Data processing: measuring – calibrating – or testing – Measurement system – Performance or efficiency evaluation
Reexamination Certificate
1998-06-26
2001-03-20
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Performance or efficiency evaluation
C702S100000, C700S282000, C700S266000, C137S486000
Reexamination Certificate
active
06205409
ABSTRACT:
BACKGROUND OF THE INVENTION
Mass Flow Controllers (MFC) are used extensively in modern semiconductor manufacturing to control the flow of various gases into a wide variety of process equipment, such as etchers, deposition reactors, implanters, etc. Many individual pieces of equipment have more than one such MFC, with six to eight MFCs per major piece of semiconductor manufacturing equipment not being uncommon.
A representative MFC
100
is shown in three orthogonal views in
FIGS. 1A
,
1
B, and
1
C. A source of gas is connected to and received at an input port
102
and a controlled flow is delivered to an output port
104
and to downstream equipment attached thereto. A housing
106
contains an electronic module for controlling the MFC which is connected to a system controller (not shown) by way of a cable attached to an MFC card edge connector
108
.
To more fully appreciate the invention, a brief description of the inner workings of a typical MFC is warranted. Referring now to
FIG. 2
, the representative MFC
100
includes four distinct subsystems. A mass flow sensor
120
produces an electrical signal proportional to the mass of the gas flowing therethrough. A control valve
122
acts as a variable orifice to control the total gas flowing through the MFC
100
. A flow bypass
124
diverts a small portion of the total gas flowing into the MFC
100
into and through the mass flow sensor
120
. Lastly, a printed wiring board
126
contains various circuits for controlling the MFC
100
, and typically include a sensor bridge and amplifier circuit, a control circuit, and an RC network, as described more fully below.
Referring now to
FIG. 3
, the basic operation of the MFC
100
is described. The mass flow sensor
120
is illustrated as a “capillary tube” thermal mass flow sensor which is designed to measure the mass of gas flowing through a thin stainless steel capillary sensor tube
131
. Two temperature sensing wire windings
130
,
132
are attached around the sensor tube
131
. One winding is located on the upstream side of the sensor tube
131
and the other winding is located on the downstream side of the sensor tube
131
. The wire forming these windings
130
,
132
is resistance thermal detection (RTD) type wire, which means the resistance of such wire is a function of the temperature of the wire. The sensor tube is installed in a protective cover and is usually enclosed in heat insulating material. An equal amount of heat is produced in both sensor windings either directly by a constant current source or by using a separate heater wire winding (not shown) between the upstream sensor winding
130
and the downstream sensor winding
132
.
With no gas flowing through the sensor
120
, both the upstream sensor winding
130
and the downstream sensor winding
132
are at the same temperature, and consequently, they both have the same resistance. Since each winding
130
,
132
has the same resistance, and the same current flows through each winding, the voltage drop across each winding
130
,
132
is the same. The voltage drop across each winding
130
,
132
is compared by a sensor bridge and amplifier circuit
134
to produce an MFC output voltage conveyed on sensor output terminal
140
.
With 50% gas flow through the mass flow sensor
120
, gas at room temperature flows through the sensor tube
131
and heat from the upstream sensor winding
130
is transferred to the gas molecules. This reduces the temperature of the upstream sensor winding
130
, and increases the temperature of the gas. As this hotter gas flows past the downstream sensor winding
132
it transfers less heat away from the downstream sensor winding
132
. This difference in temperature between the upstream sensor winding
130
and the downstream sensor winding
132
results in a difference in resistance between the two windings
130
,
132
, which then results in a difference in voltage across the two windings
130
,
132
. This voltage difference is amplified and linearized by sensor bridge and amplifier circuit
134
to become the MFC output voltage at sensor output terminal
140
. This output voltage is an indirect result of gas molecules flowing through the mass flow sensor
120
.
In other words, the difference in temperature between the upstream sensor winding
130
and the downstream sensor winding
132
is sensed as a small (millivolts) non-zero voltage by the sensor bridge and amplifier circuit
134
. This small voltage is amplified to a typical level of several volts and linearized to provide a 0 to 5 volt DC output voltage signal (for many commercial MFCs) which is proportional to the mass of the gas flowing through the mass flow sensor
120
. If the ratio of gas flowing through the mass flow sensor
120
and through the flow bypass
124
is correct, the output signal is proportional to the mass of the gas flowing through the MFC
100
from the input port
102
to the output port
104
.
A control circuit
136
compares the output voltage signal produced by the sensor bridge and amplifier circuit
134
against an externally supplied setpoint signal conveyed on terminal
142
. The setpoint signal is usually a 0 to 5 volt DC signal and corresponds to the actual flow desired through the MFC. The control circuit
136
drives a valve control transistor
138
which positions the control valve
122
in such a manner as to eliminate any difference between the setpoint signal and the output signal. If the actual flow (represented by the MFC output voltage) is less than the desired flow (as represented by the MFC setpoint voltage), the control circuit
136
biases the valve control transistor
138
in such a manner as to open the control valve
122
to allow more gas flow through the control valve
122
, and hence through the MFC
100
. As more gas flows through the MFC
100
, proportionally more gas flows through the mass flow sensor
120
causing the MFC output voltage to increase. This reduces the difference between the MFC setpoint voltage and the MFC output voltage.
The electrical schematic diagram of a particular manufacturer's MFC is shown in FIG.
4
. Shown is the schematic for a model FC-2950M mass flow controller/flowmeter available from Tylan General, Inc., located in San Diego and Torrance, Calif. The upstream sensor winding
130
and downstream sensor winding
132
are shown as resistors which connect into a bridge circuit
154
, the outputs of which are then amplified by an amplifier
156
to produce the MFC output voltage at terminal
140
. Various RC feedback circuitry within the amplifier
156
serve to stabilize the operation of the amplifier
156
. The bridge circuit
154
, amplifier
156
, and other feedback and reference circuits shown form the sensor bridge and amplifier circuit
134
described previously. The control circuit
136
receives the MFC output voltage and compares it to the MFC setpoint voltage on terminal
142
to control the valve control transistor
138
, whose output terminal
150
is connected to the control valve
122
(modeled on the schematic as a resistor connected to terminals V
1
and V
2
) through a current-limiting resistor
158
. Each of the signals shown on the left side of
FIG. 4
are usually available at the MFC card edge connector
108
.
As described above, the closed-loop feedback operation of the control circuit
136
causes the MFC output voltage on terminal
140
to be driven to match the MFC setpoint voltage on terminal
142
. The “valve voltage” on terminal
150
is adjusted to whatever voltage is required to adjust the gas flow in order to cause the MFC output voltage to match the MFC setpoint voltage. A system controller which is connected to the MFC card edge connector
108
presents to the MFC
100
the desired MFC setpoint voltage, and then monitors the MFC output voltage produced by the MFC
100
in response thereto.
SUMMARY OF THE INVENTION
The closed loop nature of the MFC
100
results in the MFC output voltage always matching, if at all possible, the MFC setpoint voltage. Unfortunately, subtle changes in the MFC, or in the
Advanced Micro Devices , Inc.
Shah Kamini
Skjerven Morrill & MacPherson LLP
Terrile Stephen A.
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