Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
1998-08-06
2001-01-16
Fulton, Christopher W. (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S130000, C219S405000
Reexamination Certificate
active
06174080
ABSTRACT:
BACKGROUND
The present invention relates to apparatus and methods for measuring a substrate temperature during thermal processing using polarized radiation.
In rapid thermal processing (RTP), a substrate is heated quickly and uniformly to a high temperature, such as 400° Celsius (C) or more, to perform a fabrication step such as annealing, cleaning, chemical vapor deposition, oxidation, or nitration. For example, a thermal processing system, such as the RTP tool available from Applied Materials, Inc., under the trade name “Centura
R
”, may be used to perform metal annealing at temperatures of 400° C. to 500° C., titanium silicide formation at temperatures around 650° C., or oxidation or implant annealing at temperatures around 1000° C.
The substrate temperature must be precisely controlled during these thermal processing steps to obtain high yields and process reliability, particularly given the submicron dimension of current semiconductor devices. For example, to fabricate a dielectric layer 60-80 angstroms (Å) thick with a uniformity of +/−2 Å, a typical requirement in current device structures, the temperature in successive processing runs cannot vary by more than a few °C. from the target temperature. To achieve this level of temperature control, the temperature of the substrate is measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the object's temperature can be determined. A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain the substrate temperature. The relationship between spectral intensity and temperature depends on the spectral emissivity of the substrate and the ideal blackbody intensity-temperature relationship, given by Planck's law:
I
b
(
λ
,
T
)
=
2
C
1
λ
5
(
ⅇ
c
2
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 °K. The spectral emissivity &egr;(&lgr;,T) of an object is the ratio of its emitted spectral intensity I(&lgr;,T) to that of a black body at the same temperature I
b
(&lgr;,T). That is,
ε
(
λ
,
T
)
=
I
(
λ
,
T
)
I
b
(
λ
,
T
)
(
2
)
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.
The emissivity of a substrate depends on many factors, including the characteristics of the wafer itself (e.g., temperature, surface roughness, doping level of various impurities, material composition and thickness of surface layers), the characteristics of the process chamber, and the process history of the wafer. Therefore, a priori estimation of substrate emissivity cannot provide a general purpose pyrometric temperature measurement capability. Consequently, it is necessary to measure the emissivity of the substrate in situ. Furthermore, any uncertainty in the measured emissivity introduces an uncertainty into the temperature measurement.
To reduce this uncertainty, several techniques have been developed for reducing the effect of substrate emissivity on the temperature measurement. One such technique involves placing a reflector plate beneath the back surface of a substrate to form a reflecting cavity. If the reflector plate was an ideal reflector, the reflecting cavity would act as an ideal black body. That is, the intensity of the radiation within the reflecting cavity would not be a function of the emissivity of the surface of the substrate. Thus, in the ideal case, the reflecting cavity increases the effective emissivity of the substrate to a value equal to one.
However, because the reflector plate is not an ideal reflector, the effective emissivity of the substrate will be less than one, although it will be higher than the substrate's actual emissivity. Consequently, although variations in the actual emissivity of the substrate will have less impact on the measured temperature, there will still be uncertainties in the temperature measurements.
Furthermore, different regions of the substrate may have different emissivities. Consequently, if the emissivity of the substrate is measured in only one region, there will be uncertainty in the temperature measurements of other regions of the substrate.
SUMMARY
In one aspect, the invention is directed to an apparatus for measuring the temperature of a substrate in a thermal processing chamber, where the chamber includes a reflector forming a reflecting cavity with a substrate when the substrate is positioned in the chamber. The apparatus includes a first polarizer positioned to polarize radiation reflected by the reflector, a probe having an input end positioned to receive reflected and non-reflected radiation from the reflecting cavity, a polarizing system optically coupled to an output end of the probe, and a detector apparatus optically coupled to the polarizing system. The polarizing system is configured to generate a first beam and a second beam, and it includes a second polarizer oriented such that a ratio of reflected radiation to non-reflected radiation is higher in the first beam than the second beam. The detector apparatus generates a first intensity signal from the first beam and a second intensity signal from the second beam.
Implementations of the invention may include the following. A processor may be coupled to the detector apparatus to calculate a substrate temperature from the first and second intensity signals. The first polarizer may be located on a surface of the reflector. For example, the first polarizer may be a polarizing grating, such as a ruled or holographic grating. The first polarizer may polarize the radiation reflected from by the reflector with a first axis of polarization. The second polarizer may be configured to exclude radiation from the second beam having an axis of polarization parallel or perpendicular to the first axis of polarization.
The polarizing system may include a partially reflective surface positioned in an optical path of radiation exiting the output end of the probe to split that radiation into the first and second beams. A polarizing filter may be positioned in an optical path of the first beam and oriented be to block radiation having an axis of polarization orthogonal to the first axis of polarization, and a polarizing filter may be positioned in an optical path of the second beam and be oriented to block radiation having an axis of polarization parallel to the first axis of polarization. A polarizing beam splitter may be positioned to split the radiation exiting the output end of the probe into the first and second beams. The second polarizer may include a polarizing filter movable between a first position in which the polarizing filter blocks radiation having an axis of polarization orthogonal to the first axis of polarization to form the first beam and a second position in which the polarizing filter blocks radiation having an axis of polarization parallel to the first axis of polarization to form the second beam. The second polarizer may include an electro-optic polarizing filter switchable between a first state in which the polarizing filter blocks radiation having an axis of polarization that is orthogonal to the first axis of polarization to form the first beam and a second state in which the polarizing filter transmits radiation having an axis of polarization that is orthogonal to the first axis of polarization to form the second beam. Alternately, the electo-optic polarizing filter may transmit radiation having an axis of polarization that is parallel to the first axis of polarization in the first state, and block radiation having an axis of polarization that is parallel to the f
Applied Materials Inc.
Fish & Richardson
Fulton Christopher W.
Pruchnic Jr. Stanley J.
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