Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
2002-08-22
2004-02-24
Verbitsky, Gail (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S208000, C374S187000, C374S121000, C219S390000
Reexamination Certificate
active
06695886
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor processing systems, and more particularly to a pyrometer system for measuring a wafer temperature employed in a rapid thermal processing (RTP) tool.
BACKGROUND OF THE INVENTION
Thermal processing furnaces have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace.
Both conventional types of furnaces are designed to heat semiconductor wafers to desired temperatures to promote either diffusion of implanted dopants to a desired depth while maintaining substantially small line widths (e.g., smaller than 1 micron), or to perform other conventional processing techniques, such as the application of an oxide layer to the wafer or deposition of a chemical vapor layer to the wafer. The heating requirements of the wafer during processing are known and well understood, and therefore are closely monitored.
Conventional vertical-type thermal processing furnaces, such as tube furnaces, are designed to support the processing tube within the furnace in the vertical position. The thermal furnace also typically employs a wafer boat assembly that is mounted to appropriate translation mechanisms for moving the wafer boat into and out of the processing tube. A wafer-handling assembly is deployed adjacent and parallel to the wafer-boat assembly to transfer the semiconductor wafers from wafer cassettes to the wafer-boat assembly. The wafers are then raised into a quartz or silicon heating tube. The tube is then slowly raised to the desired temperature and maintained at that temperature for some pre-determined period of time. Afterwards, the tube is then slowly cooled, and the wafers are removed from the tube to complete the processing. A drawback of this processing technique is that it places constraints on the time-at-temperature to which a wafer can be subjected.
As the critical dimensions for silicon integrated circuits are continuously scaled downward into the sub-micron regime, requirements for within wafer temperature uniformity and wafer-to-wafer temperature repeatability become more stringent. For example, in 0.18 micron technology, the required wafer-to-wafer temperature repeatability is in the order of about +/−3° C.
Pyrometry has been one method of choice for non-contact temperature measurements of a silicon wafer during processing in a thermal processing furnace. Pyrometry is based on the principle that all objects at temperatures above absolute zero emit electromagnetic radiation as a function of temperature in accordance with Planck's equation. Based upon that relationship, the temperature of an object may be determined from a distance by measuring its emitted radiation. However, the spectral emissivity value of the surface being measured must be known to calculate the actual temperature. Typically, silicon wafers have backside layers that can drastically alter the spectral emissivity of the wafer through interference effects, which can lead to temperature measurement errors during processing. Furthermore, the emissivity of the wafer is also dependent on the backside surface roughness and wafer temperature. All of these drawbacks make the determination or prediction of wafer emissivity a difficult task.
One technique employed to accurately measure the wafer temperature using pyrometry comprises modified single-color pyrometry with wafer emissivity compensation. An exemplary prior art pyrometry system using such compensation is illustrated in prior art
FIG. 1
, and designated at reference numeral
10
. The single-color pyrometer
10
includes an elevator tube
12
(not shown to scale) having a spider collar
14
coupled to a top portion
16
thereof. The spider collar
14
has several legs
18
(e.g., three (
3
)) and holds a wafer
20
at a predetermined distance from the tube
12
. The spider collar
14
may further hold an edge ring (not shown) that may be employed for wafer edge temperature uniformity control.
A pyrometer head
22
is coupled to a bottom portion
24
of the elevator tube
12
. The pyrometer head
22
contains an optical system (e.g., a number of lenses and apertures) that facilitates a flash emission to the wafer
20
and receipt of emitted and reflected light along or parallel to an optical axis
26
. The head
22
operates in conjunction with a radiometry channel
28
and a reflectometry channel
30
which communicate signals
32
to a processor
34
for determination of the wafer temperature.
In addition, the pyrometry system includes a port
36
coupled to the elevator tube
12
for the introduction of cooling gas
38
, such as nitrogen, thereto through, for example, a supply line
40
. Because a bell jar (not shown) is typically used to heat the wafer
20
, non-uniform heating of the wafer can occur, causing a center portion thereof to become hotter than peripheral areas. The elevator tube
12
delivers the cooling gas
38
to a center portion of the wafer
20
via the port
36
to assist in temperature uniformity thereat.
In operation, the pyrometry system
10
employs the radiometry channel
28
and the reflectometry channel
30
to determine the wafer temperature in the following exemplary manner. The radiometry channel
28
records the intensity of radiation emitted from the wafer
20
as well as radiation originating from stray light from the bell jar (not shown) and reflected from the wafer. The reflectometry channel
30
records the reflection intensity associated with a flash generated by the pyrometer head
22
. The channels
28
,
30
deliver the emission data and reflectivity data to the processor
34
that subtracts the stray light from the radiometry signal to obtain the black body intensity of the wafer. The processor
34
further calculates the wafer emissivity by assuming the emissivity is the complement of the wafer reflectivity, and thus extracts the wafer emissivity from the reflectometry signal. The wafer temperature is then calculated or otherwise determined by the processor
34
by dividing the black body intensity by the wafer emissivity.
As seen in prior art
FIG. 1
, the central optical element of the pyrometer system
10
is the pyrometer head
22
. Since the head
22
is mounted at the bottom of the elevator tube
12
to thermally isolate the components therein from the work piece that is at an elevated temperature, the optical path between the wafer
20
and the head
22
is relatively long, for example, about
1
meter. The relatively long separation between the wafer
20
and the head
22
requires that the wafer be extremely carefully aligned such that the wafer normal and the head
22
are aligned with the optical axis
26
. Even a small angle or offset of the wafer normal with respect to the axis
26
causes a loss in reflected and emitted signal intensity, thereby adversely impacting temperature calculation accuracy.
For example, as illustrated in prior art
FIGS. 2A and 2B
, if the wafer
20
is not warped or offset (
FIG. 2A
) the radiation
40
is fully collected at the bottom
24
of the tube
12
by the head
22
, while wafer warpage or offset (
FIG. 2B
) may cause reflected light
41
that results in signal loss. In order to correct the above problem, a lens
42
is inserted at the top portion
16
of the elevator tube
12
and is held in place via, for example, the spider collar
14
(not shown) at distance from the wafer that ideally is the lens focal length (e.g., about
50
mm), as illustrated in prior art FIG.
2
C. The lens
42
re-collects
44
any reflected light
41
that would otherwise not return to the pyrometer head
22
. The lens
42
thus allows or corrects for reflections due to warpage or offset, thus facilitating use of the pyr
Brown Douglas
Koch Mathias
Meadows Robert David
Tao David
Axcelis Technologies Inc.
Eschweiler & Associates LLC
Verbitsky Gail
LandOfFree
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