Alternative Method for Steam Generation
for Thermal Oxidation of Silicon
JEFFREY J. SPIEGELMAN
An account of how steamer technology—as opposed
to the pyrolytic torch—for the MEMS manufacturer’s
operation can eliminate hydrogen in thermal oxidation
and increase growth rate, uniformity, and reliability
Abstract
Thermal oxidation of silicon is an important process
step in MEMS device fabrication. Thicker oxide layers are
often used as structural components and can take days or
weeks to grow, resulting in high gas costs, maintenance
issues, and a process bottleneck.
Pyrolytic steam, which is generated from hydrogen
and oxygen combustion, has been the default process,
but has serious drawbacks: cost, safety, particles, permitting, reduced growth rate, rapid hydrogen consumption,
component breakdown and limited steam flow rates.
Results from data collected over a 24 month period by
a MEMS manufacturer supports replacement of pyrolytic
torches with RASIRC Steamer technology to reduce process cycle time and enable expansion previously limited
by local hydrogen permitting. Data was gathered to
determine whether steamers can meet or exceed pyrolytic torch performance.
This steamer uses de-ionized water as its steam source,
eliminating dependence on hydrogen and oxygen. A
non-porous hydrophilic membrane selectively allows
water vapor to pass. All other molecules are greatly
restricted, so contaminants in water such as dissolved
gases, ions, total organic compounds (TOC), particles,
and metals can be removed in the steam phase.
The MEMS manufacturer improved growth rate of
oxide layers by 7% over the growth range from 1µm
to 3.5µm. Over a four month period, wafer uniformity,
refractive index, wafer stress, and etch rate were tracked
with no significant difference found.
The elimination of hydrogen generated a four-month
return on investment (ROI). Mean time between failure
(MTBF) was increased from 3 weeks to 32 weeks based on
three Steamers operating over eight months.
Introduction
Thermal oxidation of silicon is an important process
step in the fabrication of semiconductor, MEMS, and
photovoltaic devices. At the time of this study, a MEMS
manufacturer was experiencing continued growth in
its MEMS product line. The fab is at an urbanized location and local ordinances had constrained its ability to
expand hydrogen storage. This is typical in older fabs
The elimination of hydrogen
generated a four-month ROI.
that often start out in remote locations, but over time
get urbanized and thus become subject to increased
local regulations.
One of the major consumers of hydrogen at this facility was the hydrolytic torches for wet oxidation. RASIRC
approached the manufacturer with the option to replace
its pyrolytic torches with steamers. The steamer uses
de-ionized water as its steam source, thus eliminating all
dependence on hydrogen and oxygen gas, allowing the
manufacturer to expand without the need to obtain additional hydrogen gas permits. Designed for semiconductor and photovoltaic applications, steamer technology
creates ultra high purity steam using controlled delivery
systems and proprietary steam purification technology.
Ultra-high purity is achieved by using a non-porous
hydrophilic membrane that selectively allows water
vapor to pass. All other molecules are greatly restricted,
so contaminants in water such as dissolved gases, ions,
total organic carbons ( TOCs), particles, and metals can
