Why does freon stick to surfaces
Further details and advantages of the present invention can be seen from the following description, the figures and the examples. The figures represent in detail:
1 shows the production of etching masks for micromechanical structures;
2 shows a photographic representation of a micromechanical structure which was worked out directly from a silicon dioxide layer, together with a drawing of the associated mask;
Fig. 3 height diagrams of silicon dioxide webs which were carved out of a 2 µm thick silicon dioxide layer:
a) wafer pretreated with HMDS,
b) wafer pretreated with HMCTS;
4 is a photographic representation of photoresist structures on a silicon oxynitride substrate;
Fig. 5 shows the structures of Fig. 4 after a hardness test:
a) wafer pretreated with HMDS,
b) Wafer pretreated with HMCTS.
As discussed above, hexaalkyldisalazanes of the general formula were previously used as adhesion promoters
with R = methyl (HMDS) or ethyl (HEDS) used.
Adhesion promoters which are used in the process according to the invention are cyclic organosilicon compounds of the general formula
in the R and R.I. each is H, alkyl, cycloalkyl, aryl, halogen-substituted alkyl or aryl, or halogen and RII each denotes H, alkyl or aryl and n is an integer and preferably 1 or 2. Further cyclic organosilicon compounds which can be used as adhesion promoters are described in EP-A-86 103 208.4.
In the process according to the invention, hexamethylcyclotrisilazane (HMCTS) is preferably used.
To produce a micromechanical structure, an oxide layer (1) is applied to a silicon body (2) as shown in FIG. 1A. This oxide layer can be produced by known methods, such as, for example, by depositing silicon dioxide from the vapor phase, by chemical oxidation of the silicon surface with oxygen, water vapor, air or other oxidizing agents, or by thermal decomposition of silane. The thickness of the oxide layer can generally be between 0.1 and 4 µm. In this example it is about 2 µm. After the oxide layer has been produced, an adhesive layer (3) made of hexamethylcyclotrisilazane (HMCTS) is applied. The adhesive can either be used directly or in the form of a solution in a solvent such as xylene or a chlorofluorocarbon ((Du Pont) FreonR.) are applied in a dilution of 1 part by volume of HMCTS to 9 parts of solvent using known coating processes. It is sufficient if the adhesive is applied in a layer thickness of a few nm, preferably only in a molecular layer thickness.
A variety of photoresist materials can be bonded to the oxide layer (1) via the adhesive layer (3). Among these photoresist materials, compositions based on phenol-formaldehyde polymers with diazoketone derivatives are particularly suitable as sensitizers, for example a novolak resin with naphthoquinone- (1,2) -diazide-sulfonic acid ester as sensitizer, which is described in US Pat. No. 3,201,239. A photoresist material containing this sensitizer is commercially available under the designation Shipley AZ-1350 J positive photoresist.
Other polymers that can be connected to inorganic substrates via HMCTS as adhesion promoters are, for example, polyvinylpyrrolidone, polyvinyl alcohol, polymers of p-hydroxystyrene, melamine polymers, homo- and copolymers of monoethylenically unsaturated acids such as copolymers of alkyl methacrylates and acrylic or methacrylic acid or crotonic acid.
The thickness of the photoresist to be applied depends on the specific material and the coating technique. Normally, when manufacturing micromechanical structures, thicknesses between 100 and 1000 nm are sufficient. The photoresist layer (4) is then exposed imagewise in the desired pattern, developed with an alkaline developer and post-cured. The surface of the wafer is then exposed to an etching solution to etch the oxide (1). A hydrofluoric acid buffered with ammonium fluoride is used, for example, as the etchant for etching the mask structure into the 2 μm silicon dioxide layer (1). Mixtures of nitric acid, acetic acid and hydrofluoric acid can also be used for etching.
It has been shown that the photoresist mask (4) remains firmly connected to the oxide surface during the etching of the mask structure in the silicon dioxide layer (1). Lifting off of the photoresist, as shown in FIG. 1B, does not take place when the adhesive layer (3) according to the invention made of HMCTS is used.
After stripping the photoresist mask (4) with a suitable for the substrate material (2) etchant, for. B. with KOH (44 wt.% Ig in water / isopropanol) through the silicon dioxide mask (1) etched the desired pattern in the silicon body (2) (Fig. 1 C).
Fig. 2 shows in a photographic representation (200: 1) a self-supporting, approximately 2 μm thick silicon dioxide tongue over an approximately 26 μm deep etching pit in the silicon substrate. To produce this micromechanical structure, a silicon dioxide mask, the pattern of which is shown below the photograph, is produced using a wet chemical method by etching in buffered hydrofluoric acid. For this purpose, a photoresist mask, which was produced in a known manner and using an HMCTS adhesive layer, is used as an etching mask. An anisotropic etchant is used to etch the (100) silicon substrate through this mask. 44% potassium hydroxide solution is used as the etching agent. Etching time and temperature are 5.3 hours and 40 ° C, respectively. Since the mask is rotated 45 ° to the <110> crystal direction, the mask is undercut to a considerable extent. In this way, during the long etching time, a self-supporting tongue made of silicon dioxide is obtained over an etching pit in the substrate material about 26 μm deep. The tongue shown in FIG. 2 on a scale of 200: 1 has a width of approximately 50 μm and a length of approximately 500 μm. It was vapor-deposited with gold in a layer thickness of 0.2 µm. The microscope image (512 ×) of this tongue shows a structure with sharp edges, which is due to the fact that no infiltration of the photoresist mask has taken place. The silicon wafers used in this example were also not pretreated with Caro's acid, so that the advantageous results can be attributed exclusively to the use of the HMCTS adhesion promoter according to the invention.
The superiority of hexamethylcyclotrisilazane (HMCTS) as an adhesion promoter can also be shown by direct comparison with hexamethyldisilazane (HMDS). FIG. 3 shows height diagrams of silicon dioxide webs which have been machined from a 2 μm thick silicon dioxide layer. The height diagrams were recorded with the mechanical stepping device α-step 200 from Tencor. The silicon dioxide bars are located on two different wafers, which were processed in identical procedures, with the exception that wafer A was treated with HMDS and wafer B with HMCTS as an adhesion promoter. The webs are nominally 25 µm wide and have a nominal undercut of 4.4 µm after the etching. For wafer B, which was pretreated with HMCTS, the measurement results almost correspond to the theoretical expectations, while for wafer A, which was pretreated with HMDS, an additional undercut of about 5 μm was measured, which can be attributed to the undercut of the photoresist mask.
With HMCTS, significantly better adhesion of the photoresist is obtained on silicon oxynitride layers than with HMDS as an adhesion promoter. To prove this, several wafers coated with oxynitride (layer thickness about 2 μm, refractive index 1.758) were coated with Shipley AZ 1350 J positive photoresist, exposed, developed and post-cured. Both HMDS and HMCTS, dissolved in Freon, were used as adhesion promotersR. , used. The smallest structures on the photomask had a line width of 2.5 µm. After development and post-curing at 130 ° C., the smallest photoresist structures with a line width of 3.0 μm were initially perfectly preserved on all wafers (FIG. 4). All wafers with the resist structures were then placed in the developer solution (sodium metasilicate / sodium phosphate) for another hour at room temperature and then rinsed thoroughly with DI water. This stress treatment clearly showed the superiority of wafers treated with HMCTS over wafers treated with HMDS. In the case of wafers treated with HMDS (FIG. 5, A), all resist structures including a line width of 4.5 μm had disappeared, while the 3.0 μm wide photoresist structures (FIG. 5, B) were still present in the wafers treated with HMCTS as adhesion promoter.
Although the invention has been described in its application for the production of semiconductor arrangements and micromechanical structures, it can also be used in other processes in which a photoresist layer has to be applied to a surface of inorganic substrate materials. For example, the method according to the invention can be used in the production of printed circuit cards, for the production of thin-film memories in which a metal film is protected by an oxide surface, in the gravure printing process, or in the production of photomasks and for coating glass plates. Data storage on optical disks, for example, requires very small (0.6 µm wide) structures that have to be etched into the glass substrate. An attempt was made to etch the 1 µm wide grooves required to test the scanning behavior of optical disks into the glass substrate by means of conventional lithography, which was not successful with HMDS as an adhesion promoter. By contrast, with the aid of the HMCTS adhesion promoter according to the invention, it was possible to etch these 1 μm wide grooves into the glass substrate.
The cyclic organosilicon compounds which are used as adhesion promoters in the inventive method must contain or be able to provide at least two functional groups which can react with the reactive groups of the polymer resist, while the remainder of the molecule is firmly attached to the inorganic substrate material adheres.
The adhesion promoter according to the invention is generally applicable and useful for improving the adhesion of polymer materials to surfaces of inorganic substrate materials.
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