BARRIERLESS HIGH-TEMPERATURE LIFT-OFF PROCESS FOR FORMING A PATTERNED INTERCONNECTION LAYER

Abstract: A lift-off metal deposition process in which a high-temperature polyimide layer (16) (i.e. a polyimide having a high imidization temperature) is applied to a first polyimide layer (14). The two layers (14, 16) are anisotropically etched through a photoresist mask to form via holes in the first polyimide layer (14). After application of a metal layer (20), the high-temperature polyimide layer (16) is lifted off the first polyimide layer (14), which remains as a passivation layer.

Description: The invention relates to a method of forming metal layers under high-temperature conditions. Many methods are known for forming a patterned conductor layer on a substrate. The two most common methods of forming such a layer are subtractive etching and lift-off techniques. In subtractive etching, after a blanket conductor layer is deposited on the substrate, the layer is etched through a photomask in order to remove undesired portions thereof. In lift-off, a layer (typically an insulator such as polyimide) is deposited on a substrate, and is patterned through a photomask. The conductive layer is then desposited on the patterned insulator, and the insulator is removed from (i.e. "lifted off" of) the substrate, taking with it the undesired portions of the conductive layer. Of these two techniques, it has been found that lift-off is more desireable in that the solvents used to remove the insulator in lift-off cause less damage to the underlaying substrate than do the etch processes (e.g. a plasma etch or a reactive ion etch) used in subtractive etching. Also, the conductor profile resulting from lift-off processing minimizes step coverage problems in subsequent conductor layers. An example of such a lift-off process is disclosed in US-A-4,451,971. As disclosed in this patent, a layer of pre-imidized polyimide (i.e. a copolymer of an aromatic cycloaliphatic diamine and a dianhydride) is coated on a semiconductor substrate, and a silicon dioxide barrier layer is formed on the polyimide. The barrier layer protects the polyimide layer during photolithographic processing. After these layers are patterned through a photoresist mask, a metal layer is deposited on the structure. The polyimide layer is then stripped off the silicon, lifting off the undesired portions of the metal layer. By use of the particular polyimide copolymer disclosed, the metal can be deposited at a temperature of 250°C-300°C, reducing physical faults in the deposited metal. Note that during both the deposition and lift-off of this polyimide copolymer, a harmful organic solvent such as methylene chloride must be used. The EP-A-0 188 735 (falling within the terms of Art. 54(3) EPC) discloses a method of forming via holes in a first polyimide layer through apertures in a second polyimide layer. The thickness of the second polyimide layer is varied in order to alter the slope of the via hole sidewalls. In the article by Homma et al, "Polyimide Liftoff Technology for High-Density LSI Metallization", IEEE Transactions on Electron Devices, Vol. ED-28, No. 5, May 1981 pp. 552-556, a lift-off metallization process is disclosed in which a high-imidization temperature polyimide, sold under the trade name "PIQ" by the Hitachi Chemical Co., Ltd of Japan, has an overlaying molybdenum barrier layer formed thereon. The PIQ serves as the lift-off structure (i.e. the layer which is lifted off from the underlaying layers). In the article by Winter, "Metal Deposition With Polyimide Lift-Off Technique", IBM Technical Disclosure Bulletin, Vol. 17, No. 5, October 1974, p. 1309, a first layer of polyimide is patterned through a photoresist mask. After the metal is deposited, the photoresist mask is removed from the first polyimide layer and a second polyimide layer is applied for passivation. As discussed above, special polyimide layers are needed in order to carry out high temperature lift-off processes. However, these special polyimides require overlaying barrier layers, which protect the polyimides from etching during the definition of a photoresist mask disposed on the barrier layer. It would be advantageous to eliminate these barrier layers, since they add to manufacturing cost. It is thus an object of the invention to provide an improved metal lift-off process for forming a patterned interconnection layer. It is another object of the invention to provide a lift-off structure which is compatible with high temperature metal deposition. It is a further object of the invention to provide an improved metal lift-off process using polyimide as the lift-off structure, wherein the polyimide layer is not protected by a barrier layer and can be processed using conventional solvents. These and other objects of the invention are realized by a process for forming a patterned interconnection layer on a processed semiconductor substrate, which comprises the steps of forming a first polyimide layer on said substrate, forming a second, high-temperature polyimide layer which does not fully imidize at temperatures below 250°C-280°C on said first polyimide layer, forming a photoresist layer on said high-temperature polyimide layer, exposing said photoresist layer and developing said photoresist layer in a solvent which does not appreciably attack said high-temperature polyimide layer, etching said high-temperature polyimide layer and said first polyimide layer through said developed photoresist layer to form via holes in said first polyimide layer, said high-temperature polyimide layer and said first polyimide layer etching at substantially the same rate, said photoresist layer being substantially removed during said etching step, forming a conductor layer in said via holes on said substrate and on said etched high-temperature polyimide layer, said conductor layer at least partially filling said via holes in said first polyimide layer, said conductor layer being formed at temperatures of 200°C-280°C, and lifting off said high-temperature polyimide layer from said first fully imidized polyimide layer in order to remove undesired portions of said conductor layer. The first polyimide layer thus remains as a passivation layer. Note that there is no barrier layer between the high-temperature polyimide layer and the photoresist mask. The high-temperature polyimide layer can be processed using conventional solvents. The foregoing and other structures and teachings of the invention will become more apparent upon a description of the best mode as rendered below. In the description to follow, reference will be made to the accompanying drawings, in which: Figs. 1-3 are cross-sectional views of a semiconductor structure undergoing the process steps of a first embodiment of the invention, and Figs. 4-8 are cross-sectional views of a semiconductor structure undergoing the process steps of a second embodiment of the invention. With reference to Figs. 1-3, a first embodiment of the invention will now be described. As shown in Fig. 1, a substrate 10 has a layer of polyimide 14 spin-applied thereon. While substrate 10 is shown as being a bare silicon substrate, it is to be understood that any one of the semiconductor structures or devices currently manufactured in the industry (e.g. FET or bipolar transistors, storage capacitors, resistors, etc.) could be arranged on substrate 10, and that the patterned conductor layer to be described is patterned so as to form an electrical contact to any one of these structures. In other words, substrate 10 is shown as being bare merely for the purposes of more clearly illustrating the invention. Polyimide layer 14 can be made up of any one of the known polyamic acid limides that are stable up to 350°c. For example, polyimides sold under the trade names "DuPont 5878" and "DuPont 2555" by the DuPont Company of Wilmington, Delaware could be used. Polyimide layer 14 should be approximately as thick as the metal layer to be applied. For second and third level metal, polyimide layer 14 should be approximately 1.8-2.0 µm thick. In addition to providing passivation, polyimide layer 14 produces a "step" for the metal layer to cover. This enhances discontinuities in the metal layer, facilitating lift-off as described below. It is to be understood that this embodiment of the invention relates to the formation of any level of metallurgy on the processed substrate. If the invention is used to provide a first level of metallurgy, an additional passivating layer (e.g. silicon nitride, silicon dioxide or sputtered quartz) could be formed between polyimide layer 14 and substrate 10 for the purpose of providing additional insultation. This additional passivating layer would have to be etched separately (i.e. etched using a separate mask) from the etching of the polyimide layers asd discussed below. Further, if this embodiment of the invention is used to provide via studs to structures formed on the substrate, a similar passivating layer could be used which would be patterned using the same mask as the polyimide layers (only the etch ambient or plasma would have to be changed). Either way, the total thickness of the combination of polyimide 14 and the additional passivating layer should approximately equal the thickness of the metal layer to be applied. For the first level metal, the combined thickness should be 1.0-1.2 µm. For second, third, etc. level metals, the combined thickness should be 1.8-2.0 µm. The additional passivation layer can be made up of any insulator formed at a temperature which is less than the annealling temperature of the underlaying conductor layers. It is emphasized that while incorporation of these additional passivation layers is preferred in that they improve reliability, they can be deleted from the process of the invention if desired. A layer of "high-temperature" polyimide 16 is then spin-coated onto the surface of polyimide layer 14. Polyimide 16 can be made of any one of the known "high temperature" polyimides which do not fully imidize at temperatures below 250°C- 280°C. Such polyimides are thus compatible with high-temperature metal deposition. An example of such a polyimide is Pyralin PI-2566, sold by the DuPont Company of Wilmington, Delaware. "Pyralin" is a trademark of the DuPont Company. Another such polyimide is sold under the trade name "PIQ" by the Hitachi Chemical Co., Ltd of Japan. These high-temperature polyimides can be distinguished from the polyimide copolymer disclosed in US-A-4,451,971 in that these polyimides are not pre-imidized, and that they can be processed using solvents commonly used in the industry. Whichever of these two high-temperature polyimides is used, polyimide layer 16 should be at least as thick as the metal layer to be deposited. The polyimide 16 should be heated to a temperature below its final cure temperature in order to facilitate subsequent etching. More specifically, it should be heated to at least 120°C in order to harden, and preferably should be heated to approximately 200°C in order to drive off excess solvent. For example, a 2 µm layer of the PI-2566 polyimide is heated to 200°C for approximately 20 minutes. This heating is sufficient to fully imidize polyimide layer 14. A layer of photoresist 18 (e.g. MIRP-HC photoresist) is then applied to the surface of high-temperature polyimide layer 16. This layer of photoresist must be thick enough (e.g. 3 µm) such that the underlying polyimide 16 will not be attacked when the via holes are etched. That is, by making the photoresist sufficiently thick, there is no need for a barrier layer in order to protect portions of the polyimide layer 16 which are not to be removed during etching. The photoresist can be any resin-based positive photoresist. Preferably, the photoresist is chosen such that it can be exposed and developed (i.e. etched in an aqueous base such as sodium metasilicate (Na₂SiO₃) as per conventional processing. After photoresist 18 has been exposed and developed, the high-temperature polyimide 16 and polyimide 14 are anisotropically reactive ion etched (RIE) in an oxygen plasma. Note that during the course of this etch step, much or all of the photoresist 18 is consumed. See Fig. 2. Thus, via holes having essentially vertical sidewalls are etched into the polyimide layer 14. Then, as shown in Fig. 2, a 1.8-2.0 µm layer of conductive material 20 (1.0-1.2 µm for first level metal) is formed on the structure, filling the via holes formed in polyimide layer 14. Conductive material 20 may consist of any of the conductive materials used in forming patterned interconnection layers in semiconductor processing (e.g. metals such as aluminum, copper, etc.; silicides of tungsten, titanium, molybdenum, etc.). A feature of the invention is that during the deposition of layer 20, substrate 10 can be heated such that the physical defects in the resulting interconnection layer (e.g. cracks, etc.) can be minimized. The substrate can be heated to temperatures of approximately 200°C-280°C. Note that the only constraint on these deposition temperatures is that they must not be greater than the "full" imidization temperature of polyimide layer 16. In other words, during high temperature metal deposition, polyimide layer 16 should not be imidized beyond an insignificant amount (e.g. 2-5%). Finally, as shown in Fig. 3, high-temperature polyimide layer 16 is lifted off from polyimide layer 14. This lift-off is performed by submerging the substrate in n-methyl pyrrolidone (NMP) solvent at approximately 80-90°C for less than 30 minutes. Thus, the undesired portions of conductor layer 20 are removed. The remaining polyimide layer 14 serves to passivate the conductive layer. Polyimide layer 16 can be removed without affecting polyimide layer 14 because of the fact that polyimide layer 16 is not fully imidized. As described above, the process of the first embodiment of the invention utilizes a high-temperature polyimide without a barrier layer. In addition, the invention provides a polyimide layer which passivates the patterned conductor layer. With reference to Fig. 4, a second embodiment of the invention will now be described. This embodiment relates to the formation of a patterned contact layer (i.e. a "pad metallization") which enhances the electrical contact between the final metallization level on the substrate and the chip pads (i.e. "solder balls") or wire bonds which receive signals from sources external to the chip. This contact layer also serves as a barrier layer, preventing the intermixing of the metallization metal with the chip pad or wire bond metals during the formation of the latter. As shown in Fig. 4, a polyimide passivation layer 30 is first applied to the surface of the substrate 10A. A metallization layer 32 is then disposed on passivation layer 30 and is patterned to form an elongated area which provides the electrical contact to the solder ball or wire bond to be subsequently formed. Patterned metallization layer 32 fills via holes formed in passivation layer 30, and contacts semiconductor structures and/or previously patterned conductor layers formed on substrate 10A. These underlying layers/structures are omitted from Figs. 4-8 in order to more clearly illustrate this embodiment of the invention. It is to be emphasized that while insulator layer 30 and metallization layer 32 could be processed in the manner of the first embodiment of the invention as described above, they are not limited thereto. In other words, metallization layer 32 and insulator layer 30 could be made up of other materials and processed in other ways in addition to those materials and process steps of the first embodiment of the present invention. Substrate 10A is then covered with a final polyimide passivation layer 34 which can be made up of the afore-mentioned DuPont 5878 polyimide or an equivalent. Since polyimide 34 constitutes the final passivation layer, it should be relatively thick (in the order of 8 µm) in order to protect the underlying structures. After polyimide 34 is spin-coated onto the substrate, it should be solidified by heating to 110°C-130°C for approximately 15 minutes. A layer of high-temperature polyimide 36 is then applied to the surface of polyimide 34. High-temperature polyimide 36 is made up of polyimides such as PI-2566 or PIQ, as discussed previously. Similar to the first embodiment of the invention, the high-temperature polyimide 36 should be approximately as thick (e.g. 2-3 µm) as the conductive layer to be subsequently formed. Similar to polyimide 34, the high-temperature polyimide 36 should be solidified by heating to 110°C - 130°C for approximately 15 minutes. A photoresist layer 38 is then deposited onto high-temperature polyimide layer 36. The photoresist should be a positive photoresist as described in the first embodiment of the invention. The positive photoresist is exposed and developed as per conventional processing, using aqueous bases such as potassium hydroxide (KOH) or tetramethyl-ammonium hydroxide (TMAH). A feature of this embodiment of the invention is that by using a positive photoresist and the and the above-described etchants, the polyimide layers can be etched as the positive photoresist is developed. In other words, the photoresist is patterned and openings are created in the underlying polyimide layers during the course of a single wet etch step. See Fig. 5. Then, as shown in Fig. 6, the positive photoresist 38 is removed using n-butyl acetate or any other solvent (e.g. iso-propyl alcohol or acetone) which removes photoresist without appreciably attacking underlaying polyamic acids such as polyimide. Note that this removal step is not necessary, in that the photoresist could be removed as polyimide layer 36 is lifted off as described below. By removing the photoresist separately, lift-off can be carried out more efficiently. After photoresist removal, the polyimide layers are heated to approximately 200°C, which is sufficient to fully imidize (i.e. achieve at least 98% imidization) polyimide layer 34 and to imidize high-temperature polyimide layer 36 by an inconsequential amount (e.g. 2-5%). The apertures formed in the polyimide layers are briefly etched, using either plasma or wet etch techniques, in order to remove any impurities. A suitable wet etchant would be chromic-phosphoric acid, and a suitable atmosphere for plasma etching would be CF₄ or CF₄+O₂. Again, while this step is not absolutely necessary, it contributes to the reliability of the overall process. As shown in Fig. 6, a conductor layer 40 is then deposited. This layer (e.g. a combination of chromium, copper and gold or titanium, copper and gold) enhances the contact between the wire bond or solder ball to be subsquently formed and patterned conductor layer 32, while also providing an intermixing barrier therebetween. As in the first embodiment of the invention, the substrate 10A should be heated to 200-280°C during deposition in order to minimize the defects in the pad metallurgy layer. Note that other metals (e.g. aluminum) could be used here. Then, as shown in Fig. 7, undesired portions of pad metallurgy 40 are removed by lifting off high-temperature polyimide layer 36 from polyimide layer 34. As in the first embodiment of the invention, the different solubility characteristics of the two polyimide films are produced by the extent to which they are imidized during the course of previous processing steps (i.e. recall the 200-250°C cure step in which polyimide layer 34 is fully imidized while high-temperature polyimide layer 36 is not appreciably imidized). Polyimide layer 36 is stripped using n-methyl-pyrrolidone at 80-95°C for 30 minutes. Finally, the lead-tin solder balls 42 are deposited on the remaining portions of conductor layer 40, using well known techniques (e.g. evaporating the solder through a metal mask), resulting in the structure as shown in Fig. 7. Alternatively, a wire bond metallurgy could be deposited on conductor layer 40, again using conventional techniques. The second embodiment of the invention as described above can be modified as follows. In order to ensure that sufficient contact is made between solder ball 42 and conductor layer 40, it may be advantageous to configure the upper surface of conductor layer 40 such that it overflows the via holes formed in passivating polyimide layer 34. In order to do this, the process can be altered by eliminating the positive resist strip. Immediately before the via holes are etched in order to remove impurities, briefly expose the structure to TMAH or KOH solvents for a time sufficient to etch back the sidewalls of the opening formed in high-temperature polyimide layer 36 without appreciably affecting the via holes in polyimide layer 34. Thus, upon application of conductor layer 40, the metal will overflow the via holes in polyimide layer 34 by a controllable amount (i.e. the extent to which the sidewalls of the aperture in polyimide layer 36 were etched back). The process is then completed as described, resulting in a structure as shown in Fig. 8. As described above, both embodiments of the invention present a method by which high-temperature polyimide layers are used to form a lift-off structure without the need for a barrier layer. The first embodiment of the invention provides an efficient method of forming patterned interconnection layers, and the second embodiment of the invention provides an efficient method of providing an interconnection between the final metallization layer and the solder balls or wire bonds.

Claims: Process of forming a patterned interconnection layer on a processed semiconductor substrate (10, 10A) comprising the steps of forming a first polyimide layer (14, 34) on said substrate (10, 10A), forming a second, high-temperature polyimide layer (16, 36) which does not fully imidize at temperatures below 250°C - 280°C on said first polyimide layer (14, 34), forming a photoresist layer (18, 38) on said high-temperature polyimide layer (16, 36), exposing said photoresist layer (18, 38) and developing said photoresist layer (18, 38) in a solvent which does not appreciably attack said high-temperature polyimide layer (16, 36), etching said high-temperature polyimide layer (16, 36), and said first polyimide layer (14, 34) through said developed photoresist layer (18, 38) to form via holes in said first polyimide layer (14, 34), said high-temperature polyimide layer (16, 36) and said first polyimide layer (14, 34) etching at substantially the same rate, said photoresist layer (18, 38) being substantially removed during said etching step, forming a conductor layer (20, 40) in said via holes on said substrate (10, 10A) and on said etched high-temperature polyimide layer (16, 36), said conductor layer (20, 40) at least partially filling said via holes in said first polyimide layer (14, 34), said conductor layer (20, 40) being formed at temperatures of 200°C - 280°C, and lifting off said high-temperature polyimide layer (16, 36), from said first fully imidized polyimide layer (14, 34) in order to remove undesired portions of said conductor layer (20, 40). Process in accordance with claim 1, characterized in that said high-temperature polyimide layer (16, 36) and said conductor layer (20, 40) have substantially the same thicknesses. Process in accordance with claim 1, characterized in that prior to said step of forming said photoresist layer (18, 38) on said high-temperature polyimide layer (16, 36), said first polyimide layer (14, 34) is substantially imidized without appreciably imidizing said high-temperature polyimide layer (16, 36). Process in accordance with claim 1, characterized in that said photoresist layer (18, 38) is developed in a solvent comprising sodium metasilicate. Process in accordance with claim 1, characterized in that said step of etching said high-temperature polyimide layer (16, 36), and said first polyimide layer (14, 34) is carried out using reactive ion etching in an oxygen ambient. Process in accordance with claim 1, characterized in that said substrate (10) is heated to a temperature below the imidization temperature of said high-temperature polyimide layer (16, 36) during said step of forming a conductor layer (20, 40) on said substrate (10). Process in accordance with claim 1, characterized in that said step of lifting off said high-temperature polyimide layer (16, 36) is carried out using n-methyl pyrrolidone as the solvent. Process in accordance with claim 1, characterized in that prior to forming said first polyimide layer (14, 34) on the substrate (10, 10A) an additional passivation layer comprising an insulator which can be formed at a temperature below the anneal temperature of any conductor layer (20, 40) is deposited on the substrate (10 10A), said insulator being selected from the group consisting of silicon nitride, silicon dioxide and sputtered quartz. Process in accordance with claim 1, characterized in that said photoresist (18, 38) is developed in aqueous base which attacks the second and first polyimide layers (16, 36; 14, 34) so as to form via holes in said first polyimide layer (14, 34), said aqueous base being selected from the group consisting of potassium hydroxide, and tetramethyl-ammonium hydroxide. Process in accordance with claim 1, characterized in that said conductor layer (20, 40) comprises a combination of chromium, copper and gold. Process in accordance with claim 1, characterized in that said conductor layer (20, 40) comprises a combination of titanium, copper and gold.

Inventors: CLODGO, DONNA JEAN, PREVITI-KELLY, ROSEMARY ANN, WALTON, ERICK GREGORY
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H05K3