METHOD OF ETCHING A SHAPED CAVITY

Abstract: The present disclosure pertains to our discovery of a method of etching a shaped cavity in a substrate, where the shaped cavity has a width that is at least as great as its depth. We have discovered that by varying the process chamber pressure during etching of the shaped cavity, we can control lateral etching of the shaped cavity, while allowing the removal of etch process byproducts from the shaped cavity during continued etching. The method of the invention can be used to etch shaped cavities having round or horizontal elliptical shapes. The method of the invention is particularly useful in the etching of buried cavities, where removal of etch byproducts from the cavity can be difficult.

Description: METHOD OF ETCHING A SHAPED CAVTTY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to a method of etching a shaped cavity in a substrate. In particular, the present invention pertains to a method for etching a shaped cavity, where the width of the shaped cavity is equal to or greater than the depth of the shaped cavity in the substrate. 2. Brief Description of the Background Art Commonly owned, copending U.S. application Serial No. 09/372,477, filed August 1 1, 1999, of Podlesnik et al., provides a general method for forming a multi-pan cavity in a substrate which is useful in many different micromachining applications. As disclosed by Podlesnik et al., a substrate is etched to a first predetermined depth to form a shaped opening. A conformal protective layer is then formed on at least the sidewall of the shaped opening. The protective layer comprises a material which has a different etch selectivity than the substrate material. If necessary, the protective layer is then aniso tropically etched to remove portions of the protective layer which overlie the bottom of the shaped opening. Typically, at least a portion of the substrate at the bottom of the shaped opening is exposed prior to proceeding with subsequent etching. The substrate is then further etched to form a shaped cavity using an etchant gas which selectively etches the substrate relative to the protective layer. For some applications it is desirous that the shaped cavity be formed to have a different shape than the shaped opening. For example, the opening to the cavity is tubular- shaped, while the shaped cavity has a width that is equal to or greater than its depth (i.e., a round or horizontal elliptical shaped cavity). A shaped cavity of such dimensions is particularly difficult to form using dry etching techniques. Formation of round shaped cavities has previously been accomplished using wet etch techniques. However, in recent years, the semiconductor manufacturing industry has been trending toward the use of dry etch techniques because of process integration and environmental considerations. Due to the difficulty of carrying the gaseous reactants into and reaction byproducts out of a cavity, the use of dry etch techniques has been limited to the widening of already formed shaped cavities by a relatively minor amount. Therefore, it would be desirable to provide a dry etch method that would result in the formation of a shaped cavity having a width that is equal to or greater than its depth. Lateral (as opposed to vertical) etch is dependent upon a number of factors during the etching process, including the incoming angle of the etchant species, the mean free path of the etchant species, and the ability of the etchant species to reach the surface to be etched. Furthermore, byproducts of the etch process must constantly be removed, and if these byproducts become trapped within a cavity being etched, etching may slow to an unacceptable rate or stop entirely, particularly in the lateral direction. When a conventional etch process is used, the desired lateral etch (if it can be achieved before etching stops) is typically accompanied by an undesired deeper vertical etch. Therefore, particularly during the etching of buried cavities, it is important to provide a means by which etch byproducts can be evacuated from the etch cavity during the etching process, in order to allow the desired amount of lateral etching without deepening of the cavity. SUMMARY OF THE INVENTION The present invention provides a method of dry etching a shaped cavity in a substrate. The method is useful for aspect ratios as high as at least 3.5 : 1, and is particularly useful where the width of the shaped cavity is equal to or greater than the depth of the shaped cavity in the substrate, i.e. where the aspect ratio is less than 1. We have discovered that it is necessary to control the process chamber pressure in a particular manner during the performance of the etching process to permit etch byproduct removal from the shaped cavity at a rate which reduces or avoids the buildup of etch byproducts on interior surfaces of the shaped cavity. This permits continued etching of the shaped cavity. In general, the method of the invention comprises etching of a shaped cavity using at least two different process chamber pressures, including an initial process chamber pressure, followed by continued etching of the shaped cavity using at least one decreased process chamber pressure. The decreased process chamber pressure should be at least 25% lower than the initial process chamber pressure, and in several embodiments is about 30 % to 50 % lower than the initial process chamber pressure. An optional finishing or rounding of the etched cavity may be carried out after shaping of the cavity, in which the process pressure is increased ; the amount of increase is typically up to about 90 % of the initial process chamber pressure. During the etching of a shaped cavity, the process chamber pressure may be lowered and then subsequently raised and relowered, as well, to provide for the removal of etch byproducts during a given etching step. The method of the invention is particularly useful in the etching of buried cavities. The shaped cavity is typically formed to underlie a previously formed shaped opening. The method of the invention can be performed as a continuous process, where the process chamber pressure is gradually lowered or raised and lowered, or as a multi-step process. When the substrate is (single crystal)silicon, etching is typically performed using a piasma containing reactive fluorine species. The etchant plasma is commonly generated from a source gas comprising SF 6 and argon, provided at a flow rate which is process equipment dependent, and at an SF 6 : argon ratio within the range of about 10 : 1 to about 2.5 : 1. When a round shaped cavity is desired, an SF 6 : argon ratio of about 4 : 1 provides excellent results. Argon is used as an inert carrier for the SF 6 , and ionized argon in conjunction with a substrate bias may be used to drive reactive fluorine species generated from SF 6 down through the shaped opening and into the shaped cavity. Alternative primary etchant gases include gases such as CF 4 , Cl 2 , and HBr. Any of the primary etchant gases may be used alone or in combination with a compatible other primary etchant gas. For example, CF 4 may be used to replace SF 6 or may be added to SF b to obtain a desired effect. A primary etchant gas may be used in combination with an additive gas. such as. for example, Ar, 0 2 , HBr, Cl 2 , or N : . to provide better control over the surface finish or etch profile of the shaped cavity. Depending on the particular effect desired, the additive gas may be present during the entire duration of the method of the invention, or it may be present only during a certain step or steps. The plasma source gas may further include a substantially nonreactive, diluent gas, such as Ar, He, or Xe. Examples of source gas combinations which are preferred for etching a silicon substrate, and not by way of limitation, include SF 6 /Ar/O 2 ; SF 6 /Ar/HBr; SF 6 /Ar/Cl 2 ; and SF 6 /C1 2 . CF 4 may replace SF 6 or may be added to SF 6 in any of these source gas combinations. When the substrate is polysilicon, the plasma source gas typically includes SF 6 and/or Cl 2 . For a silicon dioxide substrate, the plasma source gas typically includes CF 4 or NF 3 , and etching is typically performed at a temperature within the range of about 50°C to about 100°C. When the substrate is silicon nitride, the plasma source gas typically includes SF 6 . When the substrate is a metal (such as aluminum or an aluminum alloy), the plasma source gas typically includes Cl 2 . When the substrate is polyimide, the plasma source gas typically includes CF 4 and O 2 . All of these examples are intended to be non-limiting. When a fluorine-containing etchant plasma is used at a temperature below 50°C, the protective/ masking layer preferably comprises silicon oxide. A silicon nitride protective/masking layer may be used if the plasma source gas does not contain fluorine. A metal or alloy protective/masking layer may be used if the plasma source gas does not contain chlorine. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A - ID illustrate one embodiment of the method of the invention. Figure 1 A shows a beginning structure 100 for practicing the general method of the invention. This structure is a shaped opening 104 etched in a substrate 102 to a first predetermined depth A. Figure IB shows the structure of Figure 1A following formation of a conformal protective layer 106 on the sidewall 108 and bottom 110 of shaped opening 104. Figure 1C shows the structure of Figure IB after the protective layer has been anisotropically etched to remove a portion of the protective layer which overlies the bottom 108 of shaped opening 104. Figure ID shows the structure of Figure 1C after the underlying substrate 102 has been etched using the multi-step etch method of the invention to form a shaped cavity 1 10 which underlies shaped opening 104. The shaped cavity 110 is etched to have a width B and a depth C, where the width B is equal to or greater than the depth C. In particular, here, width B is equal to depth C, so that the shaped cavity 110 is formed in a round, as shown. Figure 2 shows a graph 200 of eccentricity (i.e., the width of a shaped cavity divided by its depth) as a function of the width of the overlying shaped opening, where curve 202 represents shaped cavities etched using a prior art etch process, and curve 204 represents shaped cavities etched using the method of the invention. Figures 3A and 3B, respectively, show examples of round and horizontal elliptical shaped cavity profiles that can be obtained using the multi-step etch method of the invention. Figure 3C shows a double spherical profile which is obtained by continued isotropic etching of closely adjacent shaped cavities 302 and 304, in order to remove the substrate material beuveen the shaped cavities. By removing the substrate material between the shaped cavities, a silicon "island" 306 is formed between shaped openings 308 and 310. Such silicon islands are useful in, for example, DRAM capacitors, optical interconnects in optoelectronic integrated circuits, micromachining, and submicron silicon-on-insulator (SOI) applications. Figure 4 is a schematic representation of an apparatus for practicing the present invention which includes elements for computerized control of the apparatus used to carry out the method of the invention. DETAILED DESCRIPTION OF THE INVENTION I. DEFINITIONS As a preface to the detailed description, it should be noted that, as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents, unless the context clearly dictates otherwise. All percentages (%) listed for gas constituents are % by volume, unless otherwise indicated. Specific terminology of particular importance to the description of the present invention is discussed below. The term "cavity" typically refers to, but is not limited to, any three-dimensional depression or buried space in a substrate, including, for example and not by way of limitation, trenches, bowls, contacts, vias, tubes, holes, squares and other geometric shapes. The term "eccentricity" refers to the ratio of the width of a shaped cavity to its depth. For example, a perfectly round shaped cavity (i.e., having a width exactly equal to its depth) would have an eccentricity of 1. The term "polysilicon" refers to polycrystalline silicon, which may be doped with, for example and not by way of limitation, arsenic, phosphorus, antimony, or boron. The term "substrate" refers to any material which can be etched using etching techniques known in the art. II. APPARATUS FOR PRACTICING THE INVENTION The method of the invention is generally performed in a plasma etching apparatus that is capable of enabling both anisotropic and isotropic etching, and is capable of switching back and forth between the two types of etching. An example of such an apparatus is the CENTURA® DPS™ polysilicon etch system, available from Applied Materials, Santa Clara. California. U.S. Patent No. 5,753,044 issued to Hanawa et al. on May 19, 1998 contains a general description of an RF plasma reactor of the kind which may be used to practice the invention described herein. III. A METHOD OF FORMING A SHAPED CAVITY IN A SUBSTRATE The present invention provides a method of dry etching a shaped cavity in a substrate. Although the method is useful for aspect ratios as high as at least 3.5 : 1, it is particularly useful where the width of the shaped cavity is equal to or greater than the depth of the shaped cavity in the substrate, i.e. where the aspect ratio is less than 1. For example, the shaped cavity can be etched to have a round shape, that is, the width of the shaped cavity is substantially equal to the depth of the shaped cavity. Alternatively, the shaped cavity can be etched to have a horizontal elliptical shape, that is, the width of the shaped cavity is substantially greater than the depth of the shaped cavity. We have discovered that it is necessary to control the process chamber pressure in a particular manner during the performance of the etching process to permit etch byproduct removal from the shaped cavity at a rate which reduces or avoids the buildup of etch byproducts on interior surfaces of the shaped cavity. This permits continued etching of the shaped cavity. In general, the method of the invention comprises etching of a shaped cavity using at least two different process chamber pressures, including an initial process chamber pressure, followed by continued etching of the shaped cavity using at least one decreased process chamber pressure. The decreased process chamber pressure should be at least 25% lower than the initial process chamber pressure, and in several embodiments is about 30 % to 50 % lower than the initial process chamber pressure. The process chamber pressure may be lowered and then subsequently raised and relowered, as well. Such pressure changes permit etch byproducts formed during the etching process to be removed from the shaped cavity during continued etching. The method of the invention is particularly useful in the etching of buried cavities. The shaped cavity is typically formed to underlie a previously formed shaped opening. In a preferred embodiment method of the invention, the following steps are typically performed prior to etching the shaped cavity: First, a substrate is etched to a predetermined depth to form a shaped opening. Then, a conformal protective layer is formed over at least a portion of the sidewall of the shaped opening. The protective layer comprises a material which has a different etch selectivity than the substrate material. If necessary, the protective layer is then anisotropically etched to remove portions of the protective layer which overlie the bottom of the shaped opening. Typically, a portion of the substrate at the bottom of the shaped opening is exposed prior to proceeding with subsequent etching, which occurs laterally beneath the bottom of the shaped opening. However, etching through the substrate laterally above the bottom of the shaped opening may be used, where the protective layer covers only the upper portion of the shaped opening. The protective/masking layer effectively preserves the profile of the protected portion of the shaped opening during subsequent etching of the shaped cavity. One method for formation of the protective layer is described in detail in commonly owned, copending U.S. application Serial No. 09/372,477, filed August 11, 1999, of Podlesnik et al., which is hereby incorporated by reference. The substrate is typically a semiconductor substrate, which is frequently silicon; however, other substrates such as polysilicon, silicon dioxide, silicon nitride, metal (such as aluminum or an aluminum alloy), polyimide, gallium arsenide, cadmium indium telluride, doped silicon, doped polysilicon, doped silicon dioxide, tungsten, spin-on-glass (SOG), polymers, metal alloys (such as Al/Si/Cu or Al/Ti); other specialized substrates (which depend on the end use application) are contemplated as well. The silicon substrate may include doped regions (e.g. , a buried dopant layer). Doped substrates may etch more readily or less readily than undoped substrates, depending on the dopant and on the etchant species. If silicon is used as the substrate material, a protective layer of silicon oxide can easily be formed using techniques known in the art (such as those described by Podlesnik et al.). The method of the invention can be performed as a continuous process, where the process chamber pressure is gradually lowered or raised and lowered, or as a multi-step process. One embodiment of the method of the invention comprises a multi-step etching method. In an initial cavity etch step, gaseous etchant species are introduced through a shaped opening, having a protective layer over at least a portion of its surface, in order to etch a shaped cavity. This embodiment is described with reference to a CENTURA® DPS™ process chamber or a similar process chamber, where etching of the shaped cavity is typically performed using an initial process chamber pressure within the range of about 20 mTorr to about 50 mTorr. Depending on the desired, predetermined shape of the cavity, a substrate bias may be applied to assist in obtaining the desired shape. When increased lateral etching is desired, a lower bias power, typically ranging from about 0 W to about 15 W, is commonly used. W en increased vertical etching is desired, an increase in bias power is used, so that the bias power typically ranges from about 20 W to about 200 W. When the substrate is (single crystal)silicon, etching is typically performed using a plasma containing reactive fluorine species. The etchant plasma is commonly generated from a source gas comprising SF 6 and argon, provided at a flow rate which is process equipment dependent, and at an SF 6 : argon ratio within the range of about 10 : 1 to about 2.5 : 1. When a round shaped cavity is desired, an SF 6 : argon ratio of about 4 : 1 provides excellent results. Argon is used as an inert carrier for the SF 6 , and ionized argon in conjunction with a substrate bias may be used to drive reactive fluorine species generated from SF 6 down through the shaped opening and into the shaped cavity. When vertical etching is desired, a bias power within the range of about 20 W to about 200 W is typically used; when lateral etching is desired, a bias power within the range of about 0 W to about 15 W is typically used. The newly formed shaped cavity may be used as a reaction chamber during subsequent etching steps. Typical process conditions for the initial cavity etch step fall within the following 1 ranges: a process chamber pressure ranging from about 20 m Torr to about 50 mTorr; a 2 source power ranging from about 600 W to about 900 W; a bias power ranging from about 3 0 W to about 15 W for more lateral etching, and ranging from about 20 W to about 200 W 4 for more vertical etching; a substrate temperature ranging from about 20°C to about 50°C; 5 and. a total gas flow of at least 100 seem for a chamber volume of 35, 000 cubic centimeters. 6 When the primary etchant gas source is SF 6 , the SF 6 is commonly combined with Ar, at an 7 SF 6 : Ar ratio ranging from about 2.5 : 1 to about 10 : 1, with the flow rate of SF 6 , typically 8 ranges from about 50 - 150 seem. 9 Alternative primary etchant gases include gases such as CF 4 , Cl 2 , and HBr. Any of 10 the primary etchant gases may be used alone or in combination with a compatible other 1 1 primary etchant gas. For example. CF 4 may be used to replace SF 6 or may be added to SF 6 12 to obtain a desired effect. A primary etchant gas may be used in combination with an 13 additive gas, such as, for example, Ar, O 2 , HBr, Cl 2 , or N 2 , to provide better control over the 14 surface finish or etch profile of the shaped cavity. Depending on the particular effect 15 desired, the additive gas may be present during the entire duration of the method of the 16 invention, or it may be present only during a certain step or steps. The plasma source gas 17 may further include a substantially nonreactive, diluent gas, such as Ar, He, or Xe. I S Examples of source gas combinations which are preferred for etching a silicon 19 substrate, and not by way of limitation, include SF 6 /Ar/O 2 ; SF 6 /Ar/HBr; SF 6 /Ar/Cl 2 ; and 0 SF 6 /C1 . CF 4 may replace SF 6 or may be added to SF 6 in any of these source gas 1 combinations. 2 When the substrate is polysilicon, the plasma source gas typically includes SF 6 3 and'or Cl 2 . When the substrate is silicon dioxide, the plasma source gas typically includes 4 CF 4 or NF 3 , and etching is typically performed at a temperature within the range of about 5 50°C to about 100°C. When the substrate is silicon nitride, the plasma source gas typically 6 includes SF 6 . When the substrate is a metal (such as aluminum or an aluminum alloy), the 7 plasma source gas typically includes Cl 2 . When the substrate is polyimide, the plasma source gas typically includes CF 4 and O 2 . When a fluorine-containing etchant plasma is used at a temperature below 50 °C, the protective/masking layer preferably comprises silicon oxide. A silicon nitride protective/masking layer may be used if the plasma source gas does not contain fluorine. A metal or alloy protective/masking layer may be used if the plasma source gas does not contain chlorine. Returning to the embodiment described above pertaining to the initial cavity etch step in a silicon substrate, after the initial cavity etch step, a second etching step is performed in which the shaped cavity is enlarged in specific directions. The process chamber pressure used in the second etching step is preferably about 25 - 50%. lower than the initial process chamber pressure. When a rounded shaped cavity is desired, decreasing the process chamber pressure by about 30% provides excellent results. A process chamber pressure within the range of about 10 mTorr to about 25 mTorr is typically employed during the second etching step. To expand the cavity in a lateral (horizontal) direction relative to the protected shaped opening, a bias power within the range of about 0 W to about 15 W is typically used. The plasma source gas used in the second etching step is typically the same as that used in the first etching step. Typical process conditions for the second etch step are as follows: a process chamber pressure ranging from about 10 mTorr to about 25 mTorr: a source power ranging from about 500W to about 800 W; a bias power ranging from about 0 W to about 15 W ; a substrate temperature ranging from about 20°C to about 50°C; and a total gas flow of at least 100 seem total gas flow (for a chamber volume of 35,000 cc). For efficiency purposes the etchant source gas is SF 6 / Ar, where the ratio of SF 6 : Ar ranges from about 2.5 : 1 to about 10 : 1, where the SF 6 flow rate ranges between about 50 seem to about 150 seem. In an optional third etching step, the process chamber pressure is decreased still further. Typically this step is a finishing or rounding step. The process chamber pressure used is typically about 40 - 50% lower than the process chamber pressure used during the second etching step. When a rounded shaped cavity is desired, decreasing the process chamber pressure by about 50% from the process chamber pressure used in the second etching step provides excellent results. A process chamber pressure within the range of about 5 mTorr to about 12 mTorr is typically employed, and excellent results were obtained using about 8 mTorr. The plasma source gas used in the optional third etching step is typically the same as that used in the first and second etching steps. Typical process conditions for the third etch step are as follows: a process chamber pressure ranging from about 5 mTorr to about 12 mTorr; a source power ranging from about 350 W to about 650 W; a bias power ranging from about 0 W to about 10 W; a substrate temperature ranging from about 20 °C to about 50°C; an SF 6 /Ar etchant gas with an SF 6 : Ar ratio ranging from about 2.5 : 1 to about 10 : 1, at an SF 6 flow rate ranging from about 50 seem to about 150 seem; and at least 100 seem total gas flow (for a chamber volume of 35,000 cc). In an optional etch finishing or rounding step, the process chamber pressure is increased to provide the shaped cavity with an increased concentration of reactive species at the surface of the cavity, after the removal of the etch process byproducts during the second and (if applicable) third etching steps. The process chamber pressure used during the etch finishing step need not be, but typically is lower than the initial process chamber pressure for the first step. When a round shaped cavity is desired, increasing the process chamber pressure to about 90%), of the initial process chamber pressure provides excellent results. A process chamber pressure within the range of about 18 mTorr to about 45 mTorr is typically employed during the optional etch finishing step. The plasma source gas used in the etch finishing step may be of any composition which provides a desired surface finish (roughness) or chemical composition. When surface roughness is the only concern, the plasma source gas for a finishing step typically compπses the same components as that used in previous etching steps. However, relative amounts of the plasma source gas components may be adjusted. For example, the etchant species may be the fluorine-containing species previously described, used in combination with argon; however, the ratio of fluorine species to argon in the plasma source gas used in the etch finishing step is typically lower than that used in the previous steps. In the etch finishing step, the SF 6 : Ar ratio of the plasma source gas is typically within the range of about 1.5 : 1 and about 5 : 1. When a round shaped cavity is desired, an SF 6 : Ar ratio of about 2.5 : 1 to about 3 : 1 provides excellent results. Surprisingly, we discovered that an SF 6 : Ar ratio of 2.0 : 1 does not provide good results. Typical process conditions for the etch finishing step are as follows: a process chamber pressure ranging from about 18 m Torr to about 45 mTorr; a source power ranging from about 500 W to about 800 W; a bias power ranging from about 0 W to about 15 W: a substrate temperature ranging from about 20 °C to about 50 °C; the SF 6 : Ar ratio described above, where the SF 6 gas flow ranges from 50 seem to about 150 seem; and, at least 110 seem total gas flow (for a chamber volume of 35,000 cc). In more general terms within the method of the invention, a shaped opening is formed in a substrate, then the profile (i.e., shape) of the shaped opening is preserved by the formation of a protective layer (typically a conformal layer) over at least a portion of the inner surfaces of the shaped opening. Subsequently, a shaped cavity, underlying the shaped opening, is etched using a multi-step method performed at varying process chamber pressures. Formation of the protective layer over at least a portion of the shaped opening generally preserves the shape of the opening and permits better control over the etch of the shaped cavity. Figures 1A - ID illustrate the general method of the invention. Referring to Figure 1A, a shaped opening 104 is etched in a substrate 102 to a predetermined depth A using etching techniques known in the art, depending on the substrate material. For example, but not by way of limitation, the substrate 102 may be pattern etched to form shaped opening 104 using a conventional organic photoresist, an inorganic hard mask, or a combination thereof. As previously mentioned, the substrate 102 may comprise any material which can be etched using techniques known in the art. The Examples provided herein are with reference to silicon, although the general concepts of the invention can be applied to the use of other substrate materials. Referring to Figure IB. a conformal protective layer 106 is formed on at least the sidewall 108 of the shaped opening 104 using techniques known in the art, depending on the composition of the protective layer. The protective layer 106 may reach all the way to the bottom 1 10 of the shaped opening. However, in certain situations, such as for very high aspect ratios (> 20 : 1), the protective layer 106 may not be deposited on the bottom 110 of the shaped opening, depending on the feature size. The conformal protective layer is deliberately formed on interior surfaces of the opening, as distinguished from layers made up of reaction byproducts which are incidentally deposited during various processing steps. To the extent that a byproduct layer is present on the interior surface of the opening, the byproduct layer may be left there if its presence serves a useful purpose, or it may be removed prior to application of the protective layer if it would interfere with subsequent processing steps. The protective layer 106 comprises a material having a different etch selectivity than the substrate. In the case of a silicon substrate, a typical protective/masking layer may comprise, but is not limited to, silicon oxide, silicon nitride, silicon oxynitride. titanium nitride, aluminum, diamond, and polyimide. Silicon oxide and silicon nitride provide excellent etch selectivity relative to silicon. In the case of a silicon oxide substrate, a typical protective/masking layer may comprise, but is not limited to, polysilicon, silicon nitride, titanium nitride, aluminum, diamond, and polyimide. Polysilicon and silicon nitride provide excellent etch selectivity relative to silicon oxide. In the case of a silicon nitride substrate, a typical protective/masking layer may compπse. but is not limited to, polysilicon, silicon oxide, titanium nitπde. aluminum, diamond, and polyimide Polysilicon and silicon oxide provide excellent etch selectivity relatι\ e to silicon nitπde In the case of an aluminum substrate, a typical protective/masking layer may compπse. but is not limited to, polyimide, titanium nitπde, silicon oxide, and silicon nitπde Polyimide, silicon oxide, and silicon nitπde provide excellent etch selectivity relative to aluminum In the case of an organic substrate, such as a polyimide, SILK, or BCB substrate, a typical protective/masking layer may compπse, but is not limited to. silicon oxide, silicon nitπde. aluminum, and titanium mtπde Silicon oxide and silicon nitπde provide excellent etch selectivity relative to polyimide, SILK, and BCB The thickness of the protective layer 106 on inteπor surfaces of the shaped opening 104 is dependent on the size of the feature to be etched, the aspect ratio of the feature, and the difference in etch rate between the protective layer and the substrate For a en feature size of 1 - 2 μm, a typical protective layer has a thickness ranging from about 1000 A to about 2000 A The required protective layer thickness for a particular application can be determined with minimal expeπmentation, in view of the known art Refernng to Figure 1C, the protective layer 106 may be amstropical etched to remo\ e portions of the protective layer which overlie the bottom 108 of the shaped opening 104 Anisotropic etching is performed using apparatus and techniques known m the art A shaped cavity is then etched in the substrate underlying the shaped opening according to the method of the invention An etchant is selected which selectπ ely etches the substrate relative to the protective layer (ι e , the substrate is etched at a much faster rate than the protective layer) dunng the performance of the method of the invention The protects e layer effectiveh preserves the profile of the shaped opening dunng etching of the underlying shaped cavity According to the first step of a prefeπed embodiment method of the invention where the substrate is silicon, gaseous etchant species are introduced through the shaped opening in order to etch an underlying shaped cavity. Etching of the shaped cavity is generally performed using a relatively high process chamber pressure within the range of about 20 mToπ and about 50 mTon. Typically, a substrate bias voltage is applied, with the amount of voltage depending on the degree of anisotropicity (i.e., vertical relative to horizontal etching) desired. The higher the voltage, the more vertical the etch. When increased vertical etching is desired, a higher bias power, typically ranging from about 20 W to about 200 W, is typically used. When increased lateral etching is desired, a lower bias power, typically ranging from about 0 W to about 15 W, is typically used. When the substrate is silicon and the protective layer is silicon oxide, etching is typically performed using a plasma containing reactive fluorine species. The etchant plasma is commonly generated from a source gas comprising SF 6 and argon, provided at a flow rate which is process equipment dependent, and at an SF 6 : argon ratio within the range of about 10 : 1 and about 2.5 : 1. When a round shaped cavity is desired, an SF 6 : argon ratio of about 4 : 1 provides excellent results. Argon is used as a chemically inert carrier for the SF 6 , and ionized argon in conjunction with a substrate bias may be used to drive reactive fluorine species generated from SF 6 down through the shaped opening and into the shaped cavity. Further, high energy argon species provide physical bombardment of the substrate, assisting in the etch process. The newly formed shaped cavity may be used as a reaction chamber during subsequent etching steps. The plasma source gas may further include an additive gas for the purpose of controlling various properties of the shaped cavity, such as the shape or the surface finish of the shaped cavity. Typical additive gases include, but are not limited to, O 2 , HBr, Cl 2 , and N 2 . For example, the addition of O 2 to the plasma source gas will result in a shaped cavity having a roughened internal surface, whereas the addition of Cl 2 will lead to a smooth internal surface. HBr provides effective profile control during etching of the shaped cavity. Depending on the particular effect desired, the additive gas may be present during the entire duration of the method of the invention, or it may be present only during a certain step or steps. For example, if a particular finish is required on the surface of the final shaped cavity, an additive gas such as 0 2 or Cl 2 is typically added during the final finishing step described below. After the initial cavity etch step, a second etch step is performed during which the process chamber pressure is decreased in order to allow the removal of etch byproducts from the shaped cavity. This permits the subsequent introduction of fresh etchant species and helps prevent the buildup of etch byproducts on interior surfaces of the shaped cavity. The process chamber pressure used in the second etching step is typically about 30% lower than the process chamber pressure during the initial cavity etch step. A process chamber pressure within the range of about 10 mTon to about 25 mTon is typically employed during the second etching step. By way of example, and not by way of limitation, when a round shaped cavity is desired, decreasing the process chamber pressure by about 25 - 50% from the process chamber pressure in the initial etch step provides excellent results. To obtain a round shaped cavity, a bias power within the range of about 0 W to about 15 W is typical. The plasma source gas used during the continued etching may be different, but for process simplicity, it is preferably the same as that used during the initial cavity etching. An additive gas, such as those described above, may further be included in the plasma source gas during continued etching. In an optional third etching step, the process chamber pressure is decreased still further. The process chamber pressure used in the optional third etching step is typically about 40 - 50% lower than the process chamber pressure used during the second etching step. By way of example, and not by way of limitation, when a round shaped cavity is desired, decreasing the process chamber pressure by about 50%) from the process chamber pressure used in the second etching step provides excellent results. A process chamber pressure within the range of about 5 mTon to about 12 mTon is typically employed during the optional third etching step. When the cavity is to be further expanded in a lateral 1 direction, a bias power within the range of about 0 W to about 10 W is typically used. For 2 process simplicity, the plasma source gas used in the optional third etching step is typically 3 the same as that used in the initial cavity etching. 4 In an optional etch finishing step, the process chamber pressure is increased to 5 provide the shaped cavity with an increased concentration of reactive species following the 6 reduction in concentration due to removal of the etch process byproducts during the second 7 and optional third etching steps. The process chamber pressure used during the etch S finishing step may be, but is not necessarily, lower than the process chamber pressure during 9 the initial cavity etch step. By way of example, and not by way of limitation, in the etch 0 finishing step, the process chamber pressure is typically increased to about 80 - 100% of the 1 process chamber pressure used during the initial cavity etch step. When a round shaped 2 cavity is desired, increasing the process chamber pressure to about 90% of the initial etch 3 step process chamber pressure provides excellent results. A process chamber pressure 4 within the range of about 18 mTon and about 45 mTon is typically employed during the 5 optional etch finishing step. When the emphasis in this step is on lateral etching, a bias 6 power within the range of about 0 W to about 15 W is typically applied. The plasma source 7 gas used in the etch finishing step is the same as that used in the first and second etching S steps. 9 The plasma source gas used in the etch finishing step typically comprises the same 0 components as that used in the first and second etching steps. However, increasing the 1 argon flow rate or lowering the ratio of fluorine species to argon in the plasma source gas 2 used in the etch finishing step provides a higher carrier flow to ensure that etchants will 3 travel into the cavity and react as desired. If no passivant species (such as HBr or 0 2 ) are 4 added to the gas mixture, and if there is an adequate dilution of SF 6 in argon (as described 5 above), smoother interior cavity surfaces will be obtained. The addition of O 2 to the plasma 6 source gas will result in some roughness on the internal cavity surface. On the other hand, 7 Cl 2 addition should result in a smoother cavity surface. In the etch finishing step, the SF 6 : argon ratio of the plasma source gas is typically within the range of about 5 : 1 and about 1.5 : 1. When a round shaped cavity is desired, an SF 6 : argon ratio of about 3 : 1 to about 2.5 : 1 provides excellent results. Figure ID shows the structure of Figure 1C after etching of shaped cavity 110 using the method of the invention. The shaped cavity 110 is etched to have a width B and a depth C. where the width B is equal to or greater than the depth C. As shown in Figure ID, the shaped cavity 1 10 directly underlies and is in continuous communication with the shaped opening 104. The protective layer 106 effectively preserves the profile of the shaped opening 104 during etching of the underlying shaped cavity 106 using the method of the invention. The shaped cavity 110 shown in Figure ID has been etched so that the width B is approximately equal to the depth C, resulting in the formation of a substantially round shaped cavity. For purposes of comparison, the typical process conditions for each step of the prefened embodiment method of the invention are presented in Table One, below. Table One. Typical Process Conditions During Etch of a Shaped Cavity Any number of reductions in process chamber pressure, changes in substrate bias voltage, and changes m plasma source gas composition and/or plasma source power may be used to etch the desired shaped cavity Figure 2 shows a graph 200 of eccentncity (ι e , the width of a shaped cavity divided by its depth) as a function of the width of the overlying shaped opening, for shaped cavities etched using a pnor art etch process, represented by curve 202, and for shaped cavities etched using the method of the invention, represented by curve 204 As shown in Figure 2. for a given shaped opening width, a shaped cavity etched using the method of the invention has a greater eccentncity than one that was etched using a conventional etch process For example, for a shaped opening width of 2 μm, a shaped cavity etched using the method of the present invention has an eccentncity of about 1 08, whereas a shaped cavity etched using a conventional etch process has an eccentncity of about 0 92 This indicates that the shaped cavity etched using the method of the invention may be wider than it is deep, whereas the shaped cavity etched using the conventional etch process must be deeper than it is wide The method of the invention is useful in many different applications The method of the invention can be applied to any application where it is necessary or desirable to provide a structure having a shaped opening and an underlying shaped cavity having varying shapes, and where it is necessary to maintain tight control over the dimensions of the shaped opemng For example, the method of the invention can be used for micromachining of test chips for performance of vaπous chemical or biological assays (e g , genome testing), wherein the testing reagents would be contained in a plurality of shaped cavities etched in the chip The method of the invention can also be used for micromachining of electrostatically controlled nozzles for use m Inkjet pπnters, wherein the ink would be contained in the shaped cavity and the shaped opening would function as a nozzle The method of the invention allows for excellent control over the cπtical dimensions of the shaped opening [e g , the diameter of the nozzle), providing for consistent and reproducible micromachining of a variety of devices. Adapting the general method of the invention for use in a particular application will be within the capability of one skilled in the art to which that particular application belongs, without the need to perform undue experimentation. Figures 3 A and 3B, respectively, show examples of round (i.e., width = depth) and horizontal elliptical (i.e., width > depth) shaped cavity profiles that can be obtained using the method of the invention. Figure 3 C shows a double spherical profile which is obtained by continued isotropic etching of closely adjacent shaped cavities 302 and 304, in order to remove the substrate material between the shaped cavities. By removing the substrate material between the shaped cavities, a silicon "island" 306 is formed between shaped openings 308 and 310. Either the silicon islands or cavities themselves can be used as waveguides for optical interconnects in optoelectronic devices, or as a cantilever for an accelerometer, for example. Using the method of the invention, with a shaped opening depth of from 0.8 μ to 2.0 μm, and a shaped opening width of from 3.5 to 4.5 μm, we have been able to form shaped cavities having a width that is from 2.5 to 4 times greater than the width of the overlying shaped opening, while maintaining a shaped cavity eccentricity of about 1. We have been successful in shaping a variety of features having an initial aspect ratio (i.e., shaped opening depth : shaped opening width) of less than about 3.5 : 1. It is likely that we would be able to accomplish similar results at even higher aspect ratios. Preferably, the apparatus used to practice the present invention is adapted to be controlled by a computer. Figure 4 shows a computer 400. Computer 400 comprises a processor 402, memory 404 adapted to store instructions 406, and one or more ports 408. Processor 402 is adapted to communicate with memory 404 and to execute instructions 406. Processor 402 and memory 404 are also adapted to communicate with one or more ports 408. Ports 408 are adapted to communicate with an etch chamber 412. Chamber 412 is adapted to carry out process steps in accordance with signals received from processor 402 via ports 408. Preferably, computer 402 can control the composition and feed rate of the etch process feed gases, the process temperature, the pressure in the chamber, the time period for each process step, and other similar functions. Preferably, computer 402 is adapted to receive measurements that describe the conditions in the chamber or a condition of the substrate being etched, and adapt the process variables accordingly. This programmed control of process variables enables production of a predetermined device etch profile as required for a given use application. One skilled in the art will recognize that a combination of etch steps at higher or lower chamber pressures, using varying ratios of active etchant to inert gas in the plasma source gas, and varying the plasma source power and substrate bias power can be used to obtain a cavity having a predetermined shape. The above described prefened embodiments are not intended to limit the scope of the present invention, as one skilled in the art can, in view of the present disclosure, expand such embodiments to conespond with the subject matter of the invention claimed below.

Claims: CLAIMS We claim 1 A method of etching a shaped cavity in a substrate, wherein initial etching of said shaped cavity is performed using an initial process chamber pressure, and wherein continued etching of the shaped ca\ ity is performed using a process chamber pressure that is at least 25%o lower than said initial process chamber pressure, whereby etch byproducts formed dunng etching of said shaped cavity are removed from said shaped cavity dunng continued etching 2 A method of etching a shaped cavity in a substrate, wherein the method compnses the steps of a) an initial cavity etch step dunng which said substrate is etched to form a shaped cavity using an initial process chamber pressure, and b) at least one additional etch step dunng which continued etching of said shaped cavity is performed using a process chamber pressure that is at least 25%o lower than said initial process chamber pressure 3 The method of Claim 2, wherein a second etch step is performed using a process chamber pressure that is \\ ithin a range of about 30%o to about 50%) lower than said initial process chamber pressure 4 The method of Claim 3, wherein said second etch is performed using a process chamber pressure that is about 30% lower than said initial process chamber pressure 5 The method of Claim 2, wherein said method further compnses an etch finishing step, wherein said etch finishing step is performed using a process chamber pressure that is 3 within a range of about 80% to about 100%) of said initial process chamber pressure. 1 6. The method of Claim 3, wherein said method further comprises a third etch step 2 during which continued etching of said shaped cavity is performed using a process chamber 3 pressure that is at least 40% lower than the process chamber pressure used during the 4 performance of said second etch step.. 1 7. The method of Claim 6, wherein said third etch step is performed using a process ? chamber pressure that is within a range of about 40%o to about 50%o lower than the process 3 chamber pressure used during the performance of step b). 1 8. The method of Claim 6, wherein said method further comprises an etch finishing 2 step, wherein said etch finishing step is performed using a process chamber pressure that is 3 within a range of about 80% to about 100%> of said initial process chamber pressure. 1 9. The method of Claim 5 or Claim 8, wherein said etch finishing step is performed ~> using a process chamber pressure that is about 90% of said initial process chamber pressure. 1 10. The method of Claim 2 or Claim 5 or Claim 6 or Claim 8, wherein said substrate 2 comprises silicon, and etching is performed using a plasma containing reactive fluorine 3 species. 1 1 1. The method of Claim 10, wherein said plasma is generated from a source gas ~> comprising SF 6 and Ar. 1 12. The method of Claim 11, wherein said plasma source gas further comprises an 2 additive gas selected from the group consisting of 0 2 , HBr, Cl 2 , N 2 , and combinations thereof. 13. The method of Claim 2 or Claim 5 or Claim 6 or Claim 8, wherein etching is performed using a plasma generated from a source gas comprising a gas selected from the group consisting of SF 6 , CF 4 , Cl 2 , HBr, and combinations thereof. 14. The method of Claim 13, wherein said plasma source gas further comprises an additive gas selected from the group consisting of Ar, 0 2 , HBr, Cl 2 , N 2 , and combinations thereof, wherein said additive gas is provided in an amount sufficient to improve profile control during etching. 15. The method of Claim 13, wherein said plasma source gas further comprises an essentially nonreactive, diluent gas selected from the group consisting of He and Xe. 16. The method of Claim 14, wherein said plasma source gas further comprises an essentially nonreactive, diluent gas selected from the group consisting of He and Xe. 17. The method of Claim 2, wherein said substrate compnses polysilicon, and etching is performed using a plasma generated from a source gas comprising a gas selected from the group consisting of SF 6 , Cl 2 , and combinations thereof. IS. The method of Claim 2, wherein said substrate comprises silicon dioxide, and etching is performed using a plasma generated from a source gas comprising a gas selected from the group consisting of CF 4 , NF 3 , and combinations thereof, and wherein etching is performed at a substrate temperature within the range of about 50°C to about 100°C. 19. The method of Claim 2, wherein said substrate comprises silicon nitride, and etching is performed using a plasma generated from a source gas comprising SF 6 . 20. The method of Claim 2, wherein said substrate comprises a metal, and etching is performed using a plasma generated from a source gas comprising Cl 2 . 21. The method of Claim 20, wherein said metal is selected from the group consisting of aluminum and an aluminum alloy. 22. The method of Claim 2, wherein said substrate comprises polyimide, and etching is performed using a plasma generated from a source gas comprising O 2 and CF 4 23. The method of Claim 2 or Claim 5 or Claim 6 or Claim 8, wherein said method includes performing the following steps prior to etching said shaped cavity: etching said substrate to a predetermined depth to form a shaped opening, then forming a conformal protective layer overlying at least the sidewall of said shaped opening, wherein said protective layer comprises a material having a different etch selectivity than said substrate, wherein said shaped cavity is etched so that said shaped cavity directly underlies and is in continuous communication with said shaped opening, and wherein said shaped cavity is etched using an etchant gas which selectively etches said substrate relative to said protective layer, whereby said protective layer effectively preserves the profile of said shaped opening during etching of said shaped cavity. 24. The method of Claim 23. wherein said substrate comprises silicon and said protective layer comprises silicon oxide. 25. An apparatus, comprising: (a) a memory that stores instructions for: 3 etching a shaped cavity in a substrate, said method comprising the steps of: 4 i) an initial cavity etch during which said substrate is etched to form a 5 shaped cavity using an initial process chamber pressure; and; 6 ii) at least one additional etch step during which continued etching of ~ : ~ said shaped cavity is performed using a process chamber pressure that is at least 25% 8 lower than said initial process chamber pressure; 9 (b) a processor adapted to communicate with the memory and to execute the 0 instructions stored by the memory; 1 (c) an etch chamber adapted to carry out said etching in accordance with the 2 instructions from said memory; and 3 (d) a port adapted to pass communications between said processor and said etch 4 chamber. 28. An article of manufacture comprising: a recordable medium having recorded thereon a plurality of programming instructions used to program an apparatus which controls the etching of a shaped cavity in a substrate to proceed by the method of Claim 1 or Claim 2.

Inventors: GIORDANI SARA, GIORDANI, SARA
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 2001
Main classification: H01L23
IPC codes: C23F4, H01L21