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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Displacement Interferometry

Working Group 5.22

Use of photomasks in lithographic production of integrated circuits


The reliable function of microelectronic devices as well as their electrical properties are crucially dependent on the dimensions and positions of the microstructures which build up an integrated circuit or chip. These in turn are defined by the microstructures on the masks which are necessary for the photolithograhic production of integrated circuits. The photomasks contain the microstructure patterns to be projected onto the silicon wafer chip within a thin chromium layer of about 100 nm thickness. The standard substrate material of the photomask is synthetic quartz glass, the standard format is a quadratic plate with 152 mm length (6 inch) and about 6 mm thickness.

In photolithographic production by means of a so-called wafer stepper (see illustration of principle) the mask is illuminated with intensive light, preferably with short wavelengths, e.g. in the UV-region. The patterns on the photomask are demagnified - e.g. to a ratio of 1:4 - by means of a high quality photolithography objective and are thus projected onto a light-sensitive resist layer on top of a silicon wafer. After develop process the resist layer contains the demagnified image of the patterns on the mask, which is used to protect parts of the silicon wafer from the impact of further process steps like etching, diffusion, etc.

In addition to the quality of the photolithography objective and the relative alignment of mask to wafer during projection, the quality of the patterns on the mask is of crucial importance for the function and properties of the integrated circuits. Important dimensional measurement parameters on the masks are the width of microstructures (critical dimension, CD) and their positions (Registration, LD).

Today more than 20 different masks are necessary to produce ICs, like e.g. microprocessors or memory chips (DRAM). The overlay tolerance e.g. for a 1 Gbit chip is about 50 nm on the wafer. In order to control these small tolerances, measuring instruments have to be used which offer even better precision. Current mask comparators allow to measure position deviations of microstructures on masks with uncertainties as small as 10 nm. Current and future requirements of IC production are described and further developed by semiconductor industry in the so-called "ITRS-Roadmap". To produce faster ICs requires the structures to be minized further. For this, resolution enhancement technologies are applied today in lithography which further increase the importance of high quality of the shape of microstructure edges on the masks.

Mask position standards are applied for the calibration of mask comparators and can be used for the control of wafer stepper properties too. In combination with standards for CD and for overlay mask position standards serve to assure traceability of dimensional measurements to the SI unit of length in production control of ICs.

An example of an 2D position photomask standard is shown here. As illustrated by differently zoomed views, this mask is a material measure of 2D microstructure positions realized by line crosses of 5 µm width in an array of 20 mm x 20 mm.

At PTB traceable calibrations of microstructure coordinates on standard 6 inch masks can be performed with uncertainties of u95 = 35 nm. More details are given in table Calibration services.