Tiêu đề 4-ASIC Design in the Silicon Sandbox A Complete Guide to Building Mixed-Signal Integrated Circuits-Keith Barr.pdf ✅

The Sandbox Analog and digital circuits combined in an IC are considered mixed-signal designs. Integrating the two types of circuitry can be challenging, but these designs can provide system-on-a-chip (SOC) functionality that, once designed into a product, can significantly impact final product cost. As a designer of commercially viable products, already buying ICs from major suppliers, you could approach a major IC company and suggest that they design a new catalog part for your application; but without some costly agreement, they would likely offer the part to your competitors as well, somewhat dulling the advantage you may be seeking. You can contract an IC design house to produce a design for you, but in the process you will be transferring specific knowledge of your business to others that you may not be able to completely control. Communication of exactly what you need is difficult without knowledge of the IC design process; it’s like a sales guy talking to an engineer, enough said? Further, the cost of having a design house do the work can easily approach a million dollars, even for a fairly simple design. If you do your own design, you can keep the details as the intellectual property of your company and get exactly what you want, at lower cost, with well-known reasons for any trade-offs. IC Overview Integrated circuits are fabricated onto silicon wafers, subsequently diced or sawn into individual die and lead bonded onto a leadframe, and then packaged with a surrounding mineral-filled thermosetting packaging material, or in the case of a ceramic package, a lid is attached. Depending on die size and wafer diameter, as few as 10 or as many as 50,000 devices could result from a single wafer. Every IC you currently purchase and Source : ASIC Design in the Silicon Sandbox Keith Barr 1 Printed from Digital Engineering Library @ McGraw-Hill (www.Digitalengineeringlibrary.com). Copyright ©2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
use in a product is produced in this way. If you take any standard, plastic-packaged IC, lay it upside down on a piece of 220 grit sandpaper and carefully grind away the top surface, you will ultimately begin to see the gold bonding wires appear and then the silicon die itself. Shifting to finer sandpaper, and carefully adjusting the pressure you apply while sanding, you will be able to prepare the part for microscopic investigation. For this, you will require an epi-illuminated microscope of high magnification power. Also, you will be able to measure the die size, which will give you an idea of how much the part cost the manufacturer to produce. A Peek Under the Hood An epi-illuminated microscope is often called a metallurgical microscope—one where light is sent to the specimen through the same optics that the image returns to the eyepieces. This is of course required in those cases where specimens are opaque, and the low demand for such microscopes makes them hard to find and expensive. My first one (for revealing the details of a DRAM design), purchased for about $1200, was a battle- worn 1960s model, retired from an unknown IC inspection line. A really good epi microscope will be mostly cast iron, weigh 50 to 100 lb, have a precision X-Y table attached with a digital readout to 0.5 μ resolution, and cost $25,000 new, and maybe $8000 used, in good condition (see Figures 1.1 and 1.2). You don’t have to have one to design projects, but it can be valuable if something goes wrong with your design and you need to probe the design to get on-chip signals. I strongly suggest you find an epi microscope, because it can open up a new world to you, providing insight into how other designers have solved problems. When shopping, look for microscopes with objective lenses that have a considerable working distance but a high numerical aperture (NA). These two characteristics are at odds from an optical design standpoint, and basically mean that such objectives are expensive. You need the high NA to get good resolution of small details, but you need a working distance of maybe 8 to 15 mm to allow enough space between the objective and your IC to allow for probe needles. An X-Y measurement table is really handy, allowing you to measure details like die size and device dimensions with good precision. Such microscopes often allow for both bright-field and dark-field illumination. Dark field means the illuminating light is traveling through the same basic path as the observed light, but at the objective it is focused to the object by a doughnut-shaped lens that surrounds the viewing objective lens, causing the light to hit the specimen at an angle. This may be useful in IC inspection, when looking for defects or seeing the crystal structure of metals, but greatly increases the cost of your THE SANDBOX Keith Barr 2 Printed from Digital Engineering Library @ McGraw-Hill (www.Digitalengineeringlibrary.com). Copyright ©2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
objective lenses. You won’t need the dark-field feature, and your objectives will cost much less if you just go for the bright field only types. Typical magnifications required would be from 100X to 1000X, which means 10X eyepieces and a few objectives, maybe 10X, 20X, 50X, and 100X. In the last case, I use an oil immersion lens intended for biological specimens, which has no working distance at all (a drop of oil spans the gap between objective and specimen), but this is the only way to get very high resolution (NA is greater than 1.0). Although the sandpaper technique is acceptable for preparing an IC for die size measurement, it often destroys the IC surface, in which case more drastic measures must be taken. The standard technique for decapsulating a plastic packaged IC is to boil it in a mixture of concentrated sulfuric and nitric acids, although I prefer near-boiling sulfuric acid alone (97%). Most IC packages, leadframes, and all, will be completely digested by this method, but the silicon nitride coating on the die, as well as the aluminum pads, will be preserved in pristine condition. Only a few milliliters of H2SO4 in a small beaker on a hot plate does the trick, THE SANDBOX Keith Barr 3 Figure 1.1 An epi microscope available at low cost. Printed from Digital Engineering Library @ McGraw-Hill (www.Digitalengineeringlibrary.com). Copyright ©2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.