Semiconductor electrochemical etching processes employing closed loop control

• US 20050199511A1
• Filed: 01/21/2005
• Published: 09/15/2005
• Est. Priority Date: 01/21/2004
• Status: Active Grant

• Current Assignee: Lake Shore Cryotronics, Inc.
• Original Assignee: Lake Shore Cryotronics, Inc.
• Inventors: Christophersen, Marc, Kochergin, Vladimir
• Granted Patent: US 7,560,018 B2
• Time in Patent Office: Days
• US Class Current: 205/641
• CPC Class Codes: C25F 3/12 of semiconducting materials
• CPC Class Codes: C25F 7/00 Constructional parts, or assemblies thereof, of cells for electrolytic removal of material from objects for both electrolytic coating and removal C25D; Servicing or operating
• CPC Class Codes: H01L 21/306 Chemical or electrical treatment, e.g. electrolytic etching to form insulating layers H01L21/31
• CPC Class Codes: H01L 21/67086 with the semiconductor substrates being dipped in baths or vessels
• CPC Class Codes: H01L 21/67253 Process monitoring, e.g. flow or thickness monitoring
• CPC Class Codes: H01L 22/26 Acting in response to an ongoing measurement without interruption of processing, e.g. endpoint detection, in-situ thickness measurement endpoint detection arrangements in CMP apparatus B24B37/013, in discharge apparatus H01J37/32

Abstract

Methods and apparatus for providing closed-loop control over an electrochemical etching process during porous semiconductor fabrication enhance the quality of the porous semiconductor materials, especially those contained structural variations (such as porosity or morphology variations) along the thickness of said porous semiconductors. Such enhancement of the control over the electrochemical etching process is highly desired for many applications of porous semiconductor materials.

Claims

1. A method of providing closed-loop control over an electrochemical etching process during porous semiconductor fabrication comprising:

providing a substrate wafer of single-crystal semiconductor having a first surface and a second surface,

disposing the substrate wafer in an electrochemical etching apparatus comprising the semiconductor wafer as an anode, electrolyte, and counter electrode as a cathode,

executing an electrochemical etching process wherein at least part of said first surface of the semiconductor wafer is exposed to electrolyte,

setting the electrochemical etching parameters to the electrochemical etching system comprising the substrate wafer, electrolyte and counter electrode, and performing the electrochemical etching process at over a process time period,

measuring the value of the resistance of the electrochemical etching system at least once during the electrochemical process time period, and

adjusting the electrochemical etching parameters according to the measurements of the resistivity of the electrochemical etching system at least once during the electrochemical process time period.

2. The method of claim 1, wherein said substrate wafer is a p-type doped silicon wafer and the electrolyte is a fluoride-containing, acidic electrolyte.

3. The method of claim 2, wherein said silicon wafer has a resistivity in the range of 0.001 to 0.999 Ω

cm, a crystal orientation selected from the group consisting of (100), (110), and (111), and the fabricated porous semiconductor is mesoporous silicon.

4. The method of claim 2, wherein said silicon wafer has a resistivity in the range of 1 to 1000 Ω

cm, a crystal orientation selected from the group consisting of (100) and (111), and the fabricated porous semiconductor is a macroporous silicon.

5. The method of claim 1, wherein said substrate wafer is an n-type doped silicon wafer with resistivity in the range of 0.001 to 0.999 Ω

cm, with crystal orientation selected from the group consisting of (100), (110), and (111), in which the electrolyte is a fluoride-containing, acidic electrolyte, and the fabricated porous semiconductor is mesoporous silicon.

6. The method of claim 1, wherein said substrate wafer is a III-V compound semiconductor wafer with a doping density in the range of 5×

10¹⁵ to 5×

10¹⁸ cm<sup>−

3</sup>, with crystal orientation selected from the group consisting of (100) and (111), in which the electrolyte is an acidic electrolyte, and the fabricated porous semiconductor is a macroporous semiconductor.

7. The method of claim 6, wherein said III-V compound semiconductor is selected from the group consisting of InP, GaAs and GaP.

8. The method of claim 1, wherein said set electrochemical etching parameters are one or more parameters selected from the group consisting of electrical current density, wafer illumination intensity, temperature of the electrolyte, voltage, the electrochemical etching process type, temporal waveform characteristics of any or all of said parameters, plus the electrochemical total process time period.

9. The method of claim 8, wherein said electrochemical etching process type is selected from the group consisting of a potentiostatic type, a galvanostatic type and a mixed type consisting of some temporal periods of potentiostatic process and some temporal periods of galvanostatic process.

10. The method of claim 1, wherein said resistivity measurements are performed by fixing all the electrochemical etching parameters except for the current and voltage, setting the one of the current or voltage to some constant value within the desired etching process range, measuring the other of the current or voltage and dividing the value of voltage by the value of the current to obtain the resistance of the process system.

11. The method of claim 10, wherein in said electrochemical etching apparatus further contains a reference electrode, distinct from the anode and cathode, at least partially immersed into the electrolyte;

said reference electrode also being utilized in measurements of resistance in a separate electrical circuit from the electrical circuit containing the etching anode and cathode.

12. The method of claim 1 wherein said electrochemical etching adjustment parameters are one or more selected from the group consisting of electrical current density, illumination intensity, temperature of the electrolyte, applied voltage, the electrochemical etching process type, temporal characteristics of any or all of said parameters, and the electrochemical total process time period.

13. The method of claim 12 wherein the electrochemical etching parameters are adjusted manually according to the measured feedback parameters.

14. The method of claim 12 wherein said electrochemical etching parameters are adjusted automatically in real time according to resistivity measurements by means of appropriate software and/or hardware.

15. The method of claim 12 wherein said adjustment of the electrochemical etching parameters is performed according to the measured relative change in the resistance of the electrochemical etching system in real time.

16. The method of claim 15 wherein the electrochemical etching parameter that is adjusted is the temporal characteristic of the applied electrical current density, more particularly said electrical current density varies in time according to a mathematical function having a characteristic temporal interval, said temporal interval of the mathematical function being adjusted according to the formula

T(t)=T(t₀)·

R(t, I)/R(t₀, I), wherein R(t, I))=U(t)/I is the measured resistance of the electrochemical etching system at time t taken at applied current density I, T is a temporal interval of said mathematical function, wherein values T(t₀), I and t₀ are set preliminary to the start of the electrochemical etching process.

17. The method of claim 12 wherein said adjustment of the electrochemical etching parameters is performed according to the measured absolute amplitude value of the resistivity of the electrochemical etching system.

18. The method of claim 17 wherein the said adjusted electrochemical etching parameter is the temporal characteristic of the applied electrical current density, and more particularly said electrical current density varies in time according to a mathematical function having a characteristic temporal interval, and further, said temporal interval of said mathematic function is adjusted according to the formula T(t)=T₀·

R(t, I)/R₀, wherein R(t, I))=U(t)/I is the measured resistance of the electrochemical etching system at time t taken at applied current density I, T is a temporal interval, T₀ is the preliminarily set starting temporal interval, R₀ is the preliminarily set reference number, and I is the preliminarily set current density.

37. The method of claim 18 wherein said the adjusted electrochemical etch parameter is the temporal characteristic of the applied electrical current density, more particularly said electrical current density varying in time according to some mathematic function having a characteristic temporal interval, said temporal interval being adjusted according to the formula T(t)=T₀·

f₀/f(t), wherein f(t) is the measured frequency of voltage self-oscillations of the electrochemical etching system at time t, T is a temporal interval, T₀ is a preliminarily set starting temporal interval and f₀ is a preliminarily set reference number.

19. A method of providing closed-loop control over an electrochemical etching process during porous semiconductor fabrication comprising:

providing a substrate wafer of single-crystal semiconductor having a first surface and a second surface,

disposing the substrate wafer in an electrochemical etching apparatus comprising the semiconductor wafer as an anode, electrolyte, counter electrode as a cathode and necessary hardware and/or software;

wherein at least part of said first surface of the semiconductor wafer is exposed to electrolyte,

setting the electrochemical etching parameters to the electrochemical etching system comprising the substrate wafer, electrolyte and counter electrode, and performing the electrochemical etching process at over some temporal length,

measuring the voltage oscillation frequency of the electrochemical etching system at least once during the electrochemical total process temporal length by setting the system to galvanostatic process conditions for at least some period of time and analyzing the temporal characteristics of the voltage needed by the electrochemical system in order to achieve a set level of electrical current density, and

adjusting the electrochemical etching parameters according to the measurements of the voltage oscillation frequency of the electrochemical etching system at least once during the electrochemical total process temporal length.

20. The method of claim 19, wherein said substrate wafer is a p-type doped silicon wafer and said electrolyte is a fluoride-containing, acidic electrolyte.

21. The method of claim 20, wherein said silicon wafer has a resistivity in the range of 1 to 1000 Ω

cm, a crystal orientation selected from the group consisting of (100) and (111), and the fabricated porous semiconductor is a macroporous silicon.

22. The method of claim 19, wherein said substrate wafer is a III-V compound semiconductor wafer with a doping density in the range of 10¹⁶ to 10¹⁸ cm⁻

3, crystal orientation selected from the group consisting of (100) and (111), the electrolyte is an acidic electrolyte, and the resultant fabricated porous semiconductor is a macroporous III-V semiconductor material.

23. The method of claim 22, wherein said III-V compound semiconductor is selected from the group consisting of InP, GaAs and GaP.

24. The method of claim 19, wherein said electrochemical etching parameters are one or more parameters selected from the group consisting of electrical current density, illumination intensity, temperature of the electrolyte, applied voltage, the type of the electrochemical etching process, the temporal characteristics of any or all of said parameters, and the electrochemical total process temporal length.

25. The method of claim 24 wherein said type of electrochemical etching process type is selected from the group consisting of potentiostatic and galvanostatic processes and further a process consisting of at least one temporal period of potentiostatic process and at least one temporal period of galvanostatic process.

26. The method of claim 19, wherein said voltage oscillation frequency measurements are performed by fixing all the electrochemical etching parameters except the applied voltage, recording the temporal dependence of the applied voltage required in order to reach the set value of applied current density and applying mathematical processing to extract the voltage oscillation frequency.

27. The method of claim 19, wherein said voltage oscillation frequency measurements are performed by fixing all the electrochemical etching parameters and by applying the small perturbation to the applied current density in a form of the periodical function with the frequency in the range of 0.001 Hz to 100 Hz, varying the frequency of said perturbation, recording the response of the electrochemical etching system as a form of frequency-dependent voltage, and extracting the voltage self-oscillation frequency by mathematical processing of the recorded data.

28. The method of claim 27 wherein said response of the system is extracted with a lock-in amplifier or other phase-locked method.

29. The method of claim 27 wherein said variation of frequency of the current density perturbation is performed around one of the higher harmonics of the voltage self-oscillation resonance.

30. The method of claim 27 wherein said voltage self-oscillation frequency is determined from the amplitude and phase of the frequency response from the electrochemical etching system.

31. The method of claim 19, wherein in said electrochemical etching apparatus further contains a reference electrode immersed at least partially into the electrolyte;

said electrode used in measurements of the frequency of voltage self-oscillations of the electrochemical etching system as a part of an electrical circuit distinct from the electrical circuit containing the electrochemical etching system anode and cathode.

32. The method of claim 19 wherein said adjustable electrochemical etching parameters are one or more selected from the group consisting of electrical current density, illumination intensity, temperature of the electrolyte, voltage, the type of the electrochemical etching process, temporal characteristics of any or all of said parameters, plus the electrochemical total process time period.

33. The method of claim 32 wherein said electrochemical etching parameters are adjusted automatically by means of appropriate software and/or hardware.

34. The method of claim 19 wherein said adjustment of the electrochemical etching parameters is performed according to the measured relative change in the voltage self-oscillation frequency of the electrochemical etching system between measurement times.

35. The method of claim 34 wherein the adjusted electrochemical etching parameter is the temporal characteristic of the applied electrical current density, more particularly said electrical current density varying in time according to some mathematic function having a characteristic temporal interval, said temporal interval of said mathematical function being adjusted according to the formula T(t)=T(t₀)·

f(t₀)/f(t), wherein f(t) is the measured voltage self-oscillation frequency of the electrochemical etching system at time t, T is a temporal interval of said mathematic function and t₀ is the temporal position of the point of reference during the electrochemical etching process, wherein values T(t₀), and t₀ are set preliminary to the electrochemical etching process.

36. The method of claim 19 wherein said adjustment of the electrochemical etching parameters is performed according to the measured absolute value of the voltage self-oscillation frequency of the electrochemical etching system

38. A method of providing closed-loop control over an electrochemical etching process during porous semiconductor fabrication comprising:

providing a substrate wafer of single-crystal semiconductor having a first surface and a second surface,

disposing the substrate wafer in an electrochemical etching apparatus comprising the semiconductor wafer as an anode, electrolyte, counter electrode as a cathode and necessary hardware and/or software, wherein at least part of said first surface of the semiconductor wafer is exposed to electrolyte,

setting the electrochemical etching parameters to the electrochemical etching system comprising the semiconductor wafer as an anode, electrolyte, and counter electrode as a cathode, and performing the electrochemical etching process over a total process temporal period,

measuring and storing the current-voltage curve of the electrochemical etching system at least once during the electrochemical total process temporal period, and

adjusting the electrochemical etching parameters according to the voltage and current values of a characteristic feature of the measured current-voltage curve of the electrochemical etching system at least once during the electrochemical process temporal length.

39. The method of claim 38, wherein said substrate wafer is an n-type doped silicon wafer with a resistivity in the range of 0.1 to 1000 Ω

cm and a crystal orientation selected from the group consisting of (100), (110) and (111), the electrolyte is a fluoride-containing, acidic electrolyte, and the resultant fabricated porous semiconductor is macroporous silicon.

40. The method of claim 38, wherein said substrate wafer is a III-V compound semiconductor wafer with a doping density in the range of 5×

10¹⁵ to 5×

10¹⁸ cm<sup>−

3</sup> and a crystal orientation selected from the group consisting of (100) and (111), the electrolyte is an acidic electrolyte, and the resultant fabricated porous semiconductor is a macroporous III-V compound semiconductor.

41. The method of claim 40, wherein said III-V compound semiconductor is selected from the group consisting of InP, GaAs and GaP.

42. The method of claim 38, wherein said electrochemical etching parameters are one or more parameter selected from the group consisting of electrical current density, illumination intensity, temperature of the electrolyte, applied voltage, type of the electrochemical etching process, temporal characteristics of those, and the electrochemical process temporal length.

43. The method of claim 42 wherein said electrochemical etching process type is selected from the group consisting of potentiostatic and galvanostatic processes and a process consisting of at least one temporal period of potentiostatic process and at least one temporal period of galvanostatic process.

44. The method of claim 38, wherein said current-voltage curve measurements are performed by fixing all the electrochemical etching parameters at preliminarily set values, except for the current and applied voltage, tuning the applied voltage within a range, and measuring and recording the current obtained at each value of applied voltage.

45. The method of claim 38, wherein said characteristic feature of the current-voltage curve is the first peak of current and the current and voltage positions of said first peak are determined by mathematically processing the measured current-voltage curve.

46. The method of claim 38, wherein said characteristic feature of the current-voltage curve is the curve inflection point positioned between zero voltage and the first peak of current and the current and voltage positions of said inflection point are determined by mathematically processing the measured current-voltage curve.

47. The method of claim 38 wherein said adjustable electrochemical etching parameters are one or more selected from the group consisting of electrical current density, illumination intensity, temperature of the electrolyte, applied voltage, the type of the electrochemical etching process and the temporal characteristics of any combination of those, plus the electrochemical total process temporal period.

48. The method of claim 38 wherein said electrochemical etching parameters are adjusted automatically by means of appropriate software and/or hardware.

49. The method of claim 38 wherein said adjustment of the electrochemical etching parameters is performed according to the measured relative change in the current and voltage positions of a characteristic feature on the measured current-voltage curve of the electrochemical etching system.

50. The method of claim 38 wherein said adjustment of the electrochemical etching parameters is performed according to the measured absolute values of current and voltage positions of a characteristic feature on measured current-voltage curve of the electrochemical etching system.