High-performance polyurethane can be engineered to produce more durable and consistent wire-cutting rollers for the production of wafers

Technical article contributed by Argonics

The manufacturing of wafers for the
microelectronics and photovoltaic industries involves a multitude of critical
processes. One of the first of these is the precision cutting of ingots or
crystals, typically composed of hard and brittle silicon, into uniformly sized wafers
that are suitable for the processing of silicon chips.

In a production environment, this cutting is typically
performed by multi-wire saws composed of diamond-coated wires precisely and evenly
spaced in a belt-like configuration. The cutting of the ingot or crystal of
silicon material is performed in one pass, resulting in dozens or even hundreds
of wafers.

To achieve the ideal spacing between wires,
as well as maintain the ideal wire tension, precisely manufactured rollers are
used. This includes smaller “tensioner” rollers along with a larger roller with
machined notches that keep the diamond-coated wire in place.

Because the accuracy of the wafer cutting
process is dependent on the durability and consistency of the rollers, those
that lack material strength or precise geometry/dimensions can fail
prematurely. This can lead to costly unplanned downtime, wasted materials, and
potential delays in subsequent operations.

Fortunately, high-performance rollers are available
that utilize unique, more durable formulations of urethane and can be machined
to meet the high tolerance requirements with consistency from part to part to ensure
the ideal multi-wire cuts.


Urethane rollers
The majority of wire-cutting rollers in use
today are composed of urethane (short for polyurethane). Shorter rollers, which
typically cut up to 30 wafers simultaneously, are made of high-performance,
cast and machined urethane. The longer rollers, which often cut up to 300 wafers,
are often coated with urethane sleeves having similar properties.

“Urethane is the preferred material for both
long and short wire-cutting rollers,” says Greg Cappaert, Technical Sales
Engineer at Argonics, one of the U.S.’s largest producers of wear-resistant
urethane products. “Cast urethanes are tough, abrasion-resistant elastic materials
that combine many of the performance advantages of high-tech plastics, metals,
and ceramics along with the resiliency and flexibility of rubber parts.”

Because urethane’s properties can be adjusted
over wide ranges of hardness, resilience, spring rate, and dynamic properties,
its versatility offers custom solutions to difficult design challenges.

“Urethane, even in relatively small quantity
production, can be adapted and tailored to prioritize the physical properties
that design engineers need,” says Cappaert.

Urethane’s properties are vital to high-volume
production operations that utilize multi-wire saws to slice wafers as thin as
80 to 100 microns. Any deviation in wafer thickness or profile will bring a
cutting operation to a halt and, unless spare rollers are on hand, create
serious production delays.

Cappaert adds that urethane
rollers used on wafer wire-cutting equipment are essentially “consumable”
items. The shorter rollers, also known as tensioners or pulleys, are replaced
several times a year. Longer rollers that have urethane linings must be
recoated as much as 40 to 50 times annually.

Cappaert says that much of the turnover of
rollers results from rollers falling out of spec prematurely due to shortcomings
in urethane formulation, casting, or machining.

Because urethane is an engineered material,
many problems can be overcome by adjusting formulation to meet the specific
requirements of an application, as well as rigorous testing and stringent
quality control of individual parts. He also notes that polyurethane rollers
can be engineered to deliver extreme accuracy in both wet and dry operations,
such as machined tolerances to ±0.004 in. (±0.10 mm).

Cappaert characterizes the production of cast
urethane rollers as complex and highly sensitive. For that reason, Argonics
casts, machines, and inspects each individually. When larger urethane-coated rollers
are approaching wear limits, these must be stripped and re-coated using the
same types of stringent procedures.

Although specific customer formulations and
design information are considered proprietary, Cappaert says that these methodologies
can lead to improvements in wafer warp, consistency of cut, and thickness
variation while also increasing roller longevity.

Another difficulty that
many suppliers and users of precision wire-cutting equipment can relate to is
supply chain issues. Depending on inventories on hand, untimely deliveries of
urethane rollers — possibly due to premature failures — can shut down wafer cutting,
resulting in costly delays down the rest of the production line. Cappaert
recommends working with a manufacturer, like Argonics, that is willing to help
solve these types of costly delays.

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