- ISOCELL HP2 (1/1.3"): f1.7
- 4/3": f2.9
- APS-C: f3.9
- Full frame: f5.9
- 4/3": 5.9
- APS-C: f7.8
- Full frame: f11.7
I really love the IMX183 from Sony. For a very good price, you get 20 million BSI pixels with photon-counting levels of read noise and negligible dark current - truly a miracle of the economies of scale. The sensor is also large enough to have good light gathering capabilities, yet small enough to work with compact optics.
The problem is taking advantage of all 20 million of those pixels. 20 MP isn't that much - 5400-pixel horizontal resolution leaves you with precious little room to crop to 4K so actually resolving 20MP is important. You can't really buy a telescope that resolves 2.4um pixels - in the center anything diffraction limited will work, but if you want a fast, wide system getting good performance in the corners isn't happening. Obviously, the answer is to design your own telescope.
Now, I have zero interest in making telescopes - grinding your own lenses is ass and generally a poor use of time. If I wanted to spend time doing repetitive tasks I'd pick up embroidering or something. As such the system is designed to be reasonably manufacturable by an overseas vendor.
The telescope is a Houghton-type design with integrated field flattener, a focal length of 150mm, an entrance pupil of 76mm (3"), and a 6-degree field of view covering a 16mm sensor. The optical performance is pristine:
The pixel scale in the capture is 560nm/pixel, so the Windows clock is only a few microns long. Performance is as expected but it is always entertaining to use the Windows desktop on a 1mm wide screen :)
It's been a year of shakeups in the high-end CPU market, what with catastrophic supply chain shortages, the rise of AMD, and Pat Gelsinger's spearheading of Intel's return to competency. Now that the dust has mostly settled, it's interesting to look at what's hot, and what's not.
The 5950X is still king...
Alder Lake i9 really gives AMD a run for the money, but the 5950X is still the king of workstation CPUs, especially now that you can buy it. It has aggressive boost clocks which allow it to beat every Skylake and Cascade Lake Xeon Platinum (including the 28-core flagships), consistent internal design which scales well in every application, consistent and manageable 142W power limit, and bonus server features like ECC support. ADL is good, but the 250W PL2 really kills it for workstation use (a good 12900K build requires serious effort to get right), and because of the heterogeneous internal layout it fails to scale on some operating systems and in some applications.
Scaling is really important in this era of 16, 32, and 64-core processors; many applications completely fail to scale past 16 cores, and even those that do exhibit much less than linear returns. As a result, those 16 highly clocked cores punch above their weight when it comes to real-world results - a 4.x GHz Zen 3 core isn't actually twice as fast as a 2.8 GHz Skylake core, but 16 4.x GHz Zen 3 cores can still outperform 28 Skylake cores because the 12 extra cores are doing less work.
Building a large telescope is hard; designing a small telescope is hard. What exactly do I mean by that? Well, there are parts of the telescope that don't scale well with size, for example, the instrument payload, the filters, or the focusing actuators. More often than not, a design which works well on a 1m-class instrument fails to scale down to a 300mm-class instrument because the payload is incompatible with the mechanics, or is so large that it fills the clear aperture of the instrument.
A small telescope should also be...small. A good example of this is the remarkable unpopularity of equatorially-mounted Newtonians; a parabolic mirror with a 3-element corrector offers fast focal ratios and good performance, but an f/4 Newtonian is four times longer than it is wide, which gets unwieldy even for a 300mm diameter instrument.
The Argument for Cassegrain Focus
Prime focus instruments are popular as survey instruments in professional observatories. However, they fail to meet the needs of small instruments because of:
The modern wafer scanner is a truck-sized contraption full of magnets, springs, and slabs of granite capable of accelerating at several g's while maintaining single-digit nanometer positioning accuracy. The motion systems contained within painstakingly try to optimize for dynamic performance by using active vibration dampening, voice coils, linear motors, and air bearings, all to increase the value of the machine for its owner (who spent a good fraction of a billion dollars on it).
As it turns out, a 80's stepper is none of these things. Scanners are immensely complex because they are dynamic systems - as the wafer moves in one direction, the reticle moves in the other direction, perfectly synchronized but four times faster. In contrast, steppers are allowed time to settle between steps, which allows for much more leeway in the motion system design. Throughput requirements were also lower; compare the 35 6" wph of an old stepper to the 230 12" wph of a modern scanner.
Old stepper stages are an instructive exercise in the design of a basic precision motion system; in fact, Dr. Trumper used to give this exact stage out as a controls exercise in 2.171. The GCA stages are also particularly interesting from a hardware perspective - they are carefully designed to achieve 40nm positioning accuracy using fairly commodity parts. The only precision parts seem to be the slides for the coarse stage, and even those are ground, not scraped.
The stage architecture
![]() |
System overview |
GCA steppers use a stacked stage architecture. Coarse positioning is done by two conventional mechanical bearing stages stacked on top of each other. Fine positioning is done by a single two-axis flexure stage. Rotational positioning, which only happens for alignment, is done using a simple open-loop, limited travel stage mounted on the fine stage. Focusing, which is done by changing the Z spacing between the lens and the wafer, is done by moving the optical column up and down with a linkage mechanism.
The position feedback system
The fine position feedback on GCA steppers is implemented through a two-axis HP 5501A heterodyne interferometer. Briefly, a stabilized HeNe laser is Zeeman split through a powerful magnet to create two adjacent lines separated by a few MHz with different polarizations. One of these lines is separated with a polarizing beam splitter and reflected off a moving mirror; this line is Doppler shifted due to the velocity of the moving mirror and beat against the stationary component to generate a signal. This signal is compared against a stationary REF signal to derive velocity and position measurements. Heterodyne interferometers are the preferred choice for metrology due to their insensitivity to ambient effects and power fluctuations.
The 5501A is the de facto choice for interferometric metrology; its successor the 5517 is still available from Keysight. A description of the system as found in the GCA steppers is as follows:
The laser points towards the rear of the stepper; a 10707A beam bender and a 10701A 50% beam splitter generate the two axes of excitation. The X and Y stages have identical measurement assemblies; the Y assembly is located to the rear of the stepper (behind the column) and the X assembly is located inside the laser housing. Both assemblies use a plane-mirror interferometer which differentially measures the wafer position against the optical column; the stationary mirror is a corner cube mounted to the column and the moving mirror is a 6" long dielectric quartz block mirror mounted to the wafer stage. The flats are precision shimmed to ensure orthogonality (since it is the orthogonality of the flats which determines the closed-loop orthogonality of the motion).
There are two additional position sensors in the system. The first is a sensor to measure the position of the fine stage relative to the coarse stage. Literature indicates that this is an LVDT, but on the 6100C it appears to be implemented as two photodiodes outputting a sin/cos type signal. The second is a brushed tachometer on each of the coarse stage drive motors, which is used for loop closure by the stock controller.
The coarse stage
The purpose of the coarse stage is to position the fine stage to within 0.001" of its final position. The stage is built as a pair of stacked plain-bearing stages; these stages are driven by brushed DC motors with brushed tachometers for velocity feedback. The motors go through a right-angle gearbox comprising of a bevel gear and several spur gear stages before being coupled by a flexible coupling to a long drive shaft which turns a pinion positioned near the center of each stage. This pinion drives a brass rack mounted to the stage which generates the final motions.
The fine stage
The fine stage is constructed as a parallel two-axis flexure stage with a few hundred microns of travel on each axis. The flexures are constructed from discrete parts; the stage is made from cast iron and the flexures themselves are constructed from blue spring steel. Actuation is by moving-coil voice coil motors with samarium-cobalt magnets, and position is read directly from the interferometer system.
The theta stage
The theta stage is a limited travel stage based on a tangent arm design. A (very small) Faulhaber Minimotor is coupled into a high reduction gearbox, which drives a worm gear that turns a segment of a worm wheel. The worm wheel pushes on a linkage which rotates the wafer stage about a pivot point.
Rotation control is entirely open-loop - the wafer is rotated once during the alignment process based on the fiducials observed through the alignment microscopes. A slow open-loop system is acceptable given that the speed of rotational alignment does not significantly affect wafer throughput.
The Z mechanism
The focusing mechanism is a limited-travel (according to literature, about 600um) flexure mechanism. The entire optical column is suspended on two large spring steel plates; a stiff spring counterbalances the weight of the column. A voice coil motor (identical to the fine stage VCMs) actuates a linkage mechanism which moves the column up and down.
Adjusting the mechanism is a bit subtle. The white rod sticking out is actually a tensioning mechanism for the counterbalance; it is possible to aggressively tension the spring to stiffen the assembly for transport. The cap at the end of the rod can be removed to reveal a nut and a piece of threaded rod with a flathead in it. You want to hold the rod in place with a screwdriver and crank on the nut with a wrench until the column just barely 'floats' in place.
Incidentally, this mechanism also reveals a fairly severe weakness of the focusing system - it is extremely undamped. Any disturbances on the column cause the whole assembly to ring like a bell, with the only source of damping being the resistance of the VCM. I think (though there is some information to the contrary) that 6000-series GCA steppers focused once per wafer, relying on wafer leveling to keep the image in resist in focus between fields. Otherwise if the focusing had to be highly dynamic there could be problems.
![]() |
The yellow lights make it look more legitimate |
I have always wanted to expose a wafer. I'd written off making my own transistors long ago (nothing that fits in a house is good for feature sizes small enough for interesting logic, and I'm not a good enough analog engineer to design interesting analog), but there are many useful optical and mechanical parts that can be made lithographically.
The usual route to home lithography is a microscope and a DLP, but the resultant ~2mm field sizes are not sufficient for mechanical parts and stitching a 20mm field out of 2mm subfields is very taxing on your motion system. Contact aligners are simple and perform well, but getting submicron resolution for interesting optical parts out of a contact aligner is challenging (the masks also get quite expensive).
The natural solution is to start with a stepper lens (which is basically a giant microscope objective with very bad color correction). There are a few variants - 1:10 lenses with a 10x10mm field, 1:5 lenses with a 14x14mm field, and 1:4 lenses, which weigh several hundred kg and have a 20x20mm field. Stepper lenses also come in several colors: g-line (436nm), i-line (365nm), and DUV (~250nm).
I wound up with a 1:5 g-line lens; the 1:5 lenses strike a good balance between performance and unwieldiness. I also had a set of stages pulled from a DNA sequencer good for a couple microns of resolution. The rough plan was to stack a fine stage on top of these and use a direct-viewing technique to perform alignment. However, the project quickly went south when I realized building an exposure tool entailed buying the parts out of an...exposure tool. Conveniently, a circa 1985 GCA DSW 6100C showed up for more or less scrap value near me, so one rigging operation later I was the proud owner of a genuine submicron stepper.
The DSW family of steppers are true classics; GCA Mann practically invented the commercial stepper in the late 70's. The GCA steppers remained more or less unchanged until the company's demise; everything from the g-line DSW 4800 to the AutoStep 200 shared a stage design, alignment system, and mechanical construction (unfortunately, they also all shared a terrible 70's-grade electronics package!). A number of GCA tools still survive in university fabs, mostly converted to manual operation. Briefly, the design consists of:
As luck would have it, practically every screw in the Maximus was loose, and the bulb was snapped in half. The rebuild took a couple hours, and was greatly improved by removing the head from the stepper - dealing with loose lenses is much easier when you are not six feet off the ground (if by some chance you are reading this and also servicing a GCA stepper, removing the Maximus is easy - just pull the four socket head screws at the base of the condenser, un-route the shutter cables and lamp cables, and the unit lifts right off).
I haven't had a chance to check performance yet, as the bulb needs replacement. The Maximus uses Ushio USH-350DP bulbs. Of critical note: the USH-350DP is a two-screw-terminal designed for aligners. The Maximus uses a screw-on "bullet" on one end to convert it to a plug-in type; if you are changing bulbs, don't throw out the plug!
Additional GCA resources