Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
1999-09-24
2002-04-16
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S723000, C438S745000, C438S753000
Reexamination Certificate
active
06372656
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of producing radiation sensors, in particular for infrared radiation, with an absorber for the radiation to be measured and a plurality of thermoelements for measuring the absorbed radiation-induced heating of the radiation absorber integrated into a semiconductor substrate. These methods can be used in particular for production of radiation absorbers with a plurality of sensor elements integrated at small distances on a substrate.
BACKGROUND INFORMATION
Such a radiation sensor with a plurality of sensor elements and a method of producing same are described, for example, by I. H. Choi and K. D. Wise, “A Linear Thermopile Infrared Detector Array with On-Chip Multiplexing,” IEEE Trans Electron. Devices (September 1985), pages 132 through 135. This article describes a method whereby boron is diffused into a <100> oriented silicon substrate in a ring pattern from the front side, a membrane of SiO
2
and Si
3
N
4
is created on the front side, and then openings are cut through the substrate from the rear side of the substrate by anisotropic wet etching. These openings end on the front side of the substrate within the ring-shaped areas doped with boron. This forms openings in the substrate that are closed only by the thin membrane. A radiation absorber is formed on the membrane in each of these openings. A plurality of thermoelements connected in series each has a hot contact in the vicinity of the radiation absorber and a cold contact on the remaining silicon substrate which functions as a heat sink.
This known manufacturing method has a number of disadvantages. The anisotropy of wet etching occurs due to the fact that the etching process takes place at different rates on the different crystal faces of the silicon substrate. The etching rate is lowest on a surface with <111> orientation. Therefore, in wet etching of a <100>surface through an opening in a mask, a recess is formed in the surface, its side walls having <111>orientations and being inclined at an angle of approx. 54° to the <100> surface. The bottom surface of the resulting recess is smaller the further the etching operation proceeds into the material, until it reaches a depth where the opposite walls of the recess abut against one another. Therefore, to produce a small opening at the level of the membrane, a mask with a much larger opening must be formed on the opposite side of the substrate.
Fluctuations in thickness between different substrates or within a substrate have a critical effect on the dimensions of the opening produced in the membrane due to the inclined orientation of the walls. It is extremely difficult to produce precision openings with small dimensions in relation to the thickness of the substrate, because fluctuations in the thickness of the substrate have a great influence on their size.
This problem is counteracted in the cited literature by the diffused ring made of boron. The boron-doped material is not attacked by etching, so the opening in the mask on the rear side of the substrate can be larger than would be necessary in view of the crystal geometry of the substrate to obtain a given opening size in the membrane. The size of the finished opening is then determined by the diameter of the undoped region inside the boron-doped ring. However, one unavoidable consequence of this method is that a portion of the rear side of the boron-doped ring which has diffused into the substrate is exposed so that the thickness of the substrate in the immediate vicinity of the opening after etching is determined by the thickness of the ring, which amounts to only approx. 20 &mgr;m. Although a greater ring thickness could be achieved, this would be possible only through long diffusion times at very high process temperatures. This leads to the problem that, in the finished infrared sensor, the boron-doped ring may be eroded to varying extents by the etching process, resulting in variations in the quality of heat transfer from the cold contacts of the thermoelement over the ring into the solid silicon substrate, which can lead to systematic measurement errors.
Another radiation sensor with a silicon substrate, a radiation absorber arranged on a membrane over an opening in the substrate and a plurality of thermoelements with a hot contact in the vicinity of the radiation absorber and a cold contact on the silicon substrate is known from German Published Patent Application No. 41 02 524. With this sensor, the walls of the opening also diverge toward the side of the substrate facing away from the membrane in the manner characteristic of anisotropic wet etching. The diameter of the opening is much greater than the thickness of the substrate.
SUMMARY OF THE INVENTION
The present invention provides methods of producing radiation sensors which make it possible to produce radiation sensors with precisely reproducible properties and permit the production of radiation sensors with a plurality of individual sensor elements which can be arranged at a small distance from one another, which is independent of the thickness of the substrate used.
According to a first aspect of the present invention, these advantages are achieved by the steps of forming an opening in the membrane in the specified area and etching the semiconductor substrate through this opening. This opening makes it possible to produce the required cavity below the radiation absorber from the front side of the substrate, thus eliminating the necessity of etching through the entire substrate in a time-consuming process. This eliminates possible sources of error in positioning the etching mask on the rear side of the substrate in relation to the position of the radiation absorber, as would otherwise be necessary; there is no danger of extensive etching beneath the edge areas of the opening where cold contacts of the thermoelement can be mounted, which would thus result in poor thermal contact with the solid silicon substrate; furthermore, there may be solid unetched semiconductor material a short distance below the radiation absorber, which increases the total mass of the heat sink formed by the semiconductor material.
According to a first variant of this method, wherever a recess is to be created, the semiconductor material is made porous in that area prior to deposition. This can be accomplished by an anodic oxidation, e.g., with an HF electrolyte, in an electrochemical process in which the wafer functions as the anode with respect to the electrolyte. This region, which has been made porous, can then be etched out selectively in a subsequent etching step.
This etching step preferably takes place after the membrane has been deposited and the thermoelements have been structured on the membrane.
To determine the area to be etched out, in this case the surface of the semiconductor substrate is preferably masked with a protective layer made of a material which is resistant to the agent used to make the semiconductor porous. This material may be chromium or gold, for example.
As an alternative, the areas to be etched out can be determined by low n-type doping (n
−
doping) of the areas that are not to be etched away, so that in contrast with the p-doped substrate and any n
++
doped areas, they are not attacked by the agent used to make the semiconductor porous. The etching step which follows the step of making the semiconductor porous may be a traditional wet etching step.
The thermoelements are preferably structured on the deposited membrane before the etching step.
No special masking is necessary for the etching step if the material to be etched out has been prepared by making it porous.
According to a second variant of this method, no preparation of the area to be etched out by making it porous is necessary, and instead the area to be etched out is determined only by the formation of the opening in the membrane. The recess can be produced easily by isotropic etching of the substrate area behind the opening.
This isotropic etching can be performed by electroc
Frey Wilhelm
Laermer Franz
Deo Duy-Vu
Kenyon & Kenyon
Robert & Bosch GmbH
Utech Benjamin L.
LandOfFree
Method of producing a radiation sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of producing a radiation sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing a radiation sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2823353