Method for photolithographic definition of recessed features on a semiconductor wafer utilizing auto-focusing alignment

• US 5,783,340 A
• Filed: 07/31/1997
• Issued: 07/21/1998
• Est. Priority Date: 09/06/1995
• Status: Expired due to Term

• Current Assignee: Sandia Corporation (Martin Marietta Corporation)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Sniegowski, Jeffry J., Montague, Stephen, McWhorter, Paul J., Smith, James H., Farino, Anthony J.
• Primary Examiner(s): Young, Christopher G.
• Application Number:
  US08/903,985
• Time in Patent Office: 355 Days
• Field of Search: 430/22, 430/311, 430/314, 430/315, 430/319, 356/399, 356/400, 356/401, 438/24
• US Class Current: 430/22
• CPC Class Codes: B81C 1/00246 Monolithic integration, i.e. micromechanical structure and electronic processing unit are integrated on the same substrate
• CPC Class Codes: B81C 2203/0728 Pre-CMOS, i.e. forming the micromechanical structure before the CMOS circuit
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: G03F 7/094 Multilayer resist systems, e.g. planarising layers
• CPC Class Codes: G03F 9/70 for microlithography measuring printed patterns for monitoring overlay G03F7/70633 or focus G03F7/70641; projection system adjustment G03F7/70258; position control G03F7/70775

Abstract

A method is disclosed for photolithographically defining device features up to the resolution limit of an auto-focusing projection stepper when the device features are to be formed in a wafer cavity at a depth exceeding the depth of focus of the stepper. The method uses a focusing cavity located in a die field at the position of a focusing light beam from the auto-focusing projection stepper, with the focusing cavity being of the same depth as one or more adjacent cavities wherein a semiconductor device is to be formed. The focusing cavity provides a bottom surface for referencing the focusing light beam and focusing the stepper at a predetermined depth below the surface of the wafer, whereat the device features are to be defined. As material layers are deposited in each device cavity to build up a semiconductor structure such as a microelectromechanical system (MEMS) device, the same material layers are deposited in the focusing cavity, raising the bottom surface and re-focusing the stepper for accurately defining additional device features in each succeeding material layer. The method is especially applicable for forming MEMS devices within a cavity or trench and integrating the MEMS devices with electronic circuitry fabricated on the wafer surface.

Claims

1. A method for photolithographically defining device features recessed below a surface of a semiconductor wafer using an auto-focusing photolithographic stepper exposure system comprising steps for:

(a) forming a plurality of equal-depth cavities within a die field on a top surface of the semiconductor wafer, including a focusing cavity formed at a predetermined position of a focusing light beam from the auto-focusing photolithographic stepper exposure system and at least one device cavity proximate to the focusing cavity;

(b) forming a material layer within each cavity, thereby raising a bottom surface of each cavity by a thickness of the material layer;

(c) covering the material layer with a photoresist layer;

(d) focusing the photolithographic stepper exposure system by reflecting the focusing light beam off the material layer in the focusing cavity and generating a detected light signal for vertically positioning the bottom surface of each cavity at a focal plane of the stepper exposure system; and

(e) exposing the photoresist layer for photolithographically defining the recessed device features to be formed in the material layer within the device cavity.

2. The method of claim 1 wherein the step for forming the plurality of equal-depth cavities comprises forming the cavities by a bulk micromachining process.

3. The method of claim 2 wherein the semiconductor wafer comprises silicon, and the bulk micromachining process comprises etching with an anisotropic etchant selected from the group consisting of potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH) and ethylenediamine pyrocatechol (EDP).

4. The method of claim 1 further including steps for providing at least one alignment mark within the die field, and laterally aligning the wafer to a projected reticle image prior to exposing the photoresist layer.

5. The method of claim 4 wherein at least one alignment feature is located within the focusing cavity.

6. The method of claim 5 wherein the step for laterally aligning the wafer to the projected reticle image comprises reflecting an alignment light beam off the alignment mark within the focusing cavity and generating a detected alignment signal for laterally positioning the semiconductor wafer.

7. The method of claim 1 further including steps for building up a device structure in the device cavity by depositing additional material layers of predetermined thicknesses and compositions within the focusing and device cavities, each additional material layer being photolithographically defined by a photoresist layer provided thereover, and each additional material layer further raising the bottom surface of each cavity so that the semiconductor wafer is vertically repositioned in the stepper by the detected light signal to locate the bottom surface of each cavity at the focal plane of the stepper exposure system prior to exposing the photoresist layer and defining the device features.

8. The method of claim 7 wherein the device structure comprises a microelectromechanical system.

9. The method of claim 7 wherein each layer of the device structure is photolithographically defined with a resolution about equal to a resolution limit of the stepper exposure system.

10. The method of claim 9 wherein the device structure comprises device features having at least one lateral dimension of <

1 micron.

11. The method of claim 7 further including steps for forming electronic circuitry on the surface of the semiconductor wafer, the electronic circuitry including electrical interconnections to the device structure.

12. The method of claim 11 wherein the device structure comprises a microelectromechanical system.

13. The method of claim 11 wherein the electronic circuitry is formed with a resolution about equal to a resolution limit of the stepper exposure system.

14. The method of claim 13 wherein the electronic circuitry comprises sub-micron device features.

15. A product formed according to the method of claim 7.

16. A method for forming a microelectromechanical system (MEMS) device in a cavity below the surface of a semiconductor wafer, comprising steps for:

(a) forming a plurality of equal-depth cavities within a die field on the semiconductor wafer, including a focusing cavity located at a predetermined position in the die field corresponding to the position of a focusing light beam from an auto-focusing photolithographic stepper, and at least one device cavity wherein the MEMS device is to be formed;

(b) depositing a plurality of material layers, one layer at a time, into the focusing and device cavities;

(c) forming a patterned mask over each material layer for defining features of the MEMS device within the material layer by automatically focusing the stepper at a top surface of the material layer in the focusing cavity; and

(d) etching the material layer through the patterned mask to form the features of the MEMS device within the device cavity.

17. The method of claim 16 wherein the semiconductor wafer comprises silicon, and the step of forming the plurality of equal-depth cavities comprises etching with an anisotropic etchant selected from the group consisting of potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH) and ethylenediamine pyrocatechol (EDP).

18. The method of claim 16 wherein the step for automatically focusing the stepper at the surface of the material layer comprises reflecting a focusing light beam off the top surface of the material layer in the focusing cavity and generating a detected light signal for vertically positioning the top surface of the material layer at a focal plane of the stepper.

19. The method of claim 16 wherein at least one feature of the MEMS device has a lateral dimension of less than one micron.

20. The method of claim 16 wherein at least one feature of the MEMS device is formed with a lateral dimension that is about equal to a resolution limit of the stepper.

21. The method of claim 20 wherein the stepper has a numerical aperture (N. A.) in the range of 0.3-0.6.

22. The method of claim 16 further including a step for laterally aligning the die field to a projected reticle image from the stepper by providing at least one alignment mark within the focusing cavity.

23. The method of claim 22 wherein the step for laterally aligning the die field to the projected reticle image from the stepper is performed automatically by reflecting an alignment light beam off the alignment mark within the focusing cavity and generating a detected alignment signal for laterally positioning the semiconductor wafer.

24. The method of claim 16 further including steps for forming electronic circuitry on the surface of the semiconductor wafer and interconnecting the electronic circuitry with the MEMS device.

25. A MEMS device formed according to the method of claim 16.

26. A MEMS device formed according to the method of claim 24.

27. A method for photolithographically defining device features recessed below a surface of a semiconductor wafer by a distance exceeding a depth of field of an auto-focusing photolithographic stepper, comprising steps for:

(a) forming a plurality of equal-depth cavities within the semiconductor wafer with a depth greater than the depth of field of the auto-focusing photolithographic stepper exposure system, the plurality of cavities including a focusing cavity centralized within a die field on the wafer, and at least one device cavity proximate to the focusing cavity;

(b) depositing a photoresist layer to blanket the bottom surfaces of each cavity; and

(c) focusing a projected light image for exposing the photoresist layer by reflecting a focusing light beam of the auto-focusing stepper off the bottom surface of the focusing cavity, thereby photolithographically defining the recessed device features within each device cavity.