Thermal measuring and testing – Thermal calibration system
Reexamination Certificate
1999-07-09
2001-08-28
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Thermal calibration system
C324S076490, C324S067000, C324S067000, C324S066000, C324S09900D, C323S316000, C327S512000, C374S121000, C374S002000
Reexamination Certificate
active
06280081
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus for calibrating temperature measurements, e.g., during rapid thermal processing of substrates, and for measuring currents.
In rapid thermal processing (RTP), a substrate is heated quickly to a high temperature, such as 1200° C., to perform one or more fabrication steps such as annealing, cleaning, chemical vapor deposition, oxidation, or nitridation. To obtain high yields and process reliability in the manufacture of submicron devices, the temperature of the substrate during such fabrication steps must be precisely controlled. For example, to fabricate a dielectric layer with a thickness of 60-80 Å and a uniformity of ±2 Å, the temperature in successive thermal processing runs should not vary by more than a few degrees centigrade (° C.) from the target temperature. To achieve this level of temperature control, the temperature of the substrate should be measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. An optical pyrometer using an optical probe samples the emitted radiation intensity from the substrate, and computes the temperature of the substrate based on the spectral emissivity of the substrate and the ideal black body radiation-temperature relationship given by Planck's law:
Ψ
b
(
λ
,
T
)
=
C
1
λ
5
(
e
c
θ
λ
T
-
1
)
1
where C
1
and C
2
are known constants, &lgr; is the radiation wavelength of interest, and T is the substrate temperature measured in degrees Kelvin (° K). The spectral emissivity &egr;(&lgr;,T) of an object is the ratio of its emitted spectral intensity &PSgr;(&lgr;,T) to the spectral intensity &PSgr;
B
(&lgr;,T) of a black body at the same temperature. That is,
ε
(
λ
,
T
)
=
Ψ
(
λ
,
T
)
Ψ
b
(
λ
,
T
)
Since C
1
and C
2
are known constants, under ideal conditions, the temperature of the substrate can be accurately determined if &egr;(&lgr;,T) is known.
When the temperature measurement system is first installed into the RTP system, the optical probe must be calibrated so that it produces an accurate temperature reading when exposed to the radiation emitted by the heated substrate. The temperature measurement system also must be periodically recalibrated because the temperature sensed by the probe may shift over time. Such temperature measurement shifts may be caused by, e.g., contamination of or damage to the light pipe that is used to sample the emitted radiation being emitted from the substrate, or by drifts in the electronic components of the pyrometer.
SUMMARY OF THE INVENTION
The invention features an inventive calibration current source and methods of calibrating temperature measurements made during, e.g., an RTP process (such as an RTP process described in U.S. Pat. No. 5,660,472). The invention also features an RTP system that incorporates these inventive calibration current source and temperature measurement calibrating methods.
In one aspect, the invention features an apparatus that includes a biasing circuit, an output transistor current source, and an offset circuit. The biasing circuit has an input, a reference voltage output and a biasing voltage output. The output transistor current source is coupled to the biasing voltage output and is configured to produce an output current. The offset circuit is coupled in a feedback loop between the reference voltage output and the biasing circuit input and is configured to generate from the reference voltage output a variable offset voltage for selectively controlling the biasing voltage applied to the output transistor current source.
Embodiments may include one or more of the following features.
The biasing circuit preferably includes first and second reference voltage circuits having respective inputs coupled to the biasing circuit input and respective outputs. The first and second reference voltage circuits each preferably includes a precision current source coupled to a transistor current source; the precision current sources substantially fixed the voltage difference between the biasing circuit input and the outputs of the first and second reference voltage circuits. The voltages generated at the outputs of the first and second reference voltage circuits preferably substantially correspond to the sum of the voltage generated by the offset circuit and the voltages substantially fixed by the precision current sources. The transistor current sources of the first and second reference voltage circuits may be matched to the output transistor current source. The biasing circuit preferably further includes a differential amplifier having first and second inputs respectively coupled to the outputs of the first and second reference voltage circuits and an output corresponding to the reference voltage output of the biasing circuit. The biasing voltage output may correspond to the output of the second reference voltage circuit.
In one embodiment, the offset circuit includes a digital to analog converter (DAC) that is coupled to the reference voltage output, and has an input configured to receive an input signal controlling the offset voltage generated from the reference voltage. The DAC input signal preferably controls the value of the output current in a one-to-one relationship. A log integrator circuit having an input coupled to receive the current output produced by the output transistor current source may be provided. A processor coupled to an input of the DAC and configured to generate a calibration table relating log integrator output voltage to DAC input signal value and a memory configured to store the calibration table generated by the processor also may be provided. A photodetector preferably is coupled to the input of the log integrator. In one embodiment, a processing chamber and a light pipe extending into the processing chamber are provided. The photodetector may be coupled directly to the light pipe.
The invention also features a current measurement circuit (a “log integrator”) that includes an amplifier having an input and an output, a capacitor coupled between the input and the output of the amplifier, and a transistor coupled in parallel with the capacitor between the input and the output of the amplifier.
In one embodiment, a switch is coupled in parallel with the capacitor and the transistor between the input and the output of the amplifier.
In another aspect, the invention features a method in which a calibration table is generated by applying a plurality of input signals to a calibration current source to produce a plurality of output signals. The calibration table is stored. When an output signal of a photodetector is received, the calibration input signal corresponding to the received photodetector output signal is determined based upon the stored calibration table.
The received photodetector output signal may correspond to a measure of radiation intensity emitted by a substrate being processed. The temperature of the substrate may be computed based upon the calibration input signal determined to correspond to the received photodetector output signal. The calibration table may be generated between substrate processing runs inside a processing chamber.
Among the advantages of the invention are the following.
The invention enables temperature measurements to be readily calibrated over a wide dynamic range (e.g., over ten decades) without requiring an external calibration current source. The calibration may be performed on-the-fly between substrate processes and when the thermal processing system otherwise is idle. This feature reduces the maintenance cycle time for the system. The invention may be integrated onto the same circuit board as the photodetector and, therefore, may be configured to compensate for temperature-induced changes in the photodetector measurements. The invention may be implemented with a small footprint and therefore may be mounted directly to the optical transmission channel (e.g.
Blau David
Jadushlever Elia
Applied Materials Inc.
Bennett G. Bradley
Pennie & Edmonds LLP
Verbitsky Gail
LandOfFree
Methods and apparatus for calibrating temperature... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods and apparatus for calibrating temperature..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods and apparatus for calibrating temperature... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2505197