Abstract:
A process for selectively removing an anti-reflective coating (ARC) in the manufacturing of semiconductor integrated circuits using an oxygen-free plasma of one or more fluorine containing compounds, chlorine and an optional inert carrier gas. The process renders effective etching of the anti-reflective coating while maintaining dimensional control of a previously etched photoresist.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improved method for etching organic anti-reflective coatings (ACR&#39;s) in the fabrication of integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Electronic, solid state microchips such as those in computer microprocessors, hold thousands of individual systems each of which is made up of thousands of individual electronic components such as transistors, resistors, and capacitors. The manufacture of solid state electronic devices relies on assembling layers of semiconducting, conducting, and insulating materials in precise patterns and uniform thicknesses, selectively etched to create electronic components and systems according to the functions desired. Because the components and systems are all made into one chip, or substrate, and interconnected, these circuits are appropriately termed “integrated circuits.” 
     The production of integrated circuits is highly competitive. Therefore improving the efficiency of the manufacturing steps means a competitive advantage. Typically, the production of integrated circuits requires fabricating one or more networks of conductive pathways interconnecting the components to form systems and interconnecting the systems to form circuits. An important manufacturing step is the formation of a network of conducting pathways, or interconnecting network, over a semiconducting substrate via photolithography and etching (collective these processes are referred to “patterning”). This is typically accomplished by coating a conductive, metallic layer, with a light sensitive coating, i.e., a photoresist coating (“photoresist”). The photoresist is then exposed to actinic light through a mask which blocks the light in a pattern corresponding to the pattern desired for the conducting interconnecting network. 
     The photoresist coating is subsequently “developed,” that is, the parts of the photoresist coating which were exposed to the actinic light are selectively removed, thereby exposing the conductive surface below in a pattern corresponding to the openings in the mask. Usually the exposed metal layer is removed by plasma etching, but may also be removed by wet chemical etching, leaving the desired interconnecting network. 
     The demand in recent years for greater miniaturization of integrated circuits has led to increased circuit density, which requires shorter wavelength light such as deep ultraviolet (UV) to expose the photoresist. While the short wavelength light theoretically should yield exposures of high resolution, unfortunately it tends to reflect off the metallic layer back through the photoresist layer. This reflection sets up interference with the incoming light which reduces resolution. A discussion of this phenomenon can be found in  Silicon Processing for the VLSI Era , S. Wolf, et al., v. 1, “Process Technology,” Lattice Press, Sunset Beach, Calif. (1987). To solve this problem, an anti-reflective coating (ARC) is typically deposited on the metallic layer before the photoresist layer to reduce the unwanted reflection and consequential loss of resolution. The ARC&#39;s widely used in the integrated circuit industry are polyimides although other organic anti-reflective material can be used. 
     After the photoresist has been exposed to UV and developed, the ARC must be removed by etching to uncover the underlying conductive, metallic layer or coating. The prior art teaches that this may be done by plasma etching. However, a system of etching agents and conditions must be used which effectively etch away the ARC but leave the photoresist relatively intact. 
     U.S. Pat. No. 5,655,110, “Krivokapic et al.,” incorporated herein by reference, discusses the importance of maintaining critical dimensions during etching operations in mass produced semiconductor wafers. U.S. Pat. No. 5,126,289, “Ziger,” teaches a method of etching the photoresist layer and the underlying aluminum layer in one operation using ionized carbon tetrachloride as the etching agent. However, Ziger is silent regarding degradation of the photoresist. 
     Thuy B. Ta in U.S. Pat. No. 5,308,742 (“Ta”), incorporated herein by reference, claims a method of manufacturing an integrated circuit which includes a step of plasma etching a polyimide ARC using trifluoromethane (CHF 3 ) and elemental oxygen (O 2 ) with argon as a carrier gas. This patent asserts that CHF 3  promotes polymer formation while the argon acts as a sputter component which removes the polymer formed by CHF 3 . The sputter component travels primarily in a vertical direction so that the polymeric material on the sidewalls of the interconnecting network etched through the photoresist is not removed. This protects the sidewalls of the photoresist from lateral attack by oxygen. Purportedly, this method provides critical dimension control because it is selective for the ARC. 
     In Ta, oxygen appears to be the actual etching agent. Oxygen is an aggressive oxidizing agent and tends to be non-specific and detrimental to some solid state components. A milder oxidizing agent would offer greater selectivity between the ARC and the photoresist, and hence, an improvement over the process of Ta. Selectivity is important because the CHF 3  would not provide protection in the vertical direction if Ar sputtering removes CHF 3  deposited on top of the photoresist. Excessive erosion impairs the photoresist&#39;s masking properties during the etching of the ARC and/or underlying conductive coating. 
     We have found an improved process for selectively and effectively removing an ARC without the use of oxygen, thus, reducing degradation of the photoresist. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a process for etching an organic ARC on a metallic substrate comprising exposing the ARC to a system of etching agents in an ionized state in a reaction chamber of a plasma generating device, the system of etching agents including one or more fluorine-containing compounds, an inert carrier and chlorine. This process is particularly useful for preserving the critical dimensions of a photoresist while removing exposed areas of an organic ARC during the manufacturing of an integrated circuit. 
     A second aspect is a formulation of one or more fluorine-containing compounds, an inert carrier gas and chlorine which is employed in the process of the first aspect. In one embodiment of the formulation, the fluorine-containing compound is selected from the group consisting of CF 4 , CHF 3 , C 2 F 6 , CH 2 F 2 , SF 6 , C n F n+4 . In a particular embodiment the fluorine-containing compound is CHF 3  and the inert carrier is argon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a schematic, perspective view of a portion of a semiconductor composite chip after the photoresist coating has been etched but before the ARC has been removed. 
     FIG. 1 b  is a schematic cross sectional view of the portion of the chip shown in FIG. 1 a.    
     FIG. 2 is a view similar to FIG. 1 a . after the ARC has been removed by a process using O 02 . 
     FIG. 3 is a view similar to FIG. 1 a . after the ARC has been removed by the process of the present invention. 
     FIG. 4 is a schematic view of a high density plasma ECR reactor which can be used to carry out the process according to the invention. 
     FIG. 5 is a schematic of a high density plasma TCP™ reactor which can be used to carry out the process according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 a , a perspective view of a greatly magnified section of a semiconductor composite chip  10  during manufacturing after the photoresist  12  has been exposed and developed, shows the interconnecting network  14  forming the circuit pattern  15 . That is, the area of the conductive coating  16  corresponding to those portions of the photoresist not exposed, will become the network of conductive pathways interconnecting the components and systems. The conductive coating or layer covers a semiconductor base  17 . FIG. 1 b , a cross sectional view corresponding to FIG. 1 a , more clearly shows that after the photoresist is developed, part of the ARC  18  covers the bottom of the interconnecting network  14 . This part of the ARC must be removed before the underlying conductive coating  16  can be removed by etching. 
     Note in FIG. 1 a  and FIG. 1 b  that the interconnecting network  14  formed by exposure and development of the photoresist  12  are clean and precise. However, if the system of agents used for removing the ARC at the bottom of the interconnecting network  14  attacks the non-etched parts of the photoresist  12 , the precision of the interconnecting network  14  may be seriously degraded. 
     In previous generations of integrated circuits, the conductive pathways formed by etching the conductive coating were relatively well separated so that some degradation of photoresist could generally be tolerated. However, in the present generation of integrated circuits, the conductive pathways are often so close together that even a small amount of degradation can be critical. Further, in order to achieve high resolution during exposure and development of deep UV photoresists, the thickness of the photoresist has been reduced from around 12000 Å to around 7000 to 8000 Å. This leaves less photoresist to protect the underlying layers during etching of the ARC. 
     FIG. 2 is a schematic illustration of the results of removing the ARC with a system of agents containing O 2  ionized in a plasma generating chamber. For example, a system which employs a mixture of O 2  and N 2 . While this system of agents containing O 2  effectively removes the ARC they also attack the photoresist  12  causing general thinning and degradation indicated as points  20  in FIG.  2 . As illustrated in FIG. 2, the degradation reduces the precision of the interconnecting network  14  and can cause undesirable exposure of the underlying layers at other locations. Because the interconnecting network  14  controls what is etched away from the conductive coating, thinning and degradation can cause voids in the conducting pathways or faulty connections leading to errors in the function of the integrated circuit. 
     FIG. 3 is a schematic illustration of the results of removing the ARC with a system of agents of the present invention. Note that the photoresist  12  has been only minimally affected and retains its precision pattern. Therefore, the precision pattern will be transferred to the formation of conductive pathways in the next step when the exposed conductive coating  16  is etched away. 
     For the present process, the carrier gas is an inert, noble gas, preferably argon (Ar), although, other noble gases such as helium, neon, krypton, xenon or mixture thereof may be used. The source of chlorine preferably is chemically pure elemental chlorine (Cl 2 ). Alternatively, it is possible to use another chlorine containing gas such as HCl or BCl 3 . Similarly, while CHF 3  is the preferred source of fluorine, other fluorocarbon gases or combination of such gases may be used. 
     FIG. 4 shows an ECR reactor  100  which can process a substrate with a high density plasma. The reactor includes a reaction chamber  102  wherein a substrate is subjected to treatment with a plasma gas. In order to generate the high density plasma, the reactor includes a plasma generating chamber  103  wherein a high density plasma is generated by the combination of microwave energy transmitted through microwave guide  104  and magnetic energy generated by electromagnetic coils  105 . The high density plasma can be generated from a suitable gas or gas mixture and an ion beam is extracted from the plasma chamber through orifice  103   a . If desired, the orifice  103   a  can have the same diameter as the chamber  103 . A substrate  106  is supported on a substrate support  107  such as an electrostatic chuck having a substrate temperature controlling mechanism associated therewith. 
     The high density plasma generated in chamber  103  can be confined within horn  108  by electromagnetic coils  118  and directed to the substrate  106  by applying an RF bias to the substrate by means of an RF source  109  and associated circuitry  110  for impedance matching, etc. The reaction chamber  102  is evacuated by a suitable vacuum arrangement represented generally by the evacuation port  111 . In order to introduce the etch reactants into the high density plasma, the horn  118  can include one or more gas injection arrangements such as gas distributing rings on the inner periphery thereof whereby reactants such as F and Cl can be introduced into the high density plasma. The reactant or reactants can be supplied through one or more passages represented generally at  112 . In order to produce a plasma in plasma generating chamber  103 , argon can be introduced into the plasma generating chamber  103  by one or more passages represented generally at  113 . 
     Microwave energy represented by arrow  114  travels through dielectric window  115  and enters the plasma generating chamber  103 , the walls of which are water cooled by water supply conduit  117 . Electromagnetic coils  118  below substrate holder  107  can be used for shaping the magnetic field in the vicinity of the substrate  106 . A DC power source  119  provides power to the substrate holder  107  for electrostatically clamping substrate  106 . 
     FIG. 5 shows a TCP™ reactor  120  which can process substrates with high density plasma. The reactor includes a process chamber  121  in which plasma  122  is generated adjacent substrate  123 . The substrate is supported on water cooled substrate support  124  and temperature control of the substrate is achieved by supplying helium gas through conduit  125  to a space between the substrate and the substrate support. The substrate support can comprise an aluminum electrode or a ceramic material having a buried electrode therein, the electrode being powered by an RF source  126  and associated circuitry  127  for providing RF matching, etc. The temperature of the substrate during processing thereof is monitored by temperature monitoring equipment  128  attached to temperature probe  129 . A similar temperature monitoring arrangement can also be used in the ECR reactor shown in FIG.  4 . 
     In order to provide a vacuum in chamber  121 , a turbo pump is connected to outlet port  130  and a pressure control valve can be used to maintain the desired vacuum pressure. Process gas containing F, Cl and an optional inert gas such as Ar can be supplied into the chamber by conduits  131 ,  132  which feed the reactant gases to a gas distribution ring extending around the underside of dielectric window  133 . Alternatively, the process gases can be supplied through a dielectric showerhead window or other suitable gas distribution system. An inductive energy source such as an antenna in the form of a spiral TCP™ coil  134  located outside the chamber in the vicinity of the window is supplied RF power by RF source  135  and associated circuitry  136  for impedance matching, etc. When a substrate is processed in the chamber, the RF source  135  supplies the TCP™ coil  134  with a RF current at 13.56 MHz and the RF source  126  supplies the lower electrode with RF current at 400 kHz. 
     The present process is carried out in an ECR or TCP™ reactor of the type described above or in any suitable reaction chamber of a plasma generator. The ARC can be on a semiconductor substrate such as a semiconductor wafer, flat panel display, etc. The process may be carried out within the following window: 
     Pressure—about 0.1 to a bout 500 millitorr 
     Temperature—about 0° to about 100° C. 
     Cl 2  flow—about 2.5 to about 200 sccm 
     Inert gas flow—0 to about 200 sccm 
     Fluorine containing compound gas flow—about 5 to about 200 sccm 
     preferably it is carried out within the following window: 
     Pressure—about 1 to about 100 millitorr 
     Temperature—about 30° to about 80° C. 
     Cl 2  flow—about 5 to about 60 sccm 
     Ar flow—about 5 to about 80 sccm 
     CHF 3  flow—about 5 to about 80 sccm 
     Process time varies with the composition and proportions of the reactants, and more importantly, the thickness of the organic ARC. The end point of the process, i.e., when all of the exposed ARC has been removed, can be determined by standard methods of this art using standard micro chip manufacturing equipment. For example, one may monitor an optical signal or the concentration of fluorine. 
     Although the present invention has been illustrated by reference to particular embodiments, those skilled in the art of integrated circuit production will appreciate variations of these embodiments may still be within the scope of the invention.