Page 330 - Sami Franssila Introduction to Microfabrication

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Tools for Microfabrication

The size of the microfabrication tools tends to be In batch processing, uniformity over the batch must
inversely proportional to the size of the structures they be added to uniformity across the wafer. Variation
make. Small tabletop instruments can pattern and etch comes from wafer position in a batch system: flow
3 µm lines, but tools for 100 nm lines require garage- patterns of gases and liquids over wafers depend on
sized behemoths with multimillion-dollar price tags. The wafer position, and the thermal environment may also be
analogy with elementary particle physics is obvious: position dependent: the first and the last wafer have only
the smaller the objects being studied, the bigger the one neighbour, but the others are sandwiched between
instruments needed. Price tags for individual tools are two wafers.
up to 10 million dollars today, even though $100 000 can During the 3 in. era, most wafer processing was
still buy a system suitable for research purposes, be it a batch processed and the major shift started at the
mask aligner, a furnace or a plasma etcher. 100 mm wafer size. Robotic loading/unloading is simple
in single-wafer systems, and they are more amenable
to factory automation, including data gathering. Film
30.1 BATCH PROCESSING VERSUS
SINGLE-WAFER PROCESSING thicknesses have been scaled down with linewidths, and
thinner films require less process time in deposition
Microfabrication economies were earlier touted to and etching, which works in favour of single-wafer
result from batch processing: tens of wafers with processing. However, single-wafer systems rarely even
hundreds of chips are processed simultaneously in, for approach batch system throughputs, which can be up to
example, a furnace or a wet etch bench. However, 200 wafers per hour (WPH) and in some simple PECVD
the scaling down of linewidths has put increasing applications (in solar cells), even 500 WPH. It may also
demands on process control, and single-wafer tools have well be that in the back end of the process, wafers are
superseded batch equipment in many process steps. so expensive that manufacturers do not want to risk a
Besides, batch equipment for large wafers can become lot by batch processing: 200 mm wafers with 300 chips
prohibitively cumbersome. selling for $10 are worth $2500 (yield is not 100%), or
Wet processing in a tank is a prototypical batch the batch of 25 is worth $60 000. If a batch is lost at
process: a full cassette of wafers is processed simul- the end of the process, it will take time to fabricate the
taneously (see Figure 12.3). Wafer cleaning and non- replacement lot, typically three to six weeks. This can be
patterning etching (e.g., removal of sacrificial oxide an even greater burden than the money loss if delivery
by HF) are widely done in batch-mode wet process- time is used as a criterion for choosing a chip supplier.
ing, even in the most advanced processes. Wet etching In single-wafer processing, wafer-to-wafer repeatabil-
for patterning (e.g., H 3 PO 4 -based aluminium etching or ity is a major issue. First-wafer effect means that the
BHF-etching of oxide) is not an option when linewidths system has not stabilized, and therefore the first wafer
are below 3 µm, because process control is difficult in experiences, for example, lower temperature or more
batch wet processing: no in situ monitoring is possi- concentrated chemicals. In addition to batch and single-
ble and wafer-to-wafer variations are often encountered. wafer processing, various combinations are being used,
However, model-based control with ionic strength and as shown in Table 30.1.
temperature measurement can be used to improve rate Single-feature processing is so slow that it is relegated
control to some extent. to special applications only. Throughputs of a few

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)