Modulated reflectance measurement system using UV probe
A modulated reflectance measurement system includes lasers for generating an intensity modulated pump beam and a UV probe beam. The pump and probe beams are focused on a measurement site within a sample. The pump beam periodically excites the measurement site and the modulation is imparted to the probe beam. For one embodiment, the wavelength of the probe beam is selected to correspond to a local maxima of the temperature reflectance coefficient of the sample. For a second embodiment, the probe laser is tuned to either minimize the thermal wave contribution to the probe beam modulation or to equalize the thermal and plasma wave contributions to the probe beam modulation.
1. A method for evaluating a silicon semiconductor sample comprising the steps of:
generating an intensity modulated pump laser beam;
exciting a region on the silicon semiconductor sample with the pump beam to produce thermal and carrier plasma effects which modify the optical reflectivity of the sample;
focusing a fixed wavelength probe beam generated by a laser onto the sample within the region that has been excited, wherein the wavelength of the probe beam is between 360 and 410 nm;
monitoring the changes in the power of the reflected probe beam induced by the pump beam; and
generating output signals in response thereto, said output signals corresponding to the changes in the optical reflectivity of the sample the output signals containing information which is used to evaluate the sample.
2. A method as recited in claim 1 , wherein the wavelength of the probe beam is in the range of 400 to 405 nm.
3. A method as recited in claim 1 , wherein the implantation dose of the sample is evaluated.
4. A method as recited in claim 1 , wherein the junction depth of an implanted and annealed sample is evaluated.
5. A method as recited in claim 1 , wherein the wavelength of the probe beam is selected to substantially maximize the output signals corresponding to the changes in the optical reflectivity of the sample.
6. A method as recited in claim 1 , wherein the wavelength of the probe beam is selected to maximize the difference between a temperature reflectance coefficient and a carrier reflectance coefficient of the sample.
7. A method as recited in claim 1 , wherein the wavelength of the probe beam is selected so that both temperature and carrier plasma effects produce changes in reflectance coefficients of the same sign leading to constructive interference between the thermal and carrier plasma contributions of the output signals.