Feb 11, 2007

The chip behind the mask

The dictionary gives the meaning of the word “mask” as a “false face used among primitive races to exercise witchcraft and sorcery.” The word “mask” also denotes a “covering worn on the face to conceal identity.” In realms far removed from masquerade balls, Infineon also uses masks – but, as one might well expect, for entirely different purposes.

The masks in our field define the structures and hence the functions of the chip. Structures signify the elementary components of a chip, i.e. transistors, diodes, resistors, capacitors and their interconnections. These elementary components, in turn, have sub-structures such as doping areas or isolation layers, which are ultimately defined by the masks.

Masks are projection patterns and are used in the photolithographic exposure units in the front-end fab. They are glass plates made of high-purity quartz glass provided with a thin layer of chrome. The light-proof chrome layer defines the structures transferred to the wafer by an exposure process. Very stringent demands on the material are made in terms of purity, transmissivity and planarity. Photo masks have to be absolutely flawless because any defect would find itself on each wafer during exposure. So it is that the mask is the blueprint with which many millions of identical chips are produced. The structures of the mask are about four to five times larger than the structure to be created on the wafer, i.e. the structures on the masks are scaled down by the factor 4 to 5 as a result of the exposure process.

A varying number of masks are required for one chip, depending on complexity. Chips for power electronics or discrete components are of low complexity and get by with some 10 to 15 masks. Memory chips and complex logic chips need about 20 to 30 masks. Highly complex logic chips with numerous metal layers and high-frequency chips in BiCMOS technology require 30 to 40 masks.

The fewer masks required for the production of a chip, the better. This is so because each mask involves numerous additional production steps, such as applying photoresist, hardening photoresist, exposing or etching off photoresist. Firstly, these all harbor some probability of failure which potentially increases the scrap rate, and secondly they prolong the time of the wafer in the front-end fab and so also the time to market. And, on top of all that, each mask also costs money. Masks are the most expensive “material” used for chip manufacture. The cost for the complete mask set of a complex chip for a 65nm process meanwhile ranges at seven-digit level.

Mask manufacture is today a science in itself. The chrome structures are vapor deposited on the glass substrate in cleanroom processes. It is not least to the credit of the masks that we can today produce chip structures far smaller than the wavelength of the laser light used. The nature of the light is exhausted by performing breathtaking physical feats. On the one hand, the undesired light diffraction phenomenon is compensated by the mask (referred to as optical proximity correction) and, on the other, a phase shift of the lightwaves during passage through the glass substrate is attained by means of areas with varied glass thicknesses (phase change masks), which is used to control light intensity on the wafer surface. These are currently hot spots in mask manufacture. The cost of research and development can hardly be borne by semiconductor manufacturers alone.

(edited from IFX magazine Jan 26, 2007)

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