The present invention relates to a method and apparatus for non-contact temperature measurement of a moving semiconductor wafer employing both a pyrometer and a reflectometer to provide temperature and reflectivity data respectively to a general purpose computer.
During the fabrication of semiconductor wafers, numerous physical parameters, such as temperature, pressure and flow rate are monitored and regulated to achieve a desired crystal growth. Maintenance of a specific temperature during the wafer fabrication process is particularly important in order to achieve a high quality semiconductor crystal. The semiconductor wafers to be grown are often placed on rapidly rotating carousels, or carriers, so as to provide a uniform surface easily accessible to circulating semiconductor gasses within a reactor chamber for the deposition of the semiconductor materials. Therefore, measurement of the wafer temperature during the deposition process is problematic in that a non-contact method of measurement is required which can accurately measure the temperature of the semiconductor wafers rotating at high speeds; often greater than 1,000 RPMs.
Non-contact temperature measurement of objects is presently possible using devices which measure the radiation reflected from a target object. These devices, known as pyrometers, are used to calculate the temperature of a physical body based on emitted radiation power from the body and a physical characteristic of the body known as its emissivity. A body""s emissivity is a measure of the ratio of the emitted radiation from a body to the incident radiation. The body""s temperature can be computed given its emissivity (E) and emitted radiation power (P) according to                     P        =                              E            w                    ⁢          2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      C            1                    ⁢                                    ∫              0              ∞                        ⁢                                          1                                                                            (                                              λ                        ⁢                                                  xe2x80x83                                                ⁢                        T                                            )                                        5                                    ⁢                                      (                                                                  ⅇ                                                                                                            C                              2                                                        /                            λ                                                    ⁢                                                      xe2x80x83                                                    ⁢                          T                                                                    -                      1                                        )                                                              ⁢                              ⅆ                λ                                                                        (        1        )            
Planck""s equation:
Where
Ew=body emissivity (dependent on body color and surface characteristics)
C1, C2=traceable universal consants (Planck""s Spectral Energy Distribution)
xcex=radiation wavelength
T=body temperature
From the emitted radiation power (P), the corresponding temperature of the body (T) may be accurately determined using the above equation if the emissivity of the body is known.
Often, however, the emissivity of the semiconductor wafers whose temperature is to be determined changes during the course of the semiconductor growth process so as to complicate its temperature determination. Therefore, a real-time time determination of the semiconductor wafer emissivity is necessary, in order to properly calculate the temperature of the semiconductor wafers during all phases of the semiconductor growth process.
Further complicating determination of the semiconductor wafer temperature is the fact that the semiconductor wafers are often placed upon a carousel within a chemical vapor deposition (CVD) chamber. The CVD chamber is a sealed environment that allows infused gases to be deposited upon the wafers to grow the semiconductor layers. Rotation of the semiconductor wafers upon the carousel permits an even deposition of infused gases upon the wafers. The rapidly rotating carousel presents difficulties in accurately measuring the semiconductor wafer temperature by the pyrometer, however, in that the pyrometer is typically fixed over a single point along the radius of the carousel such that sequential temperature measurements may include readings from the semiconductor wafers, the carousel itself, or the boundary between the semiconductor wafers and the carousel.
Several possible approaches may be used to make wafer temperature measurements using a high-speed pyrometer positioned over a rapidly rotating wafer carrier. First, a sequence of temperature measurements using a real-time algorithm to separate wafer and carousel temperatures may be used. The problem with this approach is that modem pyrometers can provide high-speed measurements with single point data acquisition times in the range of 0.1 milliseconds, but they often require a much longer time, e.g., on the order of 20 milliseconds or more, between single measurements to perform calculations and self-calibration. As a result, sequential temperature measurements of the wafer carrier and wafer are not easily obtained so as to obtain reliable temperature measurements from the wafer only.
A second approach employs a triggering function in which the pyrometer measurements are taken at a frequency close to the period of rotation for the wafer carrier. With this approach, the pyrometer provides data measurements scanned across the disk, i.e. a stroboscopic effect, to generate temperature data. The xe2x80x9cdown timexe2x80x9d between temperature measurements is used to achieve the above-mentioned calculations and self-calibration. However, this method also has its shortcomings. In particular, modern pyrometers often initiate an autonomous self-calibration with respect to the ongoing measurements of temperatures. Such autonomous self-calibration necessarily interferes with any periodic gathering of temperature data from the pyrometer and makes such temperature determinations on a periodic basis extremely difficult.
To overcome the difficulties of the above solutions, an encoder may be installed on the carousel spindle to provide a xe2x80x9ctriggerxe2x80x9d for the initiation of pyrometer measurements. This approach, however, requires additional system complexity related to the synchronization between the encoder and the wafer locations. Further, the delay in communications between the encoder and the pyrometer requires complex calibration procedures for each different rotation speed of the carousel and, therefore, requires recalibration when changing speeds.
In sum, the present level of pyrometer development does not allow for the implementation of a simple and reliable method of measuring the temperature of semiconductor wafers on a rapidly rotating carousel, particularly where the emissivity of the semiconductor wafer is varying over the measurement time.
In accordance with the present invention an apparatus has been provided for determining the real-time, non-contact temperature measurement of first and second fast moving entities. The first and second entities have first and second reflectivities, and are disposed in a fixed relationship with respect to each other. Further provided in the apparatus are a pyrometer for providing a series of temperature data related to the temperatures of the first and second entities respectively, each temperature datum having a temperature value, a reflectometer for providing a series of reflectivity data related to the first and second reflectivities, each reflectivity datum having a reflectivity value and correlated with a corresponding temperature datum so as to form a reflectivity-temperature data pair, and a computer having a memory for storing a set of computer instructions. The computer is coupled to the pyrometer and the reflectometer for receiving the series of temperature data and the series of reflectivity data and the set of computer instructions include instructions for creating a data table for storing the reflectivity-temperature data pairs according to a frequency of occurrence of each reflectivity value in the series of reflectivity data, instructions for identifying at least one reflectivity data peak representative of the first reflectivity characteristic within the data table, and instructions for determining at least the temperature of the first entity from the reflectivity data peak based upon the associated reflectivity-temperature data pairs.
In accordance with an embodiment of the present invention, the first entity is a semiconductor wafer and the second entity is a semiconductor wafer carrier, and the first reflectivity of the semiconductor wafer is higher than the second reflectivity of the semiconductor wafer carrier. Similarly, the first reflectivity of the semiconductor wafer is of a specular type and the second reflectivity of the semiconductor wafer carrier is of a diffuse type.
In accordance with another embodiment of the present invention the second entity rotates to at least 100 revolutions per minute and at least one of the series of temperature data and the series of reflectivity data are provided at a rate of at least 15 samples per second when the second entity rotates to at least 1000 revolutions per minute.
In accordance with a preferred embodiment of the present invention the set of computer instructions includes instructions for dividing a range of reflectivity values of the series of reflectivity data into a number of data bins, each data bin having a low index value and a high index value, each datum of the series of reflectivity data being assigned to one of the data bins by the computer instructions according to its reflectivity value. Further, the series of reflectivity data is a continuous series of datum points, and the set of computer instructions maintains a sliding data window including a constant number of data points from the continuous series of datum points by replacing an oldest datum point within the data window with each new datum point of the series of reflectivity data, the set of computer instructions assigning all data points within the data window to the data bins. In addition, the set of computer instructions may include a prefilter for disregarding data points within the series of reflectivity data having reflectivity values outside a given range of reflectivity values. In a further embodiment, the set of computer instructions includes a tracking filter that excludes the reflectivity-temperature data pair if the difference between the reflectivity value of a first reflectivity-temperature data pair and reflectivity values in a group of reflectivity-temperature data pair including the first reflectivity-temperature data pair exceeds a predetermined value. In a final embodiment, the set of computer instructions includes instructions for determining an alternative peak reflectivity value for the reflectivity data peak where the reflectivity peak is not easily identified.
In accordance with another embodiment of the present invention a method is provided for determining the real-time, non-contact temperature measurement of first and second fast moving entities including the steps of: providing a series of temperature data representative of a first temperature and a second temperature of the first and second entities respectively; providing a series of reflectivity data representative of a first reflectivity and a second reflectivity of the first and second entities respectively; correlating each reflectivity datum with a corresponding temperature datum so as to form reflectivity-temperature data pairs; sorting the series of reflectivity data into a data table according to a frequency of occurrence of each reflectivity datum; identifying at least one reflectivity data peak in the data table representative of the first reflectivity; and determining at least the first temperature of the first entity from the identified reflectivity data peak based upon the associated reflectivity-temperature pairs.
In accordance with another embodiment of the method of the present invention steps are provided for steps for dividing a range of reflectivity values of the series of reflectivity data into a number of data bins, each of the data bins representing a portion of the range of reflectivity values, and wherein the step of sorting the series of reflectivity data includes the step of assigning each reflectivity-temperature data pair to one of the data bins and the step of identifying at least one reflectivity data peak includes the step of selecting one of the data bins with the greatest number of reflectivity data values. Additionally, the following steps also optionally may be included: prefiltering the series of reflectivity data so as to remove data points having reflectivity values outside a given range of reflectivity values; tracking said series of temperature data and said series of reflectivity data; and excluding said reflectivity-temperature data pair if the difference between said reflectivity value of a first reflectivity-temperature data pair and the reflectivity values in a group of reflectivity-temperature data pairs including said first reflectivity-temperature data pair exceeds a predetermined value. In another embodiment of the present invention, the step of identifying at least one reflectivity data peak includes the step of determining an alternative peak value as the reflectivity data peak where the reflectivity peak is not easily identified. Finally, the method of the present invention may include the steps of windowing the series of reflectivity data so as to maintain a constant number of reflectivity data points from a continuous series of reflectivity data points for use in the steps of sorting, identifying and correlating. Finally, in a preferred embodiment of the method of the present invention, the step of identifying at least one data peak in the data table representative of the first reflectivity further includes identifying a high-valued reflectivity data peak with a greatest frequency of occurrence of reflectivity datum.
In accordance with yet another embodiment of the present invention a computer-readable medium is provided for storing a set of instructions for controlling a general purpose digital computer, the set of instructions causing the computer to provide a series of temperature data representative of a first temperature and a second temperature of the first and second entities respectively; provide a series of reflectivity data representative of a first reflectivity and a second reflectivity of the first and second entities respectively; correlate each reflectivity datum with a corresponding temperature datum so as to form reflectivity-temperature data pairs; sort the series of reflectivity data into a data table according to a frequency of occurrence of each reflectivity datum; identify at least one reflectivity data peak in the data table representative of the first reflectivity; and determine at least the first temperature of the first entity from the identified reflectivity data peak based upon the associated reflectivity-temperature pairs.
In accordance with yet another embodiment of the present invention a computer-readable medium is provided for storing a set of instructions for controlling a general purpose digital computer, the set of instructions causing the computer to divide a range of reflectivity values of said series of reflectivity data into a number of data bins, each of said data bins representing a portion of said range of reflectivity values, and wherein said instructions causing said computer to sort said series of reflectivity data include instructions causing said computer to assign each reflectivity-temperature data pair to one of said data bins and said instructions for causing said computer to identify at least one reflectivity data peak includes instructions causing said computer to select one of said data bins with the greatest number of reflectivity data values.
In accordance with yet another embodiment of the present invention, a computer-readable medium is provided for storing a set of instructions for controlling a general purpose digital computer, the set of instructions causing the computer to prefilter said series of reflectivity data so as to remove data points having reflectivity values outside a given range of reflectivity values; track said series of temperature data and said series of reflectivity data, and exclude a first reflectivity-temperature data pair if the difference between said reflectivity value of said first reflectivity-temperature data pairs and the reflectivity values in a group of reflectivity-temperature data pairs including said first reflectivity-temperature data pair exceeds a predetermined value. Further, in the instructions provided for causing said computer to identify at least one reflectivity data peak, instructions are provided to determine an alternative peak value when said reflectivity data peak is not easily identified. In addition, instructions are provided for causing said computer to window said series of reflectivity data so as to maintain a constant number of reflectivity data points from a continuous series of reflectivity data points for use in the instructions causing said computer to sort, identify and correlate. Finally, with respect to the instructions provided for causing said computer to identify at least one data peak in said data table representative of said first reflectivity, instructions are further provided for causing said computer to identify a high-valued reflectivity data peak with a greatest frequency of occurrence of reflectivity datum.