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Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 8

PHOTOMASK

Charles Howard
DuPont Photomasks
Dresden, Germany

8.1 INTRODUCTION

Photomasks are the intermediate steps between the design of an integrated circuit and the actual
wafer itself. They are the stencils used to print images on the semiconductor material. The litho-
graphic principles at play in producing wafers are the same in photomask manufacture.
During the early years of semiconductor manufacturing, photomask production was rather
straightforward. The layout of the mask was identical to that of the wafer. The image was transferred
1:1 from mask to wafer. The number of product dice on the chip was equal to the number of master
dice on the photomask. In the late seventies, so-called step and repeat systems were introduced into
semiconductor manufacturing. Such systems utilized reduction lens systems and would expose a
mask image stepped many times across the wafer plane. Initially the reduction was 10×, though in
later years 5× and 4× reductions became the norm. This innovation resulted in larger image field
sizes on the photomask (or more precisely, the reticle since the magnification is no longer 1:1 but
rather 5:1 or 4:1). The result was a relaxation of various critical specifications such as minimum fea-
ture and defect sizes. Photomask production during this period was free of many of the resolution
issues paramount to the wafer lithographer. The photomask business could properly be described as
a commodity-driven market during that time. This technical landscape was not to last, however. As
demand for faster and more powerful ICs grew, the ever-shrinking feature sizes on the wafer put
greater pressure on photomask lithographers to push engineering boundaries. The photomask busi-
ness today has evolved from a commodity to a critical enabling technology.

8.2 PHOTOMASK FUNDAMENTALS

Figure 8.1 is a flowchart outlining the manufacture of photomasks. A brief description of each step
is provided.

8.2.1 Data Preparation
Design data are received from the customer design center. Data must be conditioned prior to man-
ufacture. Generally, this includes generating a set of instructions for the pattern generator to use
to print the mask features. The pattern data also need to be sized to compensate for process bias
and converted to a file format acceptable to the pattern generation tool. Computing resources for

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