Method and apparatus for determining a material's characteristics by photoreflectance using improved computer control

• US 5,270,797 A
• Filed: 09/13/1989
• Issued: 12/14/1993
• Est. Priority Date: 07/20/1989
• Status: Expired due to Fees

• Current Assignee: Brooklyn College Research And Development Foundation
• Original Assignee: Brooklyn College Foundation
• Inventors: Shen, Hong-En, Pollak, Fred H.
• Primary Examiner(s): Rosenberger, Richard A.
• Assistant Examiner(s): HANTIS, KAREN
• Application Number:
  US07/408,903
• Time in Patent Office: 1,553 Days
• Field of Search: 356/446, 356/445, 356/432, 356/432 T, 356/417
• US Class Current: 356/417
• CPC Class Codes: G01J 2001/242 Filter wheel, i.e. absorption filter series graduated
• CPC Class Codes: G01N 2021/1719 Carrier modulation in semiconductors
• CPC Class Codes: G01N 2021/1725 Modulation of properties by light, e.g. photoreflectance
• CPC Class Codes: G01N 2021/8461 Investigating impurities in semiconductor, e.g. Silicon
• CPC Class Codes: G01N 21/1717 with a modulation of one or more physical properties of the sample during the optical investigation, e.g. electro-reflectance
• CPC Class Codes: G01N 21/55 Specular reflectivity
• CPC Class Codes: G01N 2201/0695 Supply to maintain constant beam intensity

Abstract

A method and apparatus for determining the characteristics of materials, particularly of semiconductors, semiconductor heterostructures and semiconductor interfaces by the use of photoreflectance, in which monochromatic light and modulated light beam reflected from the sample is detected to produce a d.c. signal and an a.c. signal, whereby the d.c. signal is applied to one input of a computer and the a.c. signal is used with another input of the computer which controls the light intensity of the monochromatic light impinging on the sample to maintain the d.c. signal substantially constant. A stepping motor is preferably utilized for varying the light intensity of the monochromatic light which is controlled by a computer to re-establish rapidly a predetermined d.c. signal established during normalization procedures when the light intensity of the monochromatic light changes, especially during change of its wavelength. Additionally, the modulation frequency of the modulated beam and/or the wavelength of the monochromatic light can also be varied by the computer.

Claims

1. An apparatus for determining characteristics of materials by photoreflectance, comprising monochromatic light source means, means for directing the monochromatic light onto a sample to be examined, pump beam means for directing a beam of energy onto the sample including modulation means for modulating said beam, means for directing at least a part of the non-absorbed monochromatic light and of the non-absorbed modulated beam from the sample onto a detector means operable to produce a d.c. signal and an a.c. signal in its outputs, computer means, means for applying the d.c. signal from the detector means to an input of the computer means, a lock-in amplifier means receiving at its input the a.c. signal from the detector means and operatively connected with its output to another input of the computer means, and servo control means for keeping substantially constant the operating conditions in a given experiment by maintaining the d.c. signal substantially constant at a predetermined value including variable means for varying the light intensity of the monochromatic light impinging on the sample and actuating means for controlling the variable means by an output from the computer means which is operable to cause the actuating means and therewith the variable means to move initially relatively rapidly toward re-establishing said predetermined value and to achieve thereafter at least substantially said predetermined value by slower movement of said actuating means.

2. An apparatus according to claim 1, further comprising means for changing the wavelength of the monochromatic light by said computer means.

3. An apparatus according to claim 2, wherein said variable means includes a variable neutral density filter, and wherein said actuating means includes a stepping motor controlled from the corresponding computer output.

4. An apparatus according to claim 3, further comprising means for controlling the modulation frequency of said modulation means from said computer means.

5. An apparatus according to claim 4, wherein the means for changing the wavelength of the monochromatic light includes a stepping motor controlled by said computer means and operable to change the wavelength of the monochromatic light.

6. An apparatus according to claim 1, wherein with a change in light intensity of the monochromatic light, said computer means is operable to cause the stepping motor of the actuating means to move in increments of a certain number of steps before a comparison is made again between the then produced d.c. signal with the predetermined value thereof and thereafter repeats such multi-step operation until said predetermined value is overshot by a multi-step movement of the stepping motor whereupon fine-tuning is achieved by reversing actuation of the stepping motor and movement thereof in increments of one step until the predetermined value is at least approximately reached.

7. An apparatus according to claim 6, wherein the multi-step increments involve ten steps or multiples thereof.

8. An apparatus according to claim 6, wherein said computer means includes means for disabling the servo control means during the taking of data in the apparatus.

9. An apparatus for determining characteristics of materials by photoreflectance, comprising monochromatic light source means, means for directing the monochromatic light onto a sample to be examined, pump beam means for directing a beam of energy onto the sample including modulation means for modulating said beam, means for directing at least a part of the non-absorbed monochromatic light and of the non-absorbed modulated beam from the sample onto a detector means operable to produce a d.c. signal and an a.c. signal in its outputs, computer means, means for applying the d.c. signal from the detector means to an input of the computer means including an A/D converter, a two-phase lock-in amplifier means receiving at its input the a.c. signal from the detector means and operatively connected with its output to another input of the computer means, and control means for keeping substantially constant the operating conditions in a given experiment by maintaining the d.c. signal substantially constant at a predetermined value including varying means for varying the light intensity of the monochromatic light impinging on the sample, actuating means for controlling the varying means including a stepping motor operatively connected with said varying means, and further means in said computer means for energizing said stepping motor in such a manner that in case of a change in light intensity of the monochromatic light, the stepping motor initially adjusts the varying means rapidly until proximity of said predetermined value has been reached, whereupon the stepping motor adjusts the varying means at a considerably slower speed to fine-tune the adjustment toward said predetermined value.

10. An apparatus according to claim 9, wherein said further means causes the stepping motor to adjust the varying means at a relatively faster speed in a direction toward re-establishing said predetermined value until said predetermined value is overshot, whereupon said further means causes the stepping motor to reverse its direction to approach said predetermined value at a relatively slower speed.

11. An apparatus according to claim 10, wherein said relatively faster speed involves incremental movements involving multiple steps while the relatively slower speed involves incremental movements involving individual steps.

12. An apparatus according to claim 11, wherein the multiple steps involve ten steps or multiples thereof.

13. An apparatus according to claim 9, further comprising means for controlling the modulation frequency of said modulation means from said computer means.

14. An apparatus according to claim 9, further comprising means for changing the wavelength of the monochromatic light by said computer means.

15. An apparatus according to claim 14, wherein the means for changing the wavelength of the monochromatic light includes a stepping motor means controlled by said computer means and operable to change the wavelength of the monochromatic light.

16. An apparatus according to claim 9, further comprising means for controlling the modulation frequency of said modulation means from said computer means.

17. An apparatus according to claim 9, wherein said varying means includes a variable neutral density filter.