Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
2000-03-07
2001-10-09
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S127000, C374S128000, C374S130000, C374S134000, C374S131000
Reexamination Certificate
active
06299346
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to non-contact, optical determination of the temperature of a body. More particularly, the present invention relates to a method and apparatus for non-contact, optical determination of the temperature of a wafer of semiconductor material during processing, for the manufacturing of integrated circuits. A review of the need for wafer temperature monitoring, during processing, can be found in Graham Jackson, Yael Baharav and Yaron Ish-Shalom, “The use of temperature monitoring in advanced semiconductor industry processing”,
Business Briefing of the Association of South East Asian Nations: Semiconductor Manufacturing Technology
, pp. 93-96, 1998.
Pyrometry is a well-known non-contact method of determining the temperature of a body such as a semiconductor wafer undergoing processing. As is well known, pyrometry infers the temperature of a body from the intensity of the electromagnetic radiation emitted by the body at different wavelengths (self-emission). According to Planck's radiation formula, the intensity of the radiation E
b&lgr;
d&lgr; emitted by an ideal blackbody in the wavelength band between wavelength &lgr; and wavelength &lgr;+d&lgr; is given by:
E
b
λ
ⅆ
λ
=
hc
3
λ
5
ⅆ
λ
exp
(
hc
/
k
λ
T
)
-
1
(
1
)
where h is Planck's constant, c is the speed of light, k is Boltzmann's constant, and T is the temperature of the blackbody. In the case of a real body, equation (1) must be modified as follows:
E
λ
ⅆ
λ
=
hc
3
λ
5
ⅆ
λ
exp
(
hc
/
k
λ
T
)
-
1
ϵ
(
λ
)
(
2
)
where &egr;(&lgr;) is the (usually wavelength-dependent) emissivity of the body. For an ideal blackbody, &egr;(&lgr;) is identically equal to 1 at all wavelengths. For an opaque (zero transmittance) body, the emissivity and the reflectivity R are related by a special case of the law of energy conservation, in combination with Kirchoff's law that states that the absorptivity of the body and the emissivity of the body are equal:
&egr;(&lgr;)+R(&lgr;)=1 (3)
It should be noted that equations (1) through (3) hold for each wavelength separately. This is important for the discussion below about emissivity-compensated temperature measurements of bodies with wavelength-dependent emissivity.
With regard to the angular distribution of the emitted radiation, and the behavior of &egr;(&lgr;) and R(&lgr;) as functions of angle:
(i) in the case of a blackbody, E
b&lgr;
d&lgr; is Lambertian: the radiant intensity per unit steradian is proportional to cos(&thgr;), where &thgr; is the angle between the line of sight and the normal to the radiating surface;
(ii) equation (2) expresses the total intensity integrated over all solid angles, whereas the angular dependence of E
&lgr;
d&lgr; depends on the physical properties of the radiating object;
(iii) equation (3) is valid for two different situations, that in which &egr;(&lgr;) and R(&lgr;) are integrated over an arbitrary solid angle, and that in which &egr;(&lgr;) and R(&lgr;) are measured in a specific direction, as long as both &egr;(&lgr;) and R(&lgr;) are integrated over the same solid angle, or as long as &egr;(&lgr;) and R(&lgr;) are measured in the same direction.
In active pyrometry, the emissivity of an opaque body is determined by directing radiation of a known intensity at the body, receiving reflected radiation from the body, inferring the reflectivity of the body from the intensities of the incident and reflected radiations, and subtracting the reflectivity from
1
to obtain the emissivity. A representative patent in this field is Patton, U. S. Pat. No. 5,029,117, which is incorporated by reference for all purposes as if fully set forth herein. Patton measures the temperature of a semiconductor wafer by directing radiation from a source at the back side of the wafer, and passing light reflected and emitted by the wafer to a detector via a rotating slotted disc. The detector produces signals that are alternately representative of the intensity of combined reflected and emitted radiation, representative of the intensity of emitted radiation only, and representative of background. From these signals, the temperature of the wafer is inferred.
It is well known that, if a body has a known emissivity, which is constant as a function of wavelength but which may or may not vary as a function of temperature, then a radiometric measurement of self-emission in one single wide wavelength band provides enough data to accurately derive the temperature of the body. However, if the emissivity of the body is a function of wavelength, then the temperature can be accurately derived from self-emission measurements only if the signals are acquired in one or more narrow wavelength bands. (One narrow wavelength band suffices if the emissivity at that wavelength is known; otherwise, two or more narrow wavelengths bands are needed.) This is because a wide band measurement gives a signal which is proportional to the integral of the product of two wavelength-dependent functions, the emissivity and the blackbody Planck function of equation (2), and as a result, there is in general no unique correspondence between the self-emission signal and the temperature of the measured object. Pyrometric methods such as Patton's do not use narrow wavelength bands, and so are suboptimal for measuring the temperature of semiconductor wafers undergoing processing.
The emissivity of a silicon wafer undergoing processing is often a strong function of wavelength.
FIG. 1
shows the emissivity of a silicon wafer with two layers, polysilicon above silicon dioxide, deposited on the silicon substrate of the wafer, as a function of wavelength. The thickness of the silicon dioxide layer is 1000 Å. Five different thicknesses of polysilicon are shown, as indicated. This wavelength dependence degrades the accuracy of the temperature measurement.
This variation of emissivity with wavelength has been addressed by “multi-wavelength pyrometry”, most commonly by a special case thereof, dual wavelength pyrometry. Stein, in U. S. Pat. No. 4,708,493, uses a dichroic beam splitter to gather reflected radiation and emitted radiation in two identical narrow wavelength bands. The two reflection signals from two separate diode lasers are used to estimate the emissivity of a body in the same wavelength bands. These emissivity values are then used together with the self-emission signals in the same bands to derive the temperature of the body. Gat et al., in U. S. Pat. No. 5,114,242, and Glazman, in WO 97/11340, obtain temperature and emissivity in a self-consistent manner from measurements of emitted radiation in several wavelength bands. These and similar methods require relatively complicated optical systems.
In principle, it is preferable to measure the emissivity directly at the same wavelength as the self-emitted radiation is measured. However, application considerations (specific production processes require different working temperature ranges) and engineering considerations (commercially available radiation sources and detectors yielding appropriate signal to noise ratio) may dictate that the emissivity be measured in different wavelength bands than the one in which the self-emitted radiation is measured. In general, as part of the design considerations, there is also the need to measure the self-emission from the same area on the wafer as the emissivity is measured (for the emissivity correction mentioned above), in order to avoid errors associated with non-uniformity of the thickness of films deposited on a semiconductor wafer, as a consequence of the dependence of the emissivity on the film thickness, as illustrated in FIG.
1
.
For the past several years, C. I. Systems Ltd., of Migdal HaEmek, Israel, has been developing and selling systems for measuring the temperatures of se
Baharav Yael
Ish-Shalom Yaron
C. I. Systems LTD
Friedman Mark M.
Gutierrez Diego
Verbitsky Gail
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