METHOD OF PLASMA ETCHING A SUBSTRATE

Abstract: A plasma etching process is disclosed wherein the substrate to be etched is first exposed to an etchant gas containing a volatile organohalide. When the etch rate is stabilized, the organohalide in the etchant gas is replaced by oxygen whereby the etch rate of the substrate is immediately increased to a substantially higher value.

Description: This invention relates to a process for plasma etching using an etching gas containing an organohalide. In recent years, in the filed of surface processing of substrates such as organic polymers and semiconductors, the conventional wet processing method has been extensively replaced by a dry etching processing method. In this so-called dry etching technique, the surface to be etched is exposed to a plasma established by a high frequency glow discharge. For example, U.S. Patent No. 3,795,557 (Reissue 30,505) discloses exposing a semi-conductor material such as Si, GaAs, GASsP, InSb having a relatively thin (200 to 10,000 Angströms) layer of a dielectric material e.g. SiO, SiO₂, Si₃N₄, either deposited or thermally grown onto the semiconductor, to a gaseous plasma formed from a binary mixture consisting essentially of oxygen and an organohalide compound such as tetrafluoromethane (CF₄), the mixture containing 1 to 75 percent oxygen by volume. Other patents which contain similar teachings of plasma etching using organohalide/oxygen mixtures include U.S. Patent Nos. 4,174,251, 4,180,432, 4,374,699, and 4,487,652. U.S. Patent Nos. 4,357,203, 4,460,436, and 4,482,427 are exemplary of prior art use of O₂ being the sole constituent of a plasma etchant. U.S. 4,487,652 further teaches the use of an oxygen plasma to clear underlying photoresist and polyimide layers which remain on a substrate after plasma etching of the overlying oxide and a part of the photoresist and polyimide layers with an organohalide/­ oxygen mixture is completed. One drawback to the use of organohalide/oxygen mixtures is often the gaseous mixture halogenates the surface of the substrate undergoing plasma etching and forms a halogen contaminated film surface on the substrate. The formation of the halogenated film, or the existence of a halogen contaminated substrate surface, slows the etch rate of the plasma and thereby unduly prolongs the time required for etching. Further, the presence of the halogen contaminated surface will interfere with subsequent processing steps which are intended for the etched substrate, e.g. plating, lamination and adhesion to other later applied layers of organic or inorganic materials. Because of the demand for ever-higher speeds of operation and the need for reliably prepared semiconductor components, there has been a considerable effort in the electronics industry to resolve the halogenated surface film problem. This is also the object of the present invention. This object is achieved by a process as defined in claim 1. The inventive process is based on the discovery that plasma etching process employing the use of different gaseous plasma compositions for dry etching apparently substantially eliminates the etch rate inhibiting halogen surface film previously en­ countered with organohalide containing plasmas. The plasma etch­ ing process of the present invention facilitates, in particular, the process of etching organic polymeric substrates at an extremely rapid rate that is much higher than any encountered in the prior art by using the etching properties of the halogen containing surface layers. As will hereinafter be demonstrated, by substituting a plasma consisting essentially of oxygen for the organohalide containing gas initially used to perform the dry etching process, the etch rate increases almost instantaneously upon such substitution by several orders of magnitude. Other advantageous embodiments of the inventive process are disclosed in the subclaims. The inventive process will become more apparent from the following detailed description. In effecting dry etching in accordance with the practice of the present invention, the initial etchant gas is comprised of a volatile organohalide compound such as CF₄, CCl₄, CF₂Cl₂ and CFCl₃ alone or as a binary mixture with oxygen. When binary mixtures are used as the etchant gas, the mixture generally contains on a volume basis, from about 20 to about 100 percent of the organohalide and from about 0 to about 80 percent by volume oxygen. Preferably, the gaseous binary mixture contains about 30 to about 60 volume percent of the organohalide and about 40 to about 70 volume percent oxygen. Carbon tetrafluoride (CF₄) is the organohalide gas preferred for use in the initial etching step of the plasma etching process of the present invention. The substrate receiving the sequential plasma etch treatment of the present invention is continuously etched. The substrate may be of any material which is conventionally etched using organohalide containing plasmas, as for example, organic polymers such as epoxy resin lacquers, polyimides, photoresists, semi­ conductor material such as Si, GaAs, InP, Si₃N₄, and SiO₂. The process of the present invention may also be used to treat per­ fluorinated surfaces, as in textile or synthetic fiber substrates wherein it is necessary to clean the surface to return it to its original properties. The plasma etching process of the present invention is conveniently conducted in a parallel plate reactor of the type conventionally used for plasma etching of semi-conductor materials. In such reactor an upper electrode can be powered to between 50 and 500 watts and a second, lower electrode tuned to about 0 to 100 volts with res­ pect to the ground. The total gas pressure utilized is typically 10 to 500 mTorr and the total gas flow is typically 30 to 300 sccm. In practicing the plasma etching process of the present invention, the specimen of the substrate material to be etched is mounted on the lower electrode of the reactor. The gas containing the organohalide compound with or without oxygen is introduced into the reactor and a high frequency (RF) power is applied between the first and second electrode to produce a plasma between the first electrode and second electrode. The substrate material is exposed to this organohalide containing plasma for about 0.5 to about 5 minutes or until the etch rate has stabilized at a low rate, e.g. 5 to 1000 nm/minute as indicated by etch rate measurements, such as laser interferometry. Thereafter the gas atmosphere in the reactor is changed to 100 % by volume oxygen by stopping the flow of the organohalide to the reactor and replacing the organohalide flow with oxygen. Upon the replacement of the organohalide with oxygen in the reactor, the etch rate will instantaneously increase to 3000 nm/minute or higher. This enhanced etch rate will generally last for a very short time period, i.e. less than about 2 minutes, e.g. about 10 seconds to 2 minutes and then stabilize at a consider­ ably lower rate that reflects the substrate etch rate in pure oxygen e.g. 500 nm/minute. This procedure is applicable when the etching is accomplished in the plasma reactor i.e. in the plasma mode, in the reaction ion mode or in a floating substrate mode. The following Example further illustrates the invention, it being understood that the invention is in no way intended to be limited to the details described therein. Example A series of 50-70 um thick epoxy resin coated silicon coupons were utilized as substrates. Each coupon to be etched was placed on the lower electrode of a parallel electrode plasma etching reactor. The lower electrode was RF powered and the upper electrode was either grounded or maintained at an arbitrary floating potential. The input RF power was 400 watts and the pressure during etching was 100 mTorr. The total gas flow averaged about 60 sccm. Etching was initially carried out using varying percent volume ratios of CF₄ and O₂. Etch rates were measured simultaneously by laser interferometry. After the etch rate of the epoxy resin coated coupon settled at a low steady state value which normally occurred in about 0.5 to about 5.0 minutes after admission of the CF₄/O₂ etchant gas mixture to the plasma reactor and turning on the RF power, the flow of CF₄ gas to the reactor was interrupted and re­ placed with O₂ gas. Upon replacement of the CF₄/O₂ etching gas mixture with 100 volume percent O₂, there was an immediate in­ crease in the etch rate. Maximum etch rates, with 100 volume percent O₂ atmosphere were achieved within 15-25 seconds. There­ after the etch rates declined rapidly and reached a stabilized etch rate of 300-500 nm/minute, at which time the etching process was terminated. The etch rates of the epoxy resin coated coupons when first exposed to a CF₄/O₂ etching gas mixture and the maximum rates measured when the gas mixture was replaced by 100 volume per­ cent O₂ are summarized in the Table below: By reference to the data recorded in the Table, it is immediately apparent that the replacement of the CF₄/O₂ gas mixture with O₂ unexpectedly results in a dramatic increase in etch rates which is 15-400 times greater than the etch rate previously achieved with the CF₄/O₂ gas mixture. While specific components of the present system are defined above, many other variables may be introduced which may in any way affect, enhance, or otherwise improve the system of the present invention. These are intended to be included herein. Although variations are shown in the present application, many modifications and ramifications will occur to those skilled in the art upon a reading of the present disclosure. These, too, are intended to be included herein.

Claims: 1. A process for plasma etching a substrate using an etching gas containing an organohalide, which process comprises the se­ quential steps of first exposing the substrate to a gaseous plasma containing an organohalide, etching the substrate in the gaseous plasma until the rate at which the substrate is etched reaches a stabilized value and then replacing the organohalide containing etchant gas with oxygen gas to increase the rate at which the same substrate is etched. 2. Process according to claim 1 wherein the substrate is an organic polymer coating. 3. Process according to claim 1 wherein the substrate is a semiconductor material having a dielectric material deposited thereon. 4. Process according to any one of claims 1 to 3 wherein the organohalide is CF₄. 5. Process according to any one of claims 1 to 4 wherein the etchant gas used in the first exposure of the substrate is a mixture of CF₄ and O₂. 6. Process according to claim 5 wherein the etchant gas contains on a volume basis about 20 to about 100 volume percent of the organohalide and about 0 to about 80 volume percent oxygen. 7. Process according to claim 5 or 6 wherein the substrate is first etched with etchant gas containing the organohalide for about 0.5 to about 5.0 minutes and then etched in the oxygen plasma for less than about 3.0 minutes.

Inventors: BABU, SURYADEVARA VIJAYAKUMAR, HOFFARTH, JOSEPH GERARD, KNOLL, ALLAN ROBERT, MLYNKO, WALTER EUGENE, REMBETSKI, JOHN FRANCIS
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: C23F1
IPC codes: H01L21, C09K13