Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Insulative material deposited upon semiconductive substrate
Reexamination Certificate
1999-09-20
2002-01-22
Nelms, David (Department: 1762)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Insulative material deposited upon semiconductive substrate
C438S788000, C118S410000
Reexamination Certificate
active
06340644
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for applying a protective resist to a prepatterned wafer.
BACKGROUND OF THE INVENTION
Semiconductor wafers can be composed of a substrate and membranes produced by etching the substrate from the back. Silicon is preferably used as the substrate material and thus as the material for the membranes. In a number of sensor applications, patterns must be produced from the membranes by dry etching the front of the wafer, i.e., the side containing the membranes. In the case of dry etching, it is necessary to protect the membrane undersides, which were produced by cavern etching the substrate from the back, during the dry etching process.
This back protection performs the following important functions during dry etching, preferably using an anisotropic plasma etching process as the dry etching process.
The back protection prevents etching gases, such as fluorine, from attacking the membrane underside after etching through the front of the membrane, since the caverns are sealed by the protective layer and the membrane undersides are also covered, so that no free silicon that could be attacked by the etching gases, such as fluorine, is exposed on the back.
The back protection also makes it possible to overetch patterns in the plasma that are already etched through, i.e., the etchant attack on the silicon side walls of patterns already etched from the membrane is reduced because the undersides of the membrane are protected.
In addition, a seal is provided for the back of the wafer, to which helium must be applied toward the front as a convection medium during plasma etching to cool the substrate. This cooling thus continues even after the front membrane is partially etched through because the back protection prevents the helium from escaping into the chamber interior of the vacuum system.
Furthermore, the wafer is stabilized and the patterns, which are fragile after etching, are protected mechanically by the back resist. Even in the extreme case of wafer breakage, due to the fragile cavern regions, a fractured wafer can still be extracted from the plasma etching chamber as a single unit without leaving any fragments on the substrate electrode, which would require laborious cleaning of the plasma etching system.
Finally, the back protection can serve as a defined etching barrier. Because the silicon etching process stops when it reaches the resist layer, an end point detection system equipped with optical emission spectroscopy (OES) can be used to determine when the resist layer is reached. Without the resist, silicon etching would continue to the back of the wafer, i.e., there would be no detectable end point.
However, the back protection must also satisfy the following conditions:
The protective resist should always remain inside the caverns and not spread across the remaining back of the wafer so that the wafer does not stick to or contaminate the substrate electrode or components of the load-lock device.
In addition, the protective resist should not blister or flake during the plasma etching process, when it is subjected to thermal load, thus preventing contamination of the substrate electrode or the load-lock devices and ensuring that the back of the wafer, to which the helium is applied, remains sealed against the chamber interior.
There should also be adequate selectivity toward the positive resist, which, during plasma etching, is usually located on the front to define the pattern, so that the back resist in the caverns or used to protect local areas is not damaged when the positive resist on the front is developed by photolithography or removed during or at the end of the process.
SUMMARY OF THE INVENTION
Up to now attempts have been made to protect the back by applying a positive resist all over the back of the wafer and to pattern it through photolithography so that it remains only in the caverns and not on the remaining back of the wafer. However, using a positive resist at this point does not produce the desired result. It would not be possible to reliably apply negative resists or, in particular, non-photopatternable polymers in this manner, nor could they be restricted to the cavern interior. However, those resists that are the most resistant to hydrofluoric acid vapor, for example, cannot be photopatterned, since they lack functional groups, which are an important prerequisite for sealing against HF.
The use of a positive resist for filling the caverns is also associated with the following problems: it must be spun onto the wafer very thickly and subsequently photopatterned, i.e., exposed and developed. To do this, a sticky wafer, i.e., one which has not be completely baked or hardened, must be processed with appropriate lithographic equipment. Because they contain solvents and volatile non-crosslinked monomers, however, thick positive resists blister easily during drying and at high temperatures, and they tend to flake off the undersides of the caverns in large areas. In addition, the back of the wafer, which initially is sticky all over, makes it very difficult to process the wafer. This is due to the thick resist, which initially is located all over the wafer according to this method.
Because an identical resist type does not allow for adequate selectivity between the front resist and the back resist, difficulties arise when developing the front resist because, for example, the developer used to partially strip the front resist during developing also has a very corrosive effect on the back resist. The same is true when removing the front resist from the entire surface later on, if possible without damaging the back protection. Indeed, the back protection is also damaged to the extent that the wafer, for example, cannot be further processed if any additional steps are needed.
The positive resist also cannot provide a helium seal or adequate back protection after membrane etching because it blisters and becomes permeable in the plasma due to the thermal and chemical load imposed by the plasma etching process.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a method which overcomes the described disadvantages of the related art.
According to the present invention, a method is provided for applying a protective resist to a prepatterned wafer, characterized in that the resist is applied by a distribution system, which includes a holder for the wafer, an xy sliding unit with a programming device, and a dispensing device. The dispensing device dispenses the optimum protective resist and the optimum amount of protective resist selectively into the caverns.
In one example embodiment of the present invention, the protective resist is applied to the back of the wafer in micromechanically prepatterned caverns.
The use of a negative resist or a resist made of non-photopatternable polymers as the protective resist is also preferred. In this case, benzo cyclobutadiene, polymethyl methacrylate, polyimide, or epoxide can preferably be used. Due to their high chemical and thermal stability, negative resists are especially advantageous and yet they can be removed later on cleanly and without leaving a residue, for example in an oxygen plasma ambience.
The distribution system includes the following three components:
1. It has a holder for the wafer, allowing the latter to be positioned in the distribution system in a precisely defined, reproducible manner with respect to wafer orientation. This is achieved, for example, by a plate having a recess milled into the plate and corresponding to the wafer geometry, with this plate also encompassing the wafer flat. This clearly defines the position and alignment of the wafer.
2. The distribution system also includes an xy sliding unit with a programming device, which holds the actual sprayer and can selectively move to each individual cavern position, thus scanning the entire back of the wafer, controlled by a program. Each cavern in the wafer corresponds to one explicitly entered program position.
3. Finally, the distribution system also has a dis
Becker Volker
Laermer Franz
Schilp Andrea
Dang Phuc T.
Kenyon & Kenyon
Nelms David
Robert & Bosch GmbH
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