Anisotropic metal oxide etch

Abstract: A metal oxide substrate (e.g. barium strontium titanate 34 ) is immersed in a liquid ambient (e.g. 12 molar concentration hydrochloric acid 30 ) and illuminated with radiation (e.g. collimated visible/ultraviolet radiation 24 ) produced by a radiation source (e.g. a 200 Watt mercury xenon arc lamp 20 ). A window 26 which is substantially transparent to the collimated radiation 24 allows the radiated energy to reach the metal oxide substrate 34 . An etch mask 32 may be positioned between the radiation source 20 and the substrate 34 . The metal oxide substrate 34 and liquid ambient 30 are maintained at a nominal temperature (e.g. 25 °C). Without illumination, the metal oxide is not appreciably etched by the liquid ambient. Upon illumination the etch rate is substantially increased.

Description: Field of the Invention This invention generally relates to methods of anisotropically etching metal oxide materials. Background of the Invention Without limiting the scope of the invention, its background is described in connection with current methods of etching metal oxides, as an example. Metal oxide materials, because of their electrical and mechanical properties, have found many uses in the field of electronics. Examples of these useful metal oxide materials are barium strontium titanate (BaSrTiO 3 , hereafter referred to as BST), lead zirconate titanate (PbTiZrO 3 , hereafter referred to as PZT), niobium pentoxide (Nb 2 O 5 ) and tantalum pentoxide (Ta 2 O 5 ). The very high dielectric constants exhibited by many metal oxide compounds make them useful as the dielectric material in capacitors, especially in the miniature capacitors which are built into many integrated circuits (e.g dynamic RAMs). Many metal oxide materials also have a positive temperature coefficient of electrical resistance, which allows devices to be made which protect electrical motors from damage due to over-current conditions. The piezoelectrical properties of these materials are widely used, as in the precise control of the length of laser cavities, for example. Microminiature structures which incorporate metal oxide materials are being used to sense infrared radiation, obviating the need for bandgap detector materials which require cryogenic cooling to sense the infrared. Metal oxide materials are often etched during the fabrication of the electrical devices which exploit their beneficial properties. A method used to etch metal oxides should generally do so without introducing damage which would unacceptably change the properties of the material on which the function of the eventual devices depends. In addition, an anisotropic etch method is usually desired so that detail in the etch mask pattern is preserved. Contamination of the metal oxide material (and/or nearby materials) by the etch method usually cannot be tolerated. Heretofore, in this field, metal oxides have been etched by isotropic wet etching, ion-milling, plasma etching or laser scribing. Laser scribing is a method wherein selected portions of the material are damaged and weakened by exposure to intense laser radiation and then removed. In "Applied Physics Letters, Vol. 54, No. 13, 27-03-1989, pages 1272 to 1274" a method of thermally activated laser etching is described which is suitable for anisotropically etching a surface of a metal oxide material, the surface being submersed in a liquid etchant when said etching step is carried out. In Japanese Journal of Applied Physics, Vol. 26, (1987) Supplement 26-2, pages 159-161, a method for localized etching of semiconductors and ceramics is described, which uses thermal or optical excitation of the etchant and/or workpiece. Summary of the Invention It has been discovered that current methods of etching metal oxide materials can produce unacceptable results. Isotropic wet etching can undercut the etch mask (generally by the same distance as the etch depth), leading to features in the underlying metal oxide material which are not true to the etch mask pattern. Ion-milling can generate unacceptable defects in the metal oxide material, including changes in its electrical, chemical, mechanical, optical, and/or magnetic properties. Plasma etching can cause damage similar to that caused by ion-milling, and in addition the plasma constituent elements can contaminate substrate materials and are often difficult to remove. Etching by laser scribing can result in no undercut of the material, but the laser radiation can cause damage to the metal oxide material which must be repaired by a high-temperature (approximately 1400 °C) anneal. This anneal may produce undesirable oxidation states in the metal oxide material, reducing its beneficial properties. High temperature anneals, in addition, can cause changes and damage to other materials and structures present in the substrate. The present invention discloses a method for anisotropically etching as out lined in the claims. An advantage of the invention is that it anisotropic (i.e does not cause substantial undercut). Those regions under the mask remain in shadow and are not appreciably etched. In addition, it has been found that this method does not cause unacceptable defects in the metal oxide material. Contamination from plasma species is prevented. Generally, no high temperature anneal is required. Brief Description of the Drawing The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawing. In the drawing: FIG. 1 is a representation of the apparatus and materials used to anisotropically etch metal oxide materials. Detailed Description of the Preferred Embodiments With reference to Fig. 1, a pattern is etched into a substrate 34 off barium strontium titanate (BaSrTiO 3 ) which is immersed in 12 molar concentration hydrochloric acid (HCl) 30 and illuminated with substantially collimated visible/ultraviolet radiation 24 propagating substantially orthogonal to the substrate surface 34 produced by a 200 Watt mercury xenon arc lamp 20. Radiation 21 from the radiation source 20 is collimated by collimating optics 22 and the resulting collimated radiation 24 is directed at the substrate 34 . A window 26 which is a portion of the reaction vessel 28 and which is substantially transparent to the collimated radiation allows the radiated energy to reach the BST substrate 34. An etch mask 32 defines the etched pattern by blocking the radiation at portions of the substrate. The BST substrate 34 and acid solution 30 are nominally at a temperature of 25 °C. At such a temperature and acid concentration and without illumination, BST is not appreciably etched by HCl. Upon illumination by visible/ultraviolet radiation produced by the lamp, however, the etch rate becomes approximately one micron per hour. The etch mask 32 between the radiation source 20 and the BST substrate 34 is preferably located close to or in contact with the surface to be etched. Such an etch mask 32 may be deposited onto the substrate substrate surface. The etch mask 32 is preferably made of silicon oxide. In general, any substance which is sufficiently opaque to visible and ultra violet light such as that produced by the mercury xenon lamp and which is not substantially penetrated of etched by the liquid ambient may be used, such as silicon oxide or silicon nitride. Noble metals such as platinum may be used. Those areas of the substrate not covered by the mask material will be subject to etching by the acid solution. Sidewall profile control is enhanced by introducing a passivating agent into the liquid ambient 30 that does not react with the BST to form a soluble product or only slightly soluble product. The passivating agent "poisons" the sidewalls with respect to attack by other etch reagents on the sidewall, but does not stop etching normal to the substrate because the irradiation detaches the passivating agent from the surface. An example of a passivating agent in an etch of BST is phosphoric acid. Again with reference to Fig.1, a pattern is etched into a substrate 34 of niobium pentoxide (Nb 2 O 5 ) which is immersed in 10% hydrofluoric acid acid (HF) 30 and illuminated with substantially collimated visible/ultraviolet radiation 24 propagating substantially orthogonal to the substrate surface 34 produced by a 200 Watt mercury xenon arc lamp 20 . Radiation 21 from the radiation source 20 is collimated by collimating optics 22 and the resulting collimated radiation 24 is directed at the substrate 34. A window 26 which is a portion of the reaction vessel 28 and which is substantially transparent to the collimated radiation allows the radiated energy to reach the Nb 2 O 5 substrate 34. An etch mask 32 defines the etched pattern by blocking the radiation at portions of the substrate. The Nb 2 O 5 substrate 34 and acid solution 30 are nominally at a temperature of 25 °C. At such a temperature and acid concentration and without illumination, Nb 2 O 5 is not appreciably etched. Upon illumination by vsible/ultraviolet radiation produced by the lamp, however, the etch rate is increased. Etch rate ratios as high as 9:1 (illuminated:unilluminated) have been observed. The etch mask 32 between the radiation source 20 and the Nb 2 O 5 substrate 34 is preferably located close to or in contact with the surface to be etched. Such an etch mask 32 may be deposited onto the substrate surface. The etch mask 32 is preferably an organic photoresist. In general, any substance which is sufficiently opaque to visible and ultraviolet light such as that produced by the mercury xenon lamp and which is not substantially penetrated or etched by the liquid ambient may be used. Noble metals such as platinum may be used. Those areas of the substrate not covered by the mask material will be subject to etching. Sidewall profile control is enhanced by introducing a passivating agent into the liquid ambient 30 that does not react with the Nb 2 O 5 to form a soluble product or only slightly soluble product. The passivating agent "poisons" the sidewalls with respect to attack by other etch reagents on the sidewall, but does not stop etching normal to the substrate because the irradiation detaches the passivating agent from the surface. An example of a passivating agent in an etch of Nb 2 O 5 is hydrochloric acid. Again with reference to Fig. 1, a pattern is etched into a substrate 34 of tantalum pentoxide (Ta 2 O 5 ) which is immersed in 10% hydrofluoric acid (HF) 30 and illuminated with substantially collimated visible/ultraviolet radiation 24 propagating substantially orthogonal to the substrate surface 34 produced by a 200 Watt mercury xenon arc lamp 20 . Radiation 21 from the radiation source 20 is collimated by collimating optics 22 and the resulting collimated radiation 24 is directed at the substrate 34 . A window 26 which is a portion of the reaction vessel 28 and which is substantially transparent to the collimated radiation allows the radiated energy to reach the Ta 2 O 5 substrate 34 . An etch mask 32 defines the etched pattern by blocking the radiation at portions of the substrate. The Ta 2 O 5 substrate 34 and acid solution 30 are nominally at a temperature of 25 °C. At such a temperature and acid concentration and without illumination, Ta 2 O 5 is not appreciably etched. Upon illumination by visible/ultraviolet radiation produced by the lamp, however, the etch rate is increased. Etch rate ratios as high as 50:1 (illuminated:unilluminated) have been observed. The etch mask 32 between the radiation source 20 and the Ta 2 O 5 substrate 34 is preferably located close to or in contact with the surface to be etched. Such an etch mask 32 may be deposited onto the substrate surface. The etch mask 32 is preferably an organic photoresist. In general, any substance which is sufficiently opaque to visible and ultraviolet light such as that produced by the mercury xenon lamp and which is not substantially penetrated or etched by the liquid ambient may be used. Noble metals such as platinum may be used. Those areas of the substrate not covered by the mask material will be subject to etching. Sidewall profile control is enhanced by introducing a passivating agent into the liquid ambient 30 that does not react with the Ta 2 O 5 to form a soluble product or only slightly soluble product. The passivating agent "poisons" the sidewalls with respect to attack by other etch reagents on the sidewall, but does not stop etching normal to the substrate because the irradiation detaches the passivating agent from the surface. An example of a passivating agent in an etch of Ta 2 O 5 is hydrochloric acid. With reference to Fig. 1, a pattern is etched into a substrate 34 of lead zirconate titanate (PbTiZrO 3 or PZT) which is immersed in 12 molar concentration hydrochloric acid HCl) 30 and illuminated with substantially collimated visible/ultraviolet radiation 24 propagating substantially orthogonal to the substrate surface 34 produced by a 200 Watt mercury xenon arc lamp 20. Radiation 21 from the radiation source 20 is collimated by collimating optics 22 and the resulting collimated radiation 24 is directed at the substrate 34 . A window 26 which is a portion of the reaction vessel 28 and which is substantially transparent to the collimated radiation allows the radiated energy to reach the PZT substrate 34 . An etch mask 32 defines the etched pattern by blocking the radiation at portions of the substrate. The PZT substrate 34 and acid solution 30 are nominally at a temperature of 25 °C. At such a temperature and acid concentration and without illumination, PZT is not appreciably etched by HCl. Upon illumination by visible/ultraviolet radiation produced by the lamp, however, the etch rate is increased. It is believed, in part because of the relatively low power of the light source, that the etching reaction is accelerated in the illuminated areas because of electronic excitation due to photo-irradiation rather than from thermal effects. As used herein, the term "radiation" means radiation at levels above background and this means, for example, illumination at levels substantially greater than room lighting. The etch mask 32 between the radiation source 20 and the PZT substrate 34 is preferably located close to or in contact with the surface to be etched. Such an etch mask 32 may be deposited onto the substrate surface. The etch mask 32 is preferably made of silicon oxide. In general, any substance which is sufficiently opaque to visible and ultraviolet light such as that produced by the mercury xenon lamp and which is not substantially penetrated of etched by the liquid ambient may be used, such as silicon oxide or silicon nitride. A noble metal such as platinum may be used. Those areas of the substrate not covered by the mask material will be subject to etching by the acid solution. Sidewall profile control is enhanced by introducing a passivating agent into the liquid ambient 30 that does not react with the material being etched to form a soluble product or only slightly soluble product. The passivating agent "poisons" the sidewalls with respect to attack by other etch reagents on the sidewall, but does not stop etching normal to the substrate because the irradiation detaches the passivating agent from the surface. An example of a passivating agent in an etch of PZT is phosphoric acid. For all of the above examples, it is believed, in part because of the relatively low power of the light source, that the etching reaction is accelerated in the illuminated areas because of electronic excitation due to photo-irradiation rather than from thermal effects. As used herein, the term "radiation" means radiation at levels above background and this means, for example, illumination at levels substantially greater than room lighting. For all of the above examples, the liquid ambient may be form the class of solutions that etch the substrate material without light irradiation. In tins case, the radiation accelerates the etch rate on illuminated portions of the substrate, resulting in a less isotropic etch. Still other alternate embodiments include liquid ambients containing salts and liquid ambients with pH values less than or equal to seven (i.e. acids and neutral solutions). Still other alternate embodiments include liquid ambients with pH values in the alkaline range (i.e. greater than seven), although neutral and acidic solutions are greatly preferred and have given excellent results as described in the preferred embodiments of this invention. For all of the above examples, a pattern is etched into the surface of the metal oxide material by projecting a patterned light onto the substrate (e.g. by conventional photo-lithographic techniques. Additionally, the liquid ambient may be made to flow with respect to the substrate. The flow rate of the liquid ambient may be varied. The solution temperature can be varied to achieve different etch rates and etch anisotropy. The photon flux may be varied to impact etch directionality and etch rates. The radiation wavelength can be adjusted to achieve different etch directionality and etch rates. The direction of propagation of the radiation need not be normal to the surface The etch solution may be a mixture of species (e.g. one or more passivating agents to enhance anisotropy and one or more reagents to photochemically etch the material). The sole Table, below, provides an overview of some embodiments and the drawing. Drawing Element Generic Term Preferred or Specific Terms Alternate Terms 20 Radiation Source 200 Watt mercury/xenon arc lamp 21 Radiation Visible/UV light Radiation which will penetrate the liquid ambient 22 Collimating Optics 24 Collimated Radiation Visible/UV light propagating normal to substrate surface 26 Transparent Window 28 Reaction Vessel 30 Liquid Ambient 12 Molar Hydrochloric acid Other etch constituents such as Hydrofluoric acid (HF); Passivating agents: phosphoric acid; HCl Mixtures of both. 32 Etch Mask Silicon oxide Silicon nitride Noble metals such as platinum 34 Metal oxide Substrate BaSrTiO 3 Nb 2 O 5 Ta 2 O 5 PbTiZrO 3 Titanate Compounds such as lanthanum doped lead zirconium titanate. Transition metal oxides such as LiNbO 3

Claims: A method for anisotropically etching a surface of titanate compounds such as barium strontium titanante BaSrTiO 3 or lead zirconate titanate PbTiZrO 3 submerged in hydrochloric acid HCI as liquid etchant, wherein portions of said surface are illuminated with substantially collimated radiation propagating substantially orthogonally to said surface and comprising visible or ultraviolet light or both, the temperature of said illuminated portions of said surface being maintained at substantially the same temperature as the temperature of the unilluminated portions of said surface, said liquid etchant further containing phosphoric acid as a passivating agent to form a substantially insoluble product decreasing the etch rate of said unilluminated portions of said surface, said irradiation detaching the passivating agent from said surface. A method for anisotropically etching a surface of niobium pentoxide Nb 2 O 5 or tantalum pentoxide Ta 2 O 5 submerged in hydrofluoric acid HF as liquid etchant, wherein portions of said surface are illuminated with substantially collimated radiation propagating substantially orthogonally to said surface and comprising visible or ultraviolet light or both, the temperature of said illuminated portions of said surface being maintained at substantially the same temperature as the temperature of the unilluminated portions of said surface, said liquid etchant further containing hydrochloric acid as a passivating agent to form a substantially insoluble product decreasing the etch rate of said unilluminated portions of said surface, said irradiation detaching the passivating agent from said surface. The method according to claim 1, further comprising providing said HCI as 12 molar HCI. The method according to claim 2, further comprising providing said hydrofluoric acid as 10% hydrofluoric acid. The method according to any preceding claim, further comprising forming said illuminated portions and said unilluminated portions of said surface by a photolithographic technique. The method according to any preceding claim, further comprising moving said liquid etchant with respect to said surface.

Inventors: BERATAN HOWARD R, DOUGLAS MONTE A, SUMMERFELT SCOTT R, BERATAN, HOWARD R., DOUGLAS, MONTE A., SUMMERFELT, SCOTT R.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, C04B41, G03F7