Chalcogenide layer etching method

[0001] This application claims the benefit of U.S. provisional patent application number 60/755,531 entitled Chalcogenide Layer Etching Method, filed on 30 Dec. 2005, Attorney Docket No. MXIC 1684-1.

[0002] International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation, and Infineon Technologies A.G., a German corporation, are parties to a Joint Research Agreement.

[0003] 1. Field of the Invention

[0004] The present invention relates to high density memory devices based on phase change based memory materials, including chalcogenide based materials, and to methods for manufacturing such devices.

[0005] 2. Description of Related Art

[0006] Phase change based memory materials are widely used in read-write optical disks. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.

[0007] Phase change based memory materials, like chalcogenide based materials and similar materials, also can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.

[0008] The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, and referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element.

[0009] One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued Nov. 11, 1997; Zahorik et al., “Method of Making Chalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued Aug. 4, 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued Nov. 21, 2000.

[0010] The present invention is directed to a method for etching a chalcogenide layer of a semiconductor device. A protective layer is deposited on a chalcogenide layer during the manufacture of a semiconductor device. A patterned photoresist layer is formed on the protective layer. The patterned photoresist layer and the protective layer are etched to form openings therethrough to the chalcogenide layer to create etched photoresist and etched protective layers. The etched photoresist layer is removed leaving at least a portion of the etched protective layer. The chalcogenide layer is etched through the openings in the etched protective layer. In some embodiments the patterned photoresist layer and protective layer etching step is carried out using a gas mixture comprising C₄F₈, CO and O₂. The chalcogenide layer etching step may be carried out using Cl₂. The forming step may comprise depositing a photoresist layer on the protective layer and lithographically patterning the photoresist layer to create the patterned photoresist layer.

[0011] Various features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.

[0012] FIGS. 1-5 illustrate the steps involved in the etching of a chalcogenide layer using conventional manufacturing techniques during the manufacture of a semiconductor device, such as a phase change memory device.

[0013] FIG. 1 illustrates a chalcogenide layer on a substrate.

[0014] FIG. 2 illustrates a photoresist layer deposited on the chalcogenide layer and lithographically patterning the photoresist layer.

[0015] FIG. 3 illustrates etching the patterned photoresist layer.

[0016] FIG. 4 illustrates the results of etching the photoresist layer with a fluorine based gas creating hardened, difficult to remove photoresist elements following the etching step.

[0017] FIG. 5 illustrates the results of etching the photoresist layer with a chlorine based gas creating undercut photoresist elements having downwardly and outwardly sloping walls instead of walls extending straight down to the chalcogenide layer.

[0018] FIGS. 6-11 illustrate a method for etching a chalcogenide layer of a semiconductor device according to the present invention.

[0019] FIG. 6 shows the structure of FIG. 1 with a protective layer deposited on the chalcogenide layer.

[0020] FIG. 7 illustrates a photoresist layer deposited on the protective layer and lithographically patterning the photoresist layer.

[0021] FIG. 8 illustrates etching the patterned photoresist layer and the underlying protective layer of FIG. 7.

[0022] FIG. 9 shows the results of removing the photoresist layer of FIG. 8.

[0023] FIG. 10 illustrates the result of a second etching step in which the chalcogenide layer is etched through the openings formed in the protective layer while obtaining a good profile with straight inside walls for the chalcogenide elements.

[0024] The following description of the invention will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments and methods but that the invention may be practiced using other features, elements, methods and embodiments. Like elements in various embodiments are commonly referred to with like reference numerals.

[0025] FIGS. 1-5 illustrate the steps involved in the etching of a chalcogenide layer using conventional manufacturing techniques during the manufacture of a semiconductor device, such as a phase change memory device. FIG. 1 illustrates a chalcogenide layer 10 on a substrate 12. As shown in FIG. 2, a photoresist layer 14 deposited on chalcogenide layer 10. Photoresist layer 14 is lithographically patterned as indicated at 16 to create a patterned photoresist layer 18. Patterned photoresist layer 18 is then etched as indicated at 20 in FIG. 3 to create an etched photoresist layer 22 having openings 24 formed therein.

[0026] Photomicrographs of conventional patterned photoresist layers 18 of FIG. 3 illustrate some of the problems associated with conventional techniques. FIG. 4 is a simplified view illustrating the results of etching patterned photoresist layer 18 with a fluorine based gas, such as CF₄, creating hardened, difficult to remove photoresist elements 26 following etching 20. That is, after etching chalcogenide layer 10, hardened photoresist elements 26 often prove difficult to remove. This is because some physical properties of the hardened photoresist elements 26 change by chemical reaction during etching 20, so that the hardened photoresist elements may react with chalcogenide layer 10 during removal of the hardened photoresist elements thus making it difficult to remove the hardened photoresist elements by dry stripping or wet stripping methods.

[0027] FIG. 5 illustrates the results of etching chalcogenide layer 10 with a chlorine based gas, such as Cl₂. During so creates undercut chalcogenide elements 28 having downwardly and inwardly sloping walls 30 instead of walls extending straight down to substrate 12. Downwardly and inwardly sloping walls 30 are not desirable because it is not easy to control the size of chalcogenide elements 28, and the size of the chalcogenide elements will dramatically affect the electrical properties of the resulting phase change memory device. Etched photoresist layer 22 has been removed in FIG. 5.

[0028] The present invention is described below with reference to FIGS. 6-10. The invention is directed to a method for etching a chalcogenide layer of a semiconductor device which eliminates some of the problems of conventional methods, in particular the creation of hardened photoresist elements 26 as shown in FIG. 4 and undercut chalcogenide elements 28 as shown in FIG. 5.

[0029] The method according to the present invention typically begins with the substrate layer 12 and chalcogenide layer 10 of FIG. 1. Substrate 12 is typically TiN or some other material such as W, Al, Cu or other conductive material, suitable for use as an electrode. Chalcogenide layer 10 is typically about 10 to 100 nm thick. However, instead of applying the photoresist layer directly on chalcogenide layer 10, a protective layer 32, see FIG. 6, is deposited on chalcogenide layer 10. Protective layer 32 is typically an oxide layer or a nitride layer deposited using, for example, PVD or CVD deposition procedures, and is typically about 20 to 300 nm thick. Thereafter, as shown in FIG. 7, a photoresist layer 34 is deposited on protective layer or nitride layer 32 followed by lithographically patterning 36 photoresist layer 34 to create a patterned photoresist layer 38. Photoresist layer 34 is typically made of organic or inorganic materials and is typically about 1000 to 4000 nm thick. FIG. 8 illustrates the results of etching 40 patterned photoresist layer 38 and the underlying protective layer 32 of FIG. 7, typically using C₄F₈+CO +O₂. The etching gas is chosen so that it will etch the photoresist and protective layers but stop at the chalcogenide layer. This etching step creates an etched photoresist layer 42 overlying an etched protective layer 44 having openings 46 formed in layers 42, 44. Thereafter, as shown in FIG. 9, etched photoresist layer 42 is removed leaving etched protective layer 44 with openings 46 formed in layer 44. thereafter the protective layer 44 becomes a kind of a hard mask. The present invention avoids the potential the chemical reaction which could be created among the chalcogenide layer 10, a conventional photoresist layer and conventional etch gases as can occur with conventional techniques as discussed above with reference to FIGS. 1-5.

[0030] FIG. 10 illustrates the result of a second etching step 48 which reduces the height of etched protective layer 44 to create a reduced height etched protective layer 50 and also etches chalcogenide layer 10 through openings 46 in etched protective layer 50 to create etched chalcogenide elements 52. Second etching step 48 may be accomplished using, for example, CF₄. Using this procedure, including the use of protective layer 32 between chalcogenide layer 10 and photoresist layer 34, creates chalcogenide elements 52 having a good profile with generally straight inside walls 54. Thereafter, reduced height protective layer 50 may be removed without the problems associated with removing conventional hardened photoresist elements 26.

[0031] Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te_aGe_bSb_100−(a+b).

[0032] One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky '112 patent, cols 10-11.) Particular alloys evaluated by another researcher include Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇. (Noboru Yamada, “Potential of Ge-Sb-Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v .3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky '112 at columns 11-13, which examples are hereby incorporated by reference.

[0033] Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly.

[0034] Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. A chalcogenide material useful for implementation of the method described herein is Ge₂Sb₂Te₅ commonly referred to as GST.

[0035] The invention has been described with reference to phase change materials. However, other memory materials, also sometimes referred to as programmable materials, can also be used. As used in this application, memory materials are those materials having electrical properties, such as resistance, that can be changed by the application of energy; the change can be a stepwise change or a continuous change or a combination thereof. Other programmable resistive memory materials may be used in other embodiments of the invention, including N₂ doped GST, Ge_xSb_y, or other material that uses different crystal phase changes to determine resistance; Pr_xCa_yMnO₃, PrSrMnO, ZrOx, or other material that uses an electrical pulse to change the resistance state; 7,7,8,8-tetracyanoquinodimethane (TCNQ), methanofullerene 6,6-phenyl C61-butyric acid methyl ester (PCBM),TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ, C60-TCNQ, TCNQ doped with other metal, or any other polymer material that has bistable or multi-stable resistance state controlled by an electrical pulse. Further examples of programmable resistive memory materials include GeSbTe, GeSb, NiO, Nb-SrTiO₃, Ag-GeTe, PrCaMnO, ZnO, Nb₂O₅, Cr-SrTiO₃.

[0036] For additional information on the manufacture, component materials, use and operation of phase change random access memory devices, see U.S. patent application Ser. No. 11/155,067, filed 17 Jun. 2005, entitled Thin Film Fuse Phase Change Ram And Manufacturing Method, Attorney Docket No. MXIC 1621-1.

[0037] The above descriptions may have used terms such as above, below, top, bottom, over, under, et cetera. These terms are used to aid understanding of the invention are not used in a limiting sense.

[0038] While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.

[0039] Any and all patents, patent applications and printed publications referred to above are incorporated by reference.