Patent Application
20060067605
The ability to measure the temperature of a very small device is important in many applications. For example, in the semiconductor integrated circuit (IC) industry it is often desirable to monitor the temperature of an integrated circuit to ensure that it is operating properly. Also, the manufacture of semiconductor integrated circuits depends on the growth of materials, and a strong influence on deposition rates, kinetics and composition of the final film is the deposition temperature. Because of the importance of deposition temperature, improved thermal heaters for deposition systems are being developed on a regular basis.
One important technique for measuring deposition temperature requires evaluating temperature profiles over large area heaters (greater than twelve inches). Good temperature uniformity leads to uniformity in the thickness, composition and microstructure of the final film; which, in turn leads to improvements in the yield of electronic and optical devices incorporating semiconductor integrated circuits. A thermocouple wafer having a plurality of thermocouples embedded in a silicon wafer is often used to measure the temperature profiles.
It is known that optical techniques can also be used to measure temperature. For example, it is known that by using a reflectivity signal from several dielectric materials, temperature can be determined if one of the dielectric materials exhibits a change in refractive index with temperature change. It has also been suggested that a physical parameter such as temperature can be measured with resonant frequency using resonators such as ring resonators.
In accordance with the invention, a photonic crystal optical temperature measuring system and a method for measuring the temperature of an object is provided. The photonic crystal optical temperature measuring system typically has at least one photonic crystal temperature sensor apparatus having a resonant cavity, the resonant frequency of which is a function of the temperature of the resonant cavity. Typically, a wavelength source illuminates the resonant cavity, and a detector detects the resonant frequency of the resonant cavity. A converter converts the detected resonant frequency into a temperature for providing a measure of the temperature of an object in the vicinity of the resonant cavity.
Furthermore, the invention provides embodiments and other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings.
Embodiments in accordance with the invention provide a photonic crystal optical temperature measuring system and a method for measuring the temperature of an object.
Photonic crystals are engineered structures that can be used to control the propagation of light. Photonic crystal resonators having a resonant cavity formed in a photonic crystal slab waveguide have been proposed for numerous applications including wavelength filtering, out of plane coupling and modulators. Photonic crystal resonators can provide high Q and are extremely compact, commonly having an area of less than 10 μm2.
The invention provides a photonic crystal optical temperature measuring system that includes a photonic crystal resonator that functions as a photonic crystal temperature sensor apparatus. For a photonic crystal resonator having a particular resonant cavity design, a specific resonant frequency is obtained at a particular temperature. The resonant frequency of the resonant cavity changes as the temperature of the resonant cavity changes. A converter converts a detected resonant frequency into a temperature for providing a measure of the temperature of an object in the vicinity of the resonant cavity.
Photonic crystal slab waveguide 206, may be implemented as photonic crystal slab waveguide 102 illustrated in
Some materials that satisfy the above criteria for a photonic crystal slab waveguide include Si (n=3.5, dn/dT (thermo-optic coefficient)=1.8×10−4K−1); Ge or compound semiconductors including GaAs (n=3.4, dn/dT=2.5×10−4K−1); InP (n=3.1, dn/dT=0.8×10−4K−1); CdS and CdSe.
The index of refraction of upper cladding layer 208 (nupper cladding) and the index of refraction of lower cladding layer 204 (nlower cladding) should be less than the index of refraction of photonic crystal slab waveguide 206 (nslab waveguide) in order to confine light in the plane of photonic crystal slab waveguide 206. For example, if photonic crystal slab waveguide 206 is formed of Si, lower cladding layer 204 can be formed of a material having an index of refraction of about 1.5 such as SiO2 or spin on glass, and upper cladding layer 208 can be formed of a material having an index of refraction of about 1.4-1.5, such as SiO2, spin on glass or another polymer-based material. Substrate 202 preferably comprises a Si or other semiconductor substrate.
The resonant cavity in the photonic crystal slab waveguide of each photonic crystal temperature sensor apparatus 304 in array 306 is connected to a ridge waveguide 310 that couples light from an optical fiber, schematically illustrated at 314 connecting a wavelength source 312 such as a tunable laser or a broad band light source to each resonant cavity. A detector 316 detects the resonant frequency of each of the resonant cavities. A suitable resonant frequency detector comprises a scanning monochromator for a broad band light source and a solid state semiconductor detector such as Si for visible wavelengths, Ge for visible to ˜1.5 μm and InGaAs for over infrared wavelengths. For the tunable laser, a solid state semiconductor laser is all that is needed. A converter 318 converts the detected resonant frequency to a temperature for providing a measure of the temperature of object 302. The conversion is based on the calibration of the photonic crystal resonant cavity frequency at known temperatures. A simple algorithm based on the calibration is used in the converter to perform the calculation quickly.
In the particular configuration illustrated in
According to yet further exemplary embodiments in accordance with the invention, light can be coupled into the array of photonic crystal temperature sensor apparatus by techniques that use gratings or other light coupling structures.
The out of plane output signal in photonic crystal optical temperature measuring system 400 can be enhanced by adjusting nupper cladding to be higher than nlower cladding. Several upper cladding materials can further enhance vertical coupling such as Si3N4
(n=1.9), MgO (n=1.8), Al2O3 (n=1.76), ZrSiO4 (n=1.95), SrO (n˜2.0), Ta2O5 (n=2.2), SrxBa(1-x)TiO3 (n=2.2) and TiO2 (n=2.4-2.7) as are described in commonly assigned, co-pending application Ser. No. 10/910,216, filed on Aug. 3, 2004, and entitled PHOTONIC CRYSTAL RESONATOR APPARATUS WITH IMPROVED OUT OF PLANE COUPLING, the disclosure of which is hereby incorporated by reference.
Photonic crystal temperature sensor apparatus 200 may be fabricated by patterning a resist with the design of the resonant cavity using electron beam lithography or another nano-lithography technique. The pattern is then transferred into the upper cladding layer by a selective etch technique. The reverse pattern can also be fabricated so that a metal lift-off technique can be used to prepare a hard metal mask. This reverse pattern procedure may be particularly useful in order to obtain good etch selectivity when good etch selectivity does not exist between the resist and the upper cladding layer.
A selective etch is used to etch away the Si to form the air holes and resonant cavity in the photonic crystal slab without removing the upper cladding layer. The etch may stop on the lower cladding layer or proceed through the lower cladding layer, depending on the selectivity of etches between the photonic crystal slab and the lower cladding layer.
A lithographic technique for fabricating a temperature sensitive photonic crystal slab waveguide formed of Si provides a low cost manufacturing procedure. The sensitivity and data points also increase substantially with a Si-based photonic crystal temperature sensor apparatus as compared to a thermocouple wafer. A Si-based photonic crystal temperature sensor apparatus can also enable very efficient characterization of temperature profiles in silicon manufacturing tooling that use thin film deposition chambers.
The sensitivity of the temperature measurement depends on many factors including Δn/Δλ, the ability to resolve a resonant peak position and the thermo-optic coefficient (dn/dT). For example, in order to sense a one degree change in temperature in a Si-based photonic crystal temperature sensor apparatus, (dn/dT=1.8×10−4K−1) with Δn/Δλ, it is necessary to resolve a λ of 0.27 nm. Increasing the thermo-optic coefficient by a factor of 4 requires resolving only 1 nm shifts in resonant frequency. As the Q in a cavity increases, and the full width at half maximum (FWHM) of the intensity of the resonant wavelength signal decreases, the peak position can be somewhat easier to resolve. Furthermore, by providing an array of photonic crystal temperature sensor apparatus, and by detecting the change in resonant frequency in each of the plurality of photonic crystal temperature sensor apparatus at once and/or by using different designs of resonators that are referenced to one another, temperature measuring precision can be improved. Because of the small area occupied by each photonic crystal temperature sensor apparatus (about 4 μm2, the number of photonic crystal temperature sensor apparatus incorporated into the array can be 10-20 or more, depending on the requirements of a particular application. The small size of the photonic crystal temperature sensor apparatus also suggests applications wherein a temperature sensor may be located on electrical or optical devices for in-situ temperature sensing.
Several mechanisms can be used to improve the design of the photonic crystal temperature sensor apparatus. For example, a material for the photonic crystal slab waveguide can be selected that has a larger thermo-optic effect and a refractive index change that is larger per temperature change. Another improvement to a silicon-based temperature sensor apparatus would be to select amorphous Si (a-Si:H; dn/dT=3.4×10−4K−1) or silicon carbide (SiC; dn/dT=7.1×10−4K−1) compared to Si (dn/dT=1.8×10−4K−1). In the case of SiC, however, the refractive index is approximately 2.7 and this relatively low refractive index may reduce the Q in the resonant cavity, thereby necessitating some tradeoff.
Another mechanism for improving the photonic crystal temperature sensor apparatus is to design a resonant cavity having a very high Q. A photonic crystal temperature sensor apparatus incorporating Si photonic crystal slab waveguide 102 illustrated in
While what has been described constitute exemplary embodiments in accordance with the invention, it should be recognized that the invention can be varied in numerous ways without departing from the scope thereof. For example, although in the exemplary embodiments in accordance with the invention described herein, the photonic crystal slab waveguides include air holes, the holes can also be filled with another gas or a vacuum. Because embodiments in accordance with the invention can be varied in numerous ways, it should be understood that the invention should be limited only in so far as is required by the scope of the following claims.
