Process for Etching Aluminum in a Plasma

Europaisches Patentamt J European Patent Office © Publication number: 0 295 581 Office europeen des brevets A1 EUROPEAN PATENT APPLICATION © Application number: 88109250.6 © mt. ci.4: C23F 4/00 , H01L 21/31 © Date of filing: 10.06.88 © Priority: 19.06.87 US 63872 © Applicant: TEGAL CORPORATION 11 Digital Drive © Date of publication of application: Novato California 94947(US) 21.12.88 Bulletin 88/51 @ Inventor: Krough, Ole D. © Designated Contracting States: 110 Point Lobos DE FR GB San Francisco California 941 21 (US) ® Representative: Ibbotson, Harold Motorola Ltd Patent and Licensing Operations - Europe Jays Close Viables Industrial Estate Basingstoke Hampshire RG22 4PD(GB) © Process for etching aluminum in a plasma. © The addition of bromine, or bromides, to the gas mixture in a plasma has been found to significantly improve selectivity when etching aluminum. Pro- cesses are disclosed based upon bromine etch gases and chlorine based etchants wherein bromine is a small but significant additive. 00 in in CD cm LU Xerox Copy Centre EP 0 295 581 A1 PROCESS FOR ETCHING ALUMINUM IN A PLASMA object of the present invention to provide an im- Background of the Invention proved plasma etch of aluminum. A further object of the present invention is to provide improved selectivity in the etching of alu- This invention relates to etching aluminum and, s minum using a modification of existing processes. in particular, to etching aluminum in a plasma reac- An object in accordance with another aspect of tor. the present invention is to provide a process based In the prior art, a variety of gases have been on bromine for etching aluminum. proposed or even used for etching aluminum. Since the product, aluminum chloride, is a gas at w the working pressure of the reactor, these reactant Summary of the Invention gases typically include chlorine, either as a gas or as part of the molecule of a gaseous compound, achieved in the e.g. Cl2, HCI, SiCI*, CCU, BCI3, and CHCI3. The foregoing objects are The exact mechanism for the aluminum etch is 75 present invention wherein the element bromine in not fully understood, although the understanding of the form of HBr (hydrogen bromide), BBr3 (boron it is improving. At present, it is believed that free tribromide), or Br2 (bromine), is added to the gas chlorine, atoms or molecules, actually do the etch- mixture. For chlorine based chemistries, this has ing. An etch mixture usually comprises at least two been found to substantially increase (e.g. double) While gases, at least one of which is a particularly good 20 the selectivity of aluminum to photoresist. a source of chlorine ions. The other gas(es) either bromine additive to what is essentially a chlorine promote the formation of ions or enhance the etch chemistry enables one to retain familiar gas han- by scavenging impurities. In some cases, at low dling procedures, it has further been found that an operating pressures, e.g. on the order of ten mil- all bromine chemistry retains the etch rate of chlo- litorr, the additive gases are used to lightly sputter 25 rine chemistry while greatly reducing the etch rate etch the target. The scavenging is primarily of of the photoresist. That is, an all bromine chemistry and oxygen, often present as moisture, a gaseous mol- has been found which provides remarkably ecule, or oxide. An often used scavenger is BCI3, unexpectedly improved selectivity. although it was often considered primarily as an etchant in the past. 30 Due to the unpredictable nature of plasma pro- Brief Description of the Drawings cessing, it is often difficult to categorize processes. That is, etching a given operand does not imply a the certain gas or gas mixture. Similarly, a given gas A more complete understanding of present mixture does not necessarily imply or exclude any 35 invention can be obtained by considering the fol- particular operand. In addition, one often sees lowing detailed description in conjunction with the seemingly inconsistent results using certain gases accompanying drawings, in which: on the same operand. For example, U.S. Patent FIG 1 illustrates a triode plasma reactor. 4,314,874 describes bromine trichloride as an etch- FIG. 2 summarizes gas flows for a number ant for aluminum (col. 3, lines 17-25) while U.S. 40 of experiments. Patent 4,351 ,696 describes bromine compounds as FIG. 3 lists results for a group of experi- passivating aluminum (col. 3, lines 1-20). ments selected from FIG. 2. Whether any given component of a gas is an FIG. 4 lists results for a group of experi- etchant or not, the etch rate of the operand is not ments selected from FIG. 2. the only consideration. The mask used to pattern 45 FIG. 5 lists results for a group of experi- the operand should not etch at the same rate, i.e. a ments selected from FIG. 2. selective etch is desired. In practice, a ratio of 3:1 FIG. 6 lists results for a group of experi- or greater is desired to reduce the amount of ments selected from FIG. 2. volatile mask products in the plasma reactor. As an FIG. 7 illustrates an alternative embodiment absolute, the etch rate of the operand is important so of the present invention. since it determines the production rate of the reac- FIG. 8 illustrates an alternative embodiment tor. Thus, one tries never to reduce the etch rate of of the present invention. the operand, only the relative etch rate of the mask. In view of the foregoing, it is therefore an EP 0 295 581 A1 Detailed Description of the Invention ture. In addition, the bromine gas was substituted for chlorine. As can be seen from FIG. 5 the selectivity is again significantly improved over the FIG. 1 illustrates a triode plasma reactor suit- pure chlorine chemistry. In this series of experi- able for use in carrying out the present invention. 5 ments, the etch rate of the aluminum also in- As known in the art, this type of reactor comprises creased. a central chamber 10 in which the plassna is gen- FIG. 6 illustrates a series of experiments in erated, formed by lower electrode 11, sidewall 12 which the silicon tetrachloride is gradually replaced and upper electrode 13. The electrodes and by the boron tribromide. As before, this addition of sidewalls are electrically separated from each other w the bromine significantly increased the selectivity by insulators 14 and 15. Upper electrode 13 forms of the etch. a portion of the wall of chamber 20 which collects In accordance with another aspect of the gases from reaction volume 10 and supplies them present invention, as indicated by experiment num- to an exhaust port 21. The gas or gas mixture is ber 11 in FIG. 6, the use of a bromine based supplied to volume 10 by way a peripheral supply 75 chemistry, i.e. no chlorine, provided the greatest port 18. In a preferred embodiment of the present selectivity as well as the highest etch rate. invention, upper electrode 13 is grounded, sidewall In all of the foregoing experiments, the reactor 1 2 is connected to RF power supply 31 , and lower was supplied with a 13.56 MHz signal at 600 watts, electrode 1 1 is connected to RF power supply 32. a 100 KHz signal at 50 watts. Lower electrode 11 Power supply 31 preferably produces a signal hav- 20 was maintained at a temperature of 40* C and the ing a frequency of 13.56 MHz. Source 32 prefer- experiments were performed on 100 mm. wafers ably produces a signal having a frequency of 100 having a layer of aluminum deposited thereon con- KHz. taining one percent silicon, and patterned so that As thus configured, one has three electrodes, fifty percent of the aluminum was etched. As un- hence the name triode, for causing a plasma dis- 25 derstood by those of skill in the art, varying pres- charge within volume 10. It is understood by those sure, power and flow rates involves yet a greater of skill in the art that a "diode" reactor could be matrix of experiments. It is believed that the chem- used instead. In this embodiment, the RF power istries are more significant than the pressure, pow- supplies are preferably connected together to lower er or flow rates for the present invention; i.e. the electrode 1 1 . 30 latter three parameters are readily optimized em- In accordance with the present invention, it has pirically for a given apparatus. In general, the high been found that one can greatly improve the chlo- frequency power can be from 300 to 800 watts, rine based chemistries of the prior art by adding a depending upon wafer size, loading, and the like. small amount of bromine. FIG. 2 illustrates a matrix The low frequency power can be between 30 and of experiments which was carried out in support of 35 80 watts. The total pressure can be from 100-500 this discovery. A mixture of silicon tetrachloride mtorr. and chlorine was taken as the base mixture for In addition to the foregoing, a series of experi- etching aluminum. To this mixture, boron ments were conducted comparing what were pre- tribromide, hydrogen bromide, and bromine gas viously considered analogous gas mixtures of chlo- were added, alone or in combination. 40 rine or bromine. FIG. 7 illustrates a comparison of a FIG. 3 illustrates a subset of the matrix of FIG. chlorine mixture with a mixture comprising gases in 2 wherein the effect of adding hydrogen bromide is which bromine has been substituted for the chlo- noted with respect to a base mixture referred to rine.

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