Wet etch processing

• US 20070134829A1
• Filed: 12/12/2006
• Published: 06/14/2007
• Est. Priority Date: 12/12/2005
• Status: Active Grant

• Current Assignee: University College Cork (Higher Education Authority)
• Original Assignee: University College Cork (Higher Education Authority)
• Inventors: Morrissey, Anthony, Wilke, Nicolle
• Granted Patent: US 7,651,946 B2
• Time in Patent Office: Days
• US Class Current: 438/22
• CPC Class Codes: A61M 2037/003 having a lumen
• CPC Class Codes: A61M 2037/0046 Solid microneedles
• CPC Class Codes: A61M 2037/0053 Methods for producing microneedles
• CPC Class Codes: A61M 37/0015 by using microneedles
• CPC Class Codes: B81B 2201/055 Microneedles
• CPC Class Codes: B81C 1/00626 Processes for achieving a desired geometry not provided for in groups B81C1/00563 - B81C1/00619
• CPC Class Codes: B81C 2201/0133 Wet etching
• CPC Class Codes: B81C 2201/0138 Monitoring physical parameters in the etching chamber, e.g. pressure, temperature or gas composition
• CPC Class Codes: B81C 99/0065 Process control; Yield prediction

Abstract

A method of wet etching produces high-precision microneedle arrays for use in medical applications. The method achieves precise process control over microneedle fabrication, at single wafer or batch-level, using wet etching of silicon with potassium hydroxide (KOH) solution by accurately identifying the etch time endpoint. Hence, microneedles of an exactly required height, shape, sharpness and surface quality are achieved. The outcome is a reliable, reproducible, robust and relatively inexpensive microneedle fabrication process. Microneedles formed by KOH wet etching have extremely smooth surfaces and exhibit superior mechanical and structural robustness to their dry etched counterparts. These properties afford extra reliability to such silicon microneedles, making them ideal for medical applications. The needles can also be hollowed. Wet etched silicon microneedles can then be employed as masters to replicate the improved surface and structural properties in other materials (such as polymers) by moulding.

Claims

1. A process for manufacturing a microstructure needle article having a stem and a tip, the process comprising the steps of:

providing a semiconductor bulk material,

applying a mask to the bulk material, the mask having openings,

exposing the pattern-masked bulk material to an hydroxide liquid etchant,

allowing the hydroxide liquid etchant to enter the mask openings and to etch the bulk material until a desired out-of-plane needle shape is achieved, in which the needle tip is under, and pointing towards, the mask;

wherein the process comprises the step of predicting remaining etch time by monitoring dimensions of the bulk material during etching, calculating actual etch rates in particular directions, and using said calculations to predict the remaining etch time until a stop time.

2. A process as claimed in claim 1, wherein the monitoring is performed to recognise a transition from one etching stage to a next etching stage.

3. A process as claimed in claim 1, wherein the monitoring is performed to recognise a transition from one etching stage to a next etching stage;

and

wherein etch rates for particular crystal planes for each stage are calculated.

4. A process as claimed in claim 1, wherein the prediction step comprises predicting:

(b) a time t₁ for etching to a base of the needle; and

(b) a time t₂ for etching to the tip of the needle

5. A process as claimed in claim 1, wherein the monitoring steps comprise taking measurements a plurality of times close to the end of the etching.

5-1. A process as claimed in claim 1, wherein the prediction step calculates a plurality of different etch rates.

6. A process as claimed in claim 1, wherein the monitoring steps comprise taking measurements a plurality of times close to the end of the etching;

and wherein the etching is temporarily stopped while the monitoring takes place.

6-2. A process as claimed in claim 1, wherein the prediction step predicts the stop time as being when bulk material crystal planes will intersect under the mask to form the needle tip.

7. A process as claimed in claim 1, wherein the prediction step predicts the stop time as being when the crystal planes will intersect under the mask to form the needle tip;

and wherein the end point is when the mask will detach.

8. A process as claimed in claim 1, wherein the prediction step predicts the stop time as being when the crystal planes will intersect under the mask to form a point;

and wherein eight planes intersect at the stop time.

9. A process as claimed in claim 1, wherein the prediction step predicts a stop time as being when crystal planes will intersect under the mask to form a point;

and wherein the end point is when the mask will detach; and

wherein eight planes intersect at the stop time.

10. A process as claimed in claim 1, wherein the prediction step predicts the stop time as being when crystal planes will intersect under the mask to form a point;

and wherein eight planes intersect at the stop time; and

wherein said planes are {312} planes.

11. A process as claimed in claim 1, wherein the prediction step predicts the stop time as being when crystal planes will intersect under the mask to form a point;

and wherein the article base has {121 } crystal planes.

12. A process as claimed in claim 1, wherein the prediction step predicts when certain crystal planes will be removed.

13. A process as claimed in claim 1, wherein the prediction step predicts different etching rates for different dimensions.

14. A process as claimed in claim 1, wherein the prediction step predicts different etching rates for different dimensions;

and predicts a different etching rate in the top width of the needle frustum than diagonal width of the base.

15. A process as claimed in claim 1, wherein the prediction step predicts different etching rates for different dimensions;

and predicts a different etching rate in the top width of the needle frustum than diagonal width of the base; and

wherein the process simultaneously fabricates an array of upright out-of-plane needles.

16. A process as claimed in claim 1, wherein the prediction step predicts different reduction rates for different dimensions;

and predicts a different etching rate in the top width of the needle frustum than diagonal width of the base; and

wherein the process simultaneously fabricates an array of upright out-of-plane needles; and

wherein the process simultaneously fabricates a plurality of arrays of needles.

17. A process as claimed in claim 1, wherein the monitoring step is performed with only a subset of the arrays.

18. A process as claimed in claim 1, wherein the prediction step predicts different reduction rates for different dimensions;

and predicts a different etching rate in the top width of the needle frustum than diagonal width of the base; and

wherein the process simultaneously fabricates an array of upright out-of-plane needles; and

wherein the process simultaneously fabricates a plurality of arrays of needles; and

wherein the plurality of needles are fabricated on a semiconductor wafer.

19. A process as claimed in claim 1, wherein the prediction step predicts etch time according to predicted changes in crystal plane indices.

20. A process as claimed in claim 1, comprising the further step of automatically generating an etch mask design according to target needle parameters, etch parameters, and predicted etch rates.

21. A process as claimed in claim 1, comprising the further steps of using the microstructure needle as a master to produce a plurality of microstructure needles in a polymer material.

22. A process as claimed in claim 1, comprising the further steps of using the microstructure needle as a master to produce a plurality of microstructure needles in a polymer material;

and wherein the semiconductor needle is used as a master in a mould.

23. A process as claimed in claim 1, comprising the further steps of creating a hollow in the microstructure needle.

24. A process as claimed n claim 1, wherein the mask is of nitride material.

25. A process as claimed in claim 1, wherein the mask openings are formed by plasma etching.

26. A process as claimed in claim 1, wherein the etchant is potassium hydroxide.

27. A process as claimed in claim 1, wherein the etchant temperature is above 70°

C.

28. A process as claimed in claim 1, wherein the etchant temperature is approximately 79°

C. and the etchant is potassium hydroxide.

29. A process controller comprising a processor and an input interface, the input interface being for receiving process conditions and the processor being for performing the prediction step of claim 1.

30. A computer readable medium comprising software for performing the prediction step of claim 1 when executing on a digital processor.