METHOD FOR PROTECTING A MEMS UNIT AGAINST INFRARED INVESTIGATIONS AND MEMS UNIT

Abstract

A method is provided for protecting a MEMS unit, in particular a MEMS sensor, against infrared investigations, a surface patterning being performed for at least one first area of a surface of the MEMS unit, the first area absorbing, reflecting or diffusely scattering more than 50%, in particular more than 90% of an infrared light incident upon it.

Claims

1. A method for protecting a MEMS unit against infrared investigations, the method comprising: performing a surface patterning being for at least one first area of a surface of the MEMS unit, the first area one of absorbing, reflecting or diffusely scattering more than 50% of an infrared light incident upon it.

2. The method as recited in claim 1, wherein the MEMS unit is a MEMS sensor.

3. The method as recited in claim 1, wherein the first area one of absorbs, reflects or diffusely scatters more than 90% of the infrared light incident upon it.

4. The method as recited in claim 1, wherein the surface patterning is performed prior to a bonding of parts of the MEMS unit.

5. The method as recited in claim 4, wherein the first area is an inner surface of the MEMS unit and borders on a cavity of the MEMS unit.

6. The method as recited in claim 1, wherein for the first area, the surface patterning is performed after production of the MEMS unit and the first area includes at least parts of an outer surface of the MEMS unit.

7. The method as recited in claim 1, wherein infrared light striking the first area from any angle of incidence is one of absorbed, reflected or diffusely scattered by the first area at at least 90%.

8. The method as recited in claim 1, wherein the surface patterning occurs by KOH etching.

9. The method as recited in claim 1, wherein the surface patterning increases a roughness of the first area.

10. The method as recited in claim 1, wherein the surface patterning of the first area includes grid structures.

11. The method as recited in claim 1, wherein the first area acts as one of a retroreflector or triple mirror, as a result of the surface patterning.

12. A MEMS unit having at least one first area of a surface having a surface patterning, the first area absorbing, reflecting or diffusely scattering more than of an irradiated infrared light.

13. The MEMS unit as recited in claim 12 having a base structure, a sensor structure, a cavity and a sensor cap.

14. The MEMS unit as recited in claim 13, wherein the first area is situated in such a way that light from any direction passes through the at least one area between a surface of the MEMS unit and the cavity.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0016] The present invention is explained below in greater detail with reference to the FIGURE and on the basis of exemplary embodiments.

[0017] FIG. 1 schematically shows an exemplary MEMS sensor.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0018] In measurements using laser IR vibrometry on MEMS sensors, transmission values for infrared frequencies were ascertained in the range of typically approx. 50%. With such measurements, it is possible to determine frequencies in the sensor structure and thus to make inferences regarding the physical properties of the MEMS sensor on which a PUF is based and thus inferences regarding the PUF. The present invention relates to securing MEMS sensors against an investigation using infrared spectroscopy. Aside from MEMS sensors it is also possible to use MEMS units, whose proper functional MEMS structure is optimized in such a way for PUF functionality that the MEMS unit is no longer able to perform any (significant) sensor functions, but serves primarily as a PUF base. Such MEMS units are also to be protected against infrared investigations.

[0019] FIG. 1 shows a MEMS sensor 1. MEMS sensor 1 has a base wafer 11, whose delimitation vis-a-vis the other layers 14 is shown by separating line 12. MEMS sensor 1 has additional layers 14, which together with base wafer 11 enclose a cavity 15. Layers 14 may be connected to layers 11 by a bonding method. Layers 14 are also referred to as a so-called sensor cap. A vacuum is preferably produced in cavity 15. The actual functional sensor structure 13 of MEMS sensor 1 is situated in cavity 15 and on base wafer 11.

[0020] Layers 11 and layers 14 as well as sensor structure 13 are usually made of silicon. The MEMS sensor in FIG. 1 now has surface areas 16, 17, which were processed by surface patterning. In FIG. 1, the patterned area 16 extends along an inner surface of layers 11 and 14, that is, the surface facing cavity 15. The patterned area 17 extends along an outer surface of layers 11 and 14. Because of their surface structures, surface areas 16 and 17 prevent or render more difficult an investigation of the MEMS sensor using infrared-spectroscopic methods since they are (largely) non-transparent for infrared frequencies and therefore sufficient transmission values for a spectroscopic investigation are not achieved. The depth, the surface and the type of patterning of the structured areas 16 and 17 are selected in such a way that at least 50% of an IR light irradiated upon the areas is absorbed, reflected or diffusely scattered, in particular at least 90%.

[0021] Due to the location in the interior of the MEMS sensor 1, a removal of the patterned area 16 by an attacker of the PUF is hardly possible since this would massively damage sensor 1 as well as destroy the vacuum of cavity 15. Such an intervention into the sensor structure may also have the consequence that impressed mechanical states of stress (e.g., from the molding process) or pressure ratios change and that as a result the physical properties or the precise expression of the sensor properties, on which the PUF is based, change slightly. The security against manipulation is thus greatly increased. As described, structured are 17 is located on the outer surface of MEMS sensor 1. Here, a protection against a removal of the surface structures is no longer quite as high as for the area 16. However, grinding down the area 17 continues to be laborious and may damage MEMS sensor 1 so that an attack on the PUF of the MEMS sensor is at least rendered much more difficult.