Patent Application
20080074668
The present invention provides a modulated optical reflectance measurement system with the capability to make measurements with very high sensitivity using an infrared probe beam. In particular, it has been found that for certain samples, it is preferable to have a probe beam with a wavelength of at least 800 nm and preferable greater than one micron (1000 nm). The pump beam preferably has wavelength in the near-IR range and be shorter than probe beam. Preferably, the pump beam is on the order of 670 nm to 800 nm. In certain experiments described below we used a 780/1064 nm pump/probe wavelength combination. Another useful combination would include a 670/1064 nm pump/probe wavelength system.
This particular wavelength combination was derived based on an analysis which we refer to as the Controlled Plasma-Thermal Interference (CPTI) principle. This principle is based on a deeper understanding of how the pump and probe beam wavelengths control the production and detection of the plasma and thermal waves in semiconductors. By selecting appropriate pump/probe beam wavelengths, the negative peak in the MOR signal dose dependence (
An example of CPTI-MOR signal dose dependence obtained for As-implanted Si sample using a 780/1064 nm pump/probe wavelength combination is shown in
It is believed that the position of the peak 210 on the dose axis can be changed by changing the pump and/or probe beam wavelength in a predetermined manner. Thus, the regions of a MOR signal high-sensitivity (defined as a slope of the MOR dose dependence shown in
In order to determine the best wavelengths for a particular application, one would need to use a damaged based model of the MOR response from an ion-implanted semiconductor to calculate the MOR response as a function of dose. Damaged based modeling is disclosed in our prior papers, cited above. The pump and probe wavelengths along with the modulation frequency are adjusted in the model to set the position the minimum peak (corresponding to the maximum interference between the thermal and plasma waves) at the desired point on the dose curve.
It should be noted that MOR values to the left of the minimum are dominated by plasma effects while values to the right of the minimum are dominated by thermal effects. Thus, one might want to position the minimum to be either less than (to the left of) or greater than (to the right of) the dose region of interest. In the first case, where the minimum is positioned to be less than the dose region of interest, the response in the region of interest will be dominated by the thermal effects. In the second case, where the minimum is positioned to be greater than the dose region of interest, the response in the region of interest will be dominated by plasma effects. Since the two mechanisms (plasma and thermal) are completely different physically, in some cases it would be beneficial to be able to control not only the sensitivity of the MOR response, but also its dominating physical nature.
The CPTI principle can be applied to many implantation species processed at a variety of implantation energies.
The effectiveness and uniqueness of the CPTI principle is illustrated in
The shape of the negative peak in CPTI-MOR dose dependence shown in
In the high dose range, the CPTI approach improves the MOR signal behavior to monotonic with high sensitivity as shown in
It should be noted that the method and system of the present invention could be used both as described and in combination with other improvements to a MOR instrument, i.e. a MOR system with multiple pump/probe beam wavelengths, Q-I signal processing algorithm, fiber-laser MOR system, position-modulated optical reflectance (PMOR) technique, etc.
In our initial investigation, we have found that using near-IR and IR parts of the spectrum for the pump and probe beams provides increased sensitivity in dose regions of particular interest to manufacturers for common wafer samples. We believe the use of an IR probe wavelength is of particular significance. In the preferred embodiment, the probe beam should have a wavelength of at least 1 micron (including 1.06 microns as described herein). We are in the process of testing even longer wavelengths with available lasers at 1.3 microns and 1.5 microns and believe we will find additional benefits at those wavelengths.
Referring to
In operation, the processor 750 monitors the signals generated by the filter 740. The results are typically stored and/or displayed to the user. The results could also be used for process control.
It should be noted that some of the patents assigned to Boxer Cross (for example, U.S. Pat. No. 6,049,220) include suggestions of using IR wavelengths in the 900 nm wavelength range for the pump and probe beams. However, these patents teach that the modulation frequency of the pump beam should be slow enough so that plasma waves are not created. It is believed that the benefits of the subject invention are best realized when the modulation frequency is fast enough so that plasma waves are created. In the preferred embodiment, the modulation frequency should be at least 100,000 hertz and preferably on the order of a megahertz or greater.
This patent application claims priority to U.S. Provisional Application Ser. No. 60/846,147, filed Sep. 21, 2006, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60846147 | Sep 2006 | US |
