Collimated metal deposition

Abstract: A process and structure for depositing metal lines in a lift-off process is disclosed. The process provides the deposition of a four-layer structure or lift-off stencil, comprising a first layer (12) of a lift-off polymer etchable in oxygen plasma, a first barrier layer (14) of hexamethyldisilizane (HMDS) resistant to an oxygen plasma, a second lift-off layer (16) and a second barrier layer (18). Once these layers are deposited, a layer (20) of photoresist is deposited and lithographically defined with the metal conductor pattern desired. The layers are them sequentially etched with oxygen and CF₄, resulting in a dual over­ hang lift-off structure. Metal (26, 24) is then deposited by evaporation or sputtering through the lift-off structure. Following metal deposition, the lift-off structure is dissolved or lifted-off in a solvent such as N-methylpyrrolidone (NMP).

Description: Background Information 1. Field of the Invention This invention relates to a method and structure for depositing metal conductors on a substrate. In particular, a dual overhang, collimated metal process for depositing metal lines on a semiconductor chip is described. 2. Background Art Current Very Large Scale Integrated (VLSI) circuits and Ultra Large Scale Integrated (ULSI) circuits require the deposition of metal interconnection lines less than one micrometer in width. Metal lift-off processes have been widely used for depositing narrow metal conductors in the prior art. The basic "lift-off" method is described in U. S. Patent 2,559,389. Improvements to the basic lift-off method have been made, as for example in commonly assigned U. S. Patent Nos. 3,849,136; 3,873,861 and 4,519,872. The central concept of the lift-off method involves the deposition of a non-radiation sensitive lift-off layer, followed by the deposition of a thin-film of an inorganic material as a barrier layer. The desired metallurgy pattern is formed in a top resist layer using conventional lithographic techniques and etched into the barrier layer. The barrier layer functions as an etch barrier during subsequent reactive ion etching to form openings through the lift-off layer, extending to the substrate. Metal is then deposited, by sputtering or evaporation, for example. Following the metal deposition, immersion in a solvent dissolves or releases the base lift-off film, thereby leaving the metal conductors deposited in the windows. One of the problems with this method is that commercial metal deposition tools do not allow the metal atoms to be deposited exclusively from a vertical direction. Metal ions that are sputtered or evaporated enter the lift-off stencil openings at various angles to the normal. This often results in undesired metal deposited along the inner walls of the lift-off structure, a problem known as "footing". While some footing is tolerable for line widths greater than one micrometer, it can lead to difficult lift-off and electrical shorts in the integrated circuit for line widths less than one micrometer. In EP-A-149 060 a process for forming a desired metal pattern on a substrate is disclosed which comprises forming a mask of a thermally depolymerizable polymer on the substrate with a pattern of openings complementary to the desired metal pattern, blanket coating the substrate and the mask with a metal, heating to depolymerize the thermally depolymerizable polymer, and removing the thermally depolymerizable polymer and metal thereover in a mild solvent at moderate temperatures leaving the metal having the desired pattern on the substrate. IBM TECHNICAL DISCLOSURE BULLETIN Vol.25, No.8, pages 4481-4482, January 1983 disloses a lift-off process for producing a metallisation pattern, the process including the use of a mask structure with a single overhang. Objects of the Invention It is an object of this invention to provide an improved metal lift-off process in which undesired footing is eliminated even when narrow lines and spacings are to be defined. It is a further object of the invention to provide a collimated metal deposition method and structure in which a dual overhang structure is used to eliminate undesired metal deposition. Summary of the Invention These and other objects of the invention are accomplished by the inventive process disclosed and claimed herein. The process comprises the deposition of a four-layer structure or lift-off stencil, comprising a first layer of a lift-off polymer etchable in oxygen plasma, a first barrier layer of hexamethyldisilizane (HMDS) resistant to an oxygen plasma, a second lift-off layer, and a second barrier layer. Once these layers are deposited, a layer of photoresist is deposited and lithographically defined with the metal conductor pattern desired. The layers are then sequentially etched with oxygen and CF₄, resulting in a dual overhang lift-off structure. Metal is then deposited by evaporation or sputtering through the lift-off structure. Following metal deposition, the lift-off structure is dissolved or lifted-off in a solvent such as N-methylpyrrolidone (NMP). Brief Description of the Drawing FIGs. 1A-C show a sequence of cross-sectional views of the lift-off process and structure during various stages of fabrication. Description of the Preferred Embodiment The process to be described below is based on similar principles as used in collimating a light beam, laser, X-ray beam, or other radiation sources. In such collimation systems, colinear apertures are arranged so that the radiation exiting the system has all of the rays essentially parallel to each other. This same phenomena can be applied to the deposition of metal in a lift-off process to "focus" the metal to be deposited in a narrow line. Referring to FIG. 1A, a substrate 10, such as a semiconductor wafer with integrated circuits formed therein (not shown) is coated with a polyimide layer 12. Coating can be accomplished by spinning in polyamic acid and curing to form polyimide. The polyimide layer 12 is etchable in a reactive ion etch (RIE) chamber using oxygen as the etch gas. The thickness of the polyimide should exceed the thickness of the desired metal line to be deposited in subsequent processing steps by approximately thirty percent. Alternatively, a layer of photoresist can be used as the lift-off layer 12 instead of polyimide. The thickness of the photoresist should be comparable to the polyimide thickness. A barrier layer 14 of plasma polymerized HMDS is deposited to a thickness of approximately 200nm. The HMDS barrier layer 14 while not etchable in an oxygen plasma, is etchable in a plasma of a gas selected from a group of flourocarbon (e.g., CF₄, C₂, F₆, CHF₃) gases with or without oxygen as an additive. On this barrier layer 14 is deposited a second lift-off layer 16 which can be identical to the lift-off layer 12. The thickness should also be similar. On top of the lift-off layer 16 is deposited a second barrier layer 18 similar to layer 14. The substrate 10 with the deposited layers or lift-off stencil is ready for patterning. A photoresist layer 20 having the desired radiation sensitivity is applied, exposed and developed using conventional photolithographic processes. The thickness of the photoresist layer is in the 0.5 - 1.0 micrometer range. The entire stack is then etched in a 4 step etching sequence using the patterned photoresist layer 20 as a first etch mask. In the first RIE step, fluorocarbon gas such as CF₄, CHF₃, or C₂F₆, with or without O₂ as an additive are used as the etch gases. The power level, gas flow and pressure parameters are dependent upon the thickness of the layer 18 and the type of RIE reactor. With minimal experimentation, one skilled in the art can determine the etch parameters for all of the etching steps described herein. End point for the etching can be determined by laser interferometry, spectrophotometric analysis or other commonly used means. In the second RIE step, O₂ alone or mixed with less than 2% of a fluorocarbon gas, is used to etch layer 16 in a vertical direction. This plasma alone will not significantly etch HMDS layer 18. The third RIE step to etch layer 14 is identical to the first etch step, and the fourth RIE step to etch layer 12 is identical to the second etch step. Following the completion of the 4th etching step, etching continues using oxygen and higher pressure or power such that the plasma will laterally etch the lift-off layers 16 and 12, resulting in the dual overhang HMDS structure as shown in FIG. 1B. Because the final structure has two overhangs (in layers 18 and 14), upon deposition of evaporated or sputtered metal, the polydirectional metal atoms are screened by the first overhang 18 and the aligned second overhang in layer 14. Thus, there is the equivalent of a light-beam collimator which, using two coaxial apertures, defines a precise geometrically defined beam of light. Referring to FIG. 1C, the arrows 22 depict the non-vertical paths of the metal atoms, which are prevented from reaching the surface of substrate 10 by the overhangs in layers 18 and 14. Metal is deposited using well-known sputtering or evaporation techniques known in the art. The final structure, with the metal line 24 and excess metal 26 deposited on substrate 10 is shown in FIG. 1C. Following deposition of the metal, the lift-off stencil and excess metal 26 is lifted-off in a solvent such as NMP.

Claims: A dual overhang collimating structure for depositing metal lines on a substrate comprising: a substrate (10); a first lift-off layer (12) deposited on said substrate, said first layer being etchable in a first etch gas; a first barrier layer (14) deposited on said first lift-off layer, said first barrier layer being etachable in a second etch gas, and forming the first overhang of the dual overhang collimating structure at at least one opening; a second lift-off layer (16) deposited on said first barrier layer, said second lift-off layer having etch properties similar to said first lift-off layer (12); and a second barrier layer (18) deposited on said second lift-off layer, said second barrier layer having etch properties similar to said first barrier layer (14) and forming the second overhang of the dual overhang structure aligned to said first overhang structure, whereby undesired metal footing is prevented from being deposited on the substrate by the two barrier layers during deposition by sputtering or evaporation. The structure as claimed in claim 1 wherein said first and second lift-off layers are etchable in O₂ or O₂ containing plasma. The structure as claimed in claim 1 wherein said first and second lift-off barrier layers are etchable in a plasma of gas selected from the group consisting of CF₄, C₂F₆, or CHF₃ with or without O₂ as an additive. The structure as claimed in claims 1 to 3 wherein said first lift-off layer is polyimide. The structure as claimed in claims 1 to 4 wherein said second lift-off layer is polyimide. The structure as claimed in claims 1 to 5 wherein said first and second lift-off layers are photoresist. The structure as claimed in claim 1 wherein said first barrier layer is selected from the group consisting of HMDS, SiO₂, Si₃N₄ and resin glass.

Inventors: MATHAD GANGADHARA SWANI, STANASOLOVICH DAVID, VIA GIORGIA GIULIO, MATHAD, GANGADHARA SWANI, STANASOLOVICH, DAVID, VIA, GIORGIA GIULIO
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: H01L21
IPC codes: H01L21