Wafer Cleaning System
provides safety and technological advancements to the industry
by Terry Persun
SpeedFam-IPEC, Inc. (SFI), Chandler, AZ,
is a leading OEM for Chemical Mechanical Planarization (CMP) capital equipment.
SFI designs, develops, manufactures, and supports CMP systems, which are
used in the fabrication of semiconductor devices, as well as other high-throughput
precision surface processing systems for the fiber-optic industry, These
can include single-side free-abrasive and polishing machines (FAM); single-side
gyromatic lapping and polishing machines; and double-sided lapping, grinding,
and polishing machines. When SFI contracted Tokyo Electron & Unify to
help design Clarus, its second generation post-CMP wafer cleaner, Unify's
electrical engineer Ohnishi-san recommended the use of the AlphaStep line
from Oriental Motors, Torrance, CA, for four of the machine's axes. Although
both servos and steppers were analyzed for the Clarus applications, steppers
were selected for particular axes because Clarus required very high holding
torque with relatively short stroke movement.
A SpeedFam-IPEC CMP machine
The AlphaStep system combines a specially-designed stepper motor and
driver to provide closed-loop performance. A rotor position sensor is built
into the motor itself and sends an output signal back to the motor driver.
This sensor's output is in the form of two 90-degree out-of-phase signals,
providing a resolver feedback that the driver interprets. In this way, the
drive knows precisely where the rotor position is at all times. Because
the closed-loop functionality is engaged between the motor and driver, there
is no need to interface directly with the controller. The AlphaStep driver
has an emulated encoder output to interface with the axis controller, but
because the closed-loop functionality is already engaged between the motor
and driver, this feature is used primarily for end-of-move verification.
Close-up of the Clarus wafer cleaning component
There are many advantages to using the closed-loop AlphaStep system:
high response time, low vibration at low speed, no gain tuning, and no hunting.
Like conventional stepping motors, the AlphaStep operates synchronistically
with command pulses. This makes short stroke/short time positioning possible
while reducing settling time. The driver produces smoothness due to its
microstepping driver (up to 10,000 steps/revolution), and its vibration
is very low, even when operating in the low speed range. It can provide
resolution as low as 0.036 degree per step without any damping mechanism
or other mechanical device. These features are important while moving delicate
wafers through processing.
An alternative to the step drive system would have been a servo system.
When analyzed for the particular application, SFI found that a servo drive
needs additional interfacing circuitry to operate properly, while the AlphaStep
set-up is like a traditional stepping motor system. In fact, the driver
only needs either an AC or DC power source and a 5-volt pulse stream (from
a PLC or controller). No additional time or effort is needed for gain tuning,
and because it behaves like a stepping motor, there is no potential hunting
problem at rest. Nonetheless, like other motion profiles, some adjustment
is always required, such as accel/decel, jerk and integral gain.
AlphaStep components from Oriental Motors
SFI was one of the first U.S. companies to start using the Oriental Motor's
system, which had already become popular for many Japanese closed-loop applications.
The Oriental Motor line offers small integral gear heads, which are important
in space-limited applications. In addition to the AC configurations, the
product line also provides low-voltage DC drives, which are necessary when
designing for safety in wet applications such as those in the SFI Clarus
Cleaner application. Closed loop drives offer additional current compensation,
i.e. torque, as a first defense against synchronicity loss, as compared
to similar-sized conventional open-loop stepper motors.
According to Ned Teeny, Controls Technical Manager in the CMP Group at
SpeedFam-IPEC, "I worked with both Oriental Motor's sales engineer
Steve Wade and the company's western US marketing manager Keith Watanabe.
Oriental Motor and their distributor provided excellent technical support
both in the US and in Japan. SFI tested and integrated a total of four control
axes on the new Clarus Cleaner. These particular axes required high torque
and high precision, for which the closed-loop step motors were perfect."
Although some of the specifications for the SFI project are proprietary,
the 60mm frame-size AlphaSteps can produce up to 276 oz-in of holding torque
without a gearhead, and up to 69.4 lb-in when coupled to a gearhead. Accuracy
runs from ±5 arc-min for non-geared types, down to as little as ±3
arc-min for some geared types.
Ghost view of AlphaStep built-in sensor
The emulated encoder feedback output from the AlphaStep was used to execute
coordinated multi-axis moves on the pick and place robot arm, providing
the system with fast and smooth move profiles. The high torque and quick-move
capability of the AlphaStep system provided for faster transfer between
stations that translated to an increase in the cleaner throughput.
"The time we spent on the Clarus Cleaner design was well worth the
outcome," Ned says, "because the new Clarus post-CMP cleaner not
only offers world-class safety features, but cleaning performance, reliability,
maintainability and throughput."
For more information:
Circle 480 SpeedFam-IPEC, or connect directly to
their website via Designfax's Online Reader Service Program at http://www.OneRS.net/106df-480
Circle 481 Oriental Motor USA Corp., or connect
directly to their website via Designfax's Online Reader Service Program
at http://www.OneRS.net/106df-481
See Oriental Motor at Semicon West
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