancement techniques from the IC world you can find. After that, it’s all about CD control, which means attention to detail. Let’s give the hard drive folks the credit they deserve: lithography at k1 < 0.1 is hard.
Lithography scaling pressures are at least as fierce in the hard drive world as in the IC world, so you can bet the minimum isolated line feature size will continue to shrink. It will be interesting to see how they do it.