In the additive process of depositing patterned metal films known as the lift-off process which entails depositing a resist film on a substrate and transferring a pattern to such film followed by the blanket deposition of a thin layer of metal in order to reproduce the pattern with the metal. The resist serves as a deposition mask separating the desired metal pattern from the excessive metal by the vertical thickness gap of 1-2 .mu.m (i.e., the resist thickness). Generally the resist thickness is twice the thickness of the metal film deposition thickness. Since it is difficult to create a desirable undercut profile in a single resist layer, multiple resist layer processes have been developed in which a bilayer or trilayer structure is sequentially patterned by series of exposure and development steps using ultraviolet radiation (portable conformable mask (PCM) process as is set forth in U.S. Pat. No. 4,211,834 to Lapadula et al.) or a reactive ion etch process using a patterned metallic or other inorganic masking layer over a polymeric layer for pattern transfer and metal deposition (see, for example the process described in U.S. Pat. No. 3,873,361 to Franco et al.)
In a typical PCM process as shown in FIG. 2, a substrate 10 upon which a line or feature is to be placed has a planarizing underlayer 12 deposited or coated thereon. The underlayer is a deep UV resist composition such as the polyglutarimide sold commercially as Shipley (SAL) resist. This underlayer is patternable with deep UV (200-300 nm) radiation. A top image layer 14 is deposited over the underlayer. The top image layer is typically a diazoquinonesensitized novolak resin (DQN) resist which is patternable in near UV (350-450 nm) radiation.
The bilayer resist is imaged with near UV radiation through a mask to form a latent image 16A in the top imaging layer as is shown in FIG. 2A. This image is developed and will have a profile 16B as may be seen in FIG. 2B.
The pattern formed by openings 16B in the top layer 14 serves as a contact printing mask to enable flood or blanket deep UV exposure to transfer a latent image 16C into underlayer 12. This image is developed as is best seen in FIG. 2C to form the profile of the opening 16D which is characterized in having sloped foot 18. This profile serves as the deposition and lift-off mask for forming Al-Cu metallurgy patterns of the order of 0.5-2.5 .mu.m in width and 0.5-1.0 .mu.m in thickness on semiconductor substrates. The profile formed using a typical diazonaphthoquinone sensitized AZ-1350 (DQN)/PMGI bilayer structure suffers from a protruding foot at the PMGI-silicon wafer interface.
Metallization may be by any conventional method including evaporation, sputtering, or the like to provide a metallized structure as is shown in FIG. 2D with blanket metal 20 covering the bilayer resist and the metal line or feature 20' on the substrate.
FIG. 2E shows the completion of metal lift off wherein the line or feature 20' has a protruding fence metal which may cause shorts or dielectric breakdown between it and adjacent conductive features. The protruding foot of resist leads to undesirable extra metal fences 22 at the edges of the unlifted metal circuitry. The extraneous metal tips induce short circuiting by contact with adjacent circuit lines.
An improved process was provided in U.S. Pat. No. 4,814,258 to Tam which is set forth in FIG. 3. The steps and materials are the same as those in FIG. 2 except before the planarizing underlayer 12 was soaked in chlorobenzene to increase its solubility in the developer with respect to the photoresist layer 14 which is the applied and which is exposed and developed to give the image 16B as is shown in FIG. 3A.
The image 16D is undercut at the top to enable the imaging layer 14 to overhang the planarizing layer 12 and leaves somewhat of a foot 18A. This foot leaves the opportunity for fencing as was seen in FIG. 2. The resulting profile 16 after the chlorobenzene soak exhibits lateral undercut 18A at the DQN/PMGI interface which in the case of closely spaced metal lines (FIG. 3B) defined by the DQN/PMGI interface tends to further undercut at the DQN/PMGI interface and finally topple the resist structure. The protruding foot at the PMGI/silicon interface is not effectively removed. Secondly, the process of soaking an organic resist layer in a solvent involves a separate process step using chemicals of a flammable and toxic nature with the added cost of storage, safety, and disposal. Thirdly, the soak process is isotropic and subject to many factors of diffusion such as the prebake conditions of the PMGI layer, the time, temperature of soaking, the shelf or use life of the chlorobenzene bath, and the purity of the chlorobenzene soaking material.
The formation of an image in the PMGI planarizing layer involves a deep UV exposure of a considerable dose of the order of 500-2000 mJ/cm.sup.2 and exposure times of several minutes in order to induce sufficient solubility of the PMGI in alkaline developers. Long exposure times with large 200 mm diameter wafers can add extra costs when using a PCM process. Attempts to decrease the exposure time of positive resists of the DQN type have used additives of alkaline soluble substances such as acids (U.S. Pat. No. 4,009,033). The addition of these acids also significantly result in loss of unexposed resist resulting in more pinholes, and insufficient step coverage over topography. Shorter exposure times at the expense of film thinning is to be avoided. In another system, diazoquinone sensitizers are added directly to the PMGI resist (U.S. Pat. No. 4,524,121). High doses are still required (1500-3000 mJ/cm.sup.2 in the near UV region). For a PCM application, the presence of a diazoquinone in the novolak toplayer and in the PMGI underlayer provides no discrimination to the near UV light used to image the DQN toplayer. The diazoquinone in the PMGI layer would also be exposed by the near UV light resulting in sloping resist profiles unsuitable for lift-off. The toplayer of DQN would also have to be &gt;1.5 .mu.m thick to avoid imaging the PMGI layer with near UV light. For exposure tools of high numerical aperture, the depth of focus of the tools is close to 1.5 .mu.m in range and thus thick resist films &gt;1.5 .mu.m in the DQN imaging layer are to be avoided since blurred images will be produced.