METHOD AND APPARATUS FOR ANALYZING SEMICONDUCTORS

• US 3,731,192 A
• Filed: 05/28/1971
• Issued: 05/01/1973
• Est. Priority Date: 05/28/1971
• Status: Expired due to Term

• Current Assignee: Gabriel Lorimer Miller
• Original Assignee: Gabriel Lorimer Miller
• Inventors: Miller, Gabriel Lorimer
• Primary Examiner(s): Rolinec, Rudolph V.
• Assistant Examiner(s): Karlsen, Ernest F.
• Application Number:
  US05/147,846
• Time in Patent Office: 704 Days
• Field of Search: 324/158D,158T,158R,73R 29/574
• US Class Current: 324/762.05
• CPC Class Codes: G01R 31/2644 Adaptations of individual semiconductor devices to facilitate the testing thereof

Abstract

A semiconductor wafer is analyzed by applying successive values of reverse-bias voltage V to a diode region to form successive depletion layers of different depth X. Current of frequency f1 is applied to the diode and detected to determine X. Voltage at frequency f2, which is lower than f1, is applied to the diode such as to produce a modulation Delta E2 of the electric field in the depletion layer and a modulation Delta X2 of the depletion layer depth. The voltage input at frequency f2 is controlled to maintain either Delta E2 or Delta X2 constant with changes of X, and the modulation of the f1 voltage at frequency f2 is measured to determine one of these semiconductor parameters at different values of X. In this manner, a profile of semiconductor carrier density N, or its reciprocal 1/N, can be determined. An improved method of determining X is also disclosed.

Claims

1. A process for analyzing a semiconductor wafer comprising the steps of:

forming a diode region in the wafer;

successively applying different values of reverse-bias voltage V to the diode region, thereby forming successive depletion layers of different depth X;

applying a constant A.C. cuRrent of frequency f1 to the diode region;

detecting the voltage across the diode region at frequency f1, said voltage being indicative of the depletion layer depth X;

applying a voltage of frequency f2 to the diode region, said frequency f2 being small compared to f1, said voltage resulting in a modulation parameter Delta X equal to the variation in depletion layer depth, and a modulation parameter Delta E equal to the variation of electric field in the depletion layer;

maintaining one of the modulation parameters constant with changes of depletion layer depth X, and detecting the modulation of the f1 signal at frequency f2 for different values of bias voltage V, thereby to evaluate the diode region at different depths.

2. The process of claim 1 wherein:

the step of maintaining a modulation parameter constant comprises the step of using said voltage at frequency f1 to control the voltage at frequency f2.

3. The process of claim 2 wherein:

the step of detecting frequency f1 voltage comprises the step of generating a D.C. voltage proportional to the detected frequency f1 voltage; and

the step of controlling f2 voltage comprises the step of causing the D.C. voltage to be interrupted at frequency f2, whereby the applied diode region voltage V + Delta V2 maintains a constant modulation parameter Delta E with changes of X, and the detected f2 voltage is indicative of the reciprocal of carrier density 1/N at the edge of the depletion layer.

4. The process of claim 3 wherein:

the f2 voltage across the diode is detected by phase detection.

5. The process of claim 2 wherein:

the step of detecting frequency f1 comprises the step of generating a D.C. voltage Delta V proportional to the detected f1 voltage; and

the step of modulating frequency f2 energy comprises the step of comparing Delta V with a reference voltage, generating a control signal proportional to the difference of the reference voltage and Delta V, and controlling the amplitude of frequency f2 with said control signal, whereby the parameter Delta X is maintained constant at changing values of X and the detected f2 voltage is indicative of carrier density N at the edge of the depletion layer.

6. The process of claim 2 wherein:

the step of applying to the diode region a constant A.C. current at frequency f1 comprises the steps of comparing f1 current through the diode with a reference current to generate a control signal, and controlling said f1 current with said control signal.

7. The process for measuring the free carrier density in a semiconductor diode region comprising the steps of:

applying a bias voltage V to the diode, thereby establishing a depletion region in the diode of depth X generating a voltage Delta V proportional to X;

adding Delta V to V and applying to the diode a bias voltage V + Delta V, thereby establishing a depletion region of depth X + Delta X; and

measuring the parameters X and Delta X thereby to determine respectively the depletion region depth X and the reciprocal of free carrier density 1/N at the edge of the depletion region.

8. The process of claim 7 wherein:

the step of measuring the distance X comprises the step of applying to the diode a constant A.C. current of frequency f1; and

detecting the amplitude of the voltage across the diode region at frequency f1, said amplitude being indicative of the depletion region depth X.

9. The process of claim 8 wherein:

the step of measuring Delta X comprises the steps of adding voltage Delta V to bias voltage V intermittently at a frequency f2, and detecting the amplitude of the modulation of frequency f1 acRoss the diode region at frequency f2, said amplitude being indicative of the parameter 1/N

10. The process of claim 8 further comprising the steps of:

applying progressively different bias voltages V to the diode region; and

repeating said steps after applying each different bias voltage, thereby to determine the reciprocal of the free carrier density at a succession of different depletion layer depths and to determine a carrier density profile of the diode region.

11. The process of claim 10 wherein:

the frequency f2 is much smaller than f1.

12. The process of claim 10 wherein:

the step of applying to the diode region a constant A.C. current at frequency f1 comprises the steps of comparing f1 current through the diode with a reference current to generate a control signal, and controlling said f1 current with said control signal.

13. The process of claim 10 wherein:

the step of applying to the diode region A.C. current frequency f1 comprises the steps of comparing f1 current through the diode region with a reference current to generate a control current, causing said control current to be interrupted at frequency f1, and directing said interrupted control current through the diode region.

14. The process for determining a profile of the free-carrier density in a semiconductor diode region comprising the steps of:

successively applying different values of bias voltage of V to the diode region, thereby to form successive depletion layers of different depth X;

applying A.C. current of frequency f1 to the diode region detecting the f1 frequency voltage across the diode region;

measuring the f1 frequency voltage at different values of bias voltage V to determine corresponding values of depletion layer depth X;

applying A.C. voltage of frequency f2 to the diode region, the f2 frequency being significantly smaller than f1, and resulting in a modulation Delta X2 of the depletion layer depth and in modulation Delta E2 of the electric field in the depletion layer;

maintaining the electric field modulation Delta E2 substantially constant, comprising the step of generating a voltage Delta V proportional to depletion layer depth X and using said voltage Delta V to control the magnitude of applied frequency f2 voltage; and

measuring frequency f2 modulation of the f1 voltage across the diode region at different values of bias voltage V to determine the carrier density reciprocal 1/N at successive depths in the diode region.