Combined Physical and Chemical Etch to Reduce Magnetic Tunnel Junction (MTJ) Sidewall Damage

Abstract

A process flow for forming magnetic tunnel junction (MTJ) nanopillars with minimal sidewall residue and minimal sidewall damage is disclosed wherein a pattern is first formed in a hard mask that is an uppermost MTJ layer. Thereafter, the hard mask sidewall is etch transferred through the remaining MTJ layers including a reference layer, free layer, and tunnel barrier between the free layer and reference layer. The etch transfer may be completed in a single RIE step that features a physical component involving inert gas ions or plasma, and a chemical component comprised of ions or plasma generated from one or more of methanol, ethanol, ammonia, and CO. In other embodiments, a chemical treatment with one of the aforementioned chemicals, and a volatilization at 50 C. to 450 C. may follow an etch transfer through the MTJ stack with an ion beam etch or plasma etch involving inert gas ions.

Claims

1. A method of etching a magnetic tunnel junction (MTJ) stack of layers, comprising: (a) providing a MTJ stack of layers on a first electrode wherein the MTJ stack of layers includes an uppermost hard mask layer, and a first stack of layers comprising a reference layer, a free layer, and a tunnel barrier layer between the reference layer and free layer; (b) forming a pattern in the hard mask layer with a first etch step that is an ion beam etch (IBE) with inert gas, or a reactive ion etch (RIE) with a fluorocarbon or chlorocarbon, wherein the pattern has a sidewall that extends from a hard mask top surface to a top surface of the first stack of layers; and (c) forming a pattern in the first stack of layers with a sidewall that forms a continuous surface with the sidewall in the hard mask layer, and that extends to a top surface of the first electrode, wherein the pattern in the first stack of layers is produced by a second etch step that combines a physical component in the form of noble gas ions or plasma, and a chemical component in the form of ions or plasma of one or more chemicals selected from methanol, ethanol, ammonia, N.sub.2O, H.sub.2O.sub.2, H.sub.2O, and carbon monoxide.

2. The method of claim 1 further comprised of a volatilization step to remove volatile residue on the first stack sidewall after the second etch step.

3. The method of claim 2 wherein the volatilization step is an IBE or plasma sputter etch comprised of an inert gas and a RF or DC power.

4. The method of claim 3 wherein the IBE or plasma sputter etch during the volatilization step generates inert gas ions or plasma that are directed with a certain penetration angle between 0 and 90 for IBE, or orthogonal with respect to a top surface of the first electrode for plasma sputter etching.

5. The method of claim 2 wherein the volatilization step is a thermal treatment comprised of a temperature between about 50 C. and 450 C.

6. The method of claim 1 wherein the one or more chemicals in the second etch step further includes oxygen.

7. The method of claim 1 wherein a direction of ions and plasma in the second etch step is essentially orthogonal to the top surface of the first electrode.

8. The method of claim 1 wherein the ions and plasma in the second etch step are generated with a RF power in the range of 600 to 3000 Watts.

9. The method of claim 5 wherein the volatilization step is further comprised of a flow rate of an inert gas, or a flow rate of oxygen and an inert gas.

10. A method of etching a magnetic tunnel junction (MTJ) stack of layers, comprising: (a) providing a MTJ stack of layers on a first electrode wherein the MTJ stack of layers includes an uppermost hard mask layer, and a first stack of layers comprising a reference layer, a free layer, and a tunnel barrier layer between the reference layer and free layer; (b) forming a pattern in the hard mask layer with a first etch step that is an ion beam etch (IBE) with inert gas, or a reactive ion etch (RIE) with a fluorocarbon or chlorocarbon, wherein the pattern has a sidewall that extends from a hard mask top surface to a top surface of the first stack of layers; and (c) forming a pattern in the first stack of layers with a sidewall that forms a continuous surface with the sidewall in the hard mask layer, and that extends to a top surface of the first electrode, wherein the pattern in the first stack of layers is produced by a process sequence, comprising: (1) a first step that is an IBE with an inert gas; and (2) a second step that is a chemical treatment to convert a non-volatile residue that is formed on the continuous surface during the first step to a volatile residue.

11. The method of claim 10 further comprised of performing a volatilization step to remove the volatile residue on the continuous surface after the chemical treatment.

12. The method of claim 11 wherein the volatilization step comprises a second IBE or plasma sputter etch step with an inert gas and a RF power less than 100 Watts.

13. The method of claim 11 wherein the volatilization step comprises one or both of a thermal treatment at a temperature between about 50 C. and 450 C., and introduction of oxygen into the chamber.

14. The method of claim 10 wherein the chemical treatment comprises one or more of methanol, ethanol, NH.sub.3, and CO.

15. The method of claim 14 wherein the chemical treatment further comprises oxygen.

16. The method of claim 10 wherein the second step further comprises the addition of an inert gas and a temperature in the range of 25 C. to 150 C. to remove the volatile residue.

17. The method of claim 14 wherein the chemical treatment further comprises RIE conditions with a RF power from about 100 to 800 Watts that generates a plasma.

18. A method of etching a magnetic tunnel junction (MTJ) stack of layers, comprising: (a) providing a MTJ stack of layers on a first electrode wherein the MTJ stack of layers includes an uppermost hard mask layer, and a first stack of layers comprising a reference layer, a free layer, and a tunnel barrier layer between the reference layer and free layer; (b) forming a pattern in the hard mask layer with a first etch step that is an ion beam etch (IBE) with inert gas, or a reactive ion etch (RIE) with a fluorocarbon or chlorocarbon, wherein the pattern has a sidewall that extends from a hard mask top surface to a top surface of the first stack of layers; and (c) forming a pattern in the first stack of layers with a sidewall that forms a continuous surface with the sidewall in the hard mask layer, and that extends to a top surface of the first electrode, wherein the pattern in the first stack of layers is produced by a second etch step comprising RIE conditions with one or more chemicals selected from methanol, ethanol, N.sub.2O, H.sub.2O.sub.2, H.sub.2O, ammonia, and carbon monoxide.

19. The method of claim 18 further comprised of performing a volatilization step to remove volatile residue on the first stack sidewall after the second etch step wherein the volatilization step comprises an IBE or plasma sputter etching with an inert gas.

20. The method of claim 18 further comprised of performing a volatilization step after the second etch step wherein the volatilization step comprises a temperature in the range of 50 C. to 450 C.

21. The method of claim 19 wherein the IBE or plasma sputter etching further comprises introducing a flow of oxygen with the inert gas.

22. The method of claim 20 wherein the volatilization step further comprises introducing a flow of oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a cross-sectional view of a MTJ stack of layers on which a photoresist pattern has been formed, and shows ions used to transfer the pattern through an underlying BARC or DARC during an etch sequence of the present disclosure.

[0013] FIG. 2 is a cross-sectional view of a MTJ stack in FIG. 1 after an etch process transfers the pattern through the uppermost hard mask MTJ layer according to an embodiment of the present disclosure.

[0014] FIG. 3 is cross-sectional view of a MTJ nanopillar after an etch that combines physical and chemical components is used to transfer the hard mask pattern in FIG. 2 through the remaining MTJ stack of layers according to an embodiment described herein.

[0015] FIG. 4a is a cross-sectional view of the MTJ stack in FIG. 2 after an ion beam etch transfers the hard mask pattern through the MTJ stack of layers and causes residue to form on MTJ sidewalls.

[0016] FIG. 4b is a cross-sectional view of the MTJ stack in FIG. 2 after a reactive ion etch transfers the hard mask pattern through the MTJ stack and causes residue to form on MTJ sidewalls.

[0017] FIG. 5 is a cross-sectional view of the MTJ nanopillar in FIG. 4a or FIG. 4b after a chemical treatment convents non-volatile residue to volatile residue on MTJ sidewalls according to an embodiment of the present disclosure.

[0018] FIG. 6 is a cross-sectional view of the MTJ nanopillar in FIG. 5 after a volatilization step involving IBE, plasma sputter etch, or thermal treatment is used to remove the volatile residue according to an embodiment of the present disclosure.

[0019] FIG. 7 is a cross-sectional view of the MTJ nanopillar in FIG. 6 following deposition of an encapsulation layer and planarization to electrically isolate the MTJ nanopillar from adjacent MTJ nanopillars.

[0020] FIG. 8 is a top-down view of a plurality of MTJ nanopillars having a circular shape in an array of rows and columns according to an embodiment of the present disclosure.

[0021] FIG. 9 is a flow diagram showing a sequence of steps of forming a MTJ nanopillar according to an embodiment of the present disclosure.

[0022] FIGS. 10-11 are flow diagrams showing alternative sequences of forming MTJ nanopillars having sidewalls that are essentially free of damage and residue according to embodiments of the present disclosure.

[0023] FIG. 12 is a plot of magnetoresistive ratio vs. MTJ size for MTJs that have been ion beam etched with no post-cleaning process while FIG. 13 is a similar plot for MTJ nanopillars formed by a combined physical/chemical etch followed by an Ar plasma etch volatilization step according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0024] The present disclosure is a method of etching a MTJ stack of layers wherein all layers below the hard mask are removed with a single etch process comprised of both physical and chemical components to form a MTJ nanopillar with sidewalls that have substantially less sidewall damage compared with conventional methanol based RIE, and minimal residue. An alternative process sequence is provided where the physical and chemical etchants are alternated, or are in separate steps and followed by a chemical treatment and an optional volatilization process to achieve improved MTJ performance. Although only one MTJ nanopillar is depicted in the drawings with cross-sectional views, one skilled in the art will appreciate that a plurality of MTJ nanopillars is formed in a typical memory device pattern. A process is defined as a method that includes one or more steps, and a sequence or process flow according to the present disclosure refers to two or more processes in succession.

[0025] Referring to FIG. 1, a MTJ stack of layers 1 that will eventually become a plurality of MTJ nanopillars in a MRAM or STT-MRAM is shown from a cross-sectional view. The y-axis is perpendicular to the planes of the layers in the MTJ stack. There is a substrate 10 that in one embodiment is a bottom electrode in a memory device. The bottom electrode may be a multilayer structure and is typically embedded in a dielectric layer (not shown).

[0026] MTJ stack 1 is laid down on the substrate 10 and in the exemplary embodiment has a bottom spin valve configuration wherein a seed layer 11, pinned or reference layer 12, tunnel barrier 13, free layer 14, and hard mask 15 are sequentially formed on the substrate. Each of the reference layer and free layer preferably have PMA with a magnetization aligned in a y-axis direction. In other embodiments, at least one additional layer may be included in the aforementioned MTJ stack such as a Hk enhancing layer between the free layer and hard mask that enhances PMA in the free layer. The seed layer may be comprised of one or more of NiCr, Ta, Ru, Ti, TaN, Cu, Mg, or other materials typically employed to promote a smooth and uniform grain structure in overlying layers.

[0027] The reference layer 12 may have a synthetic anti-parallel (SyAP) configuration represented by AP2/Ru/AP1 where a anti-ferromagnetic coupling layer made of Ru, Rh, or Ir, for example, is sandwiched between an AP2 magnetic layer and an AP1 magnetic layer (not shown). The AP2 layer, which is also referred to as the outer pinned layer is formed on the seed layer while AP1 is the inner pinned layer and typically contacts the tunnel barrier. AP1 and AP2 layers may be comprised of CoFe, CoFeB, Co, or a combination thereof. In other embodiments, the reference layer may be a laminated stack with inherent PMA such as (Co/Ni).sub.n, (CoFe/Ni).sub.n, (Co/NiFe).sub.n, (Co/Pt).sub.n, (Co/Pd).sub.n, or the like where n is the lamination number. Furthermore, a transitional layer such as CoFeB or Co may be inserted between the uppermost layer in the laminated stack and the tunnel barrier layer.

[0028] The tunnel barrier layer 13 is preferably a metal oxide that is one of MgO, TiOx, AITiO, MgZnO, Al.sub.2O.sub.3, ZnO, ZrOx, HfOx, or MgTaO. More preferably, MgO is selected as the tunnel barrier layer because it provides the highest magnetoresistive ratio, especially when sandwiched between two CoFeB layers, for example.

[0029] The free layer 14 may be Co, Fe, CoFe, or an alloy thereof with one or both of B and Ni, or a multilayer stack comprising a combination of the aforementioned compositions. In another embodiment, the free layer may have a non-magnetic moment diluting layer such as Ta or Mg inserted between two CoFe or CoFeB layers that are ferromagnetically coupled. In an alternative embodiment, the free layer has a SyAP configuration such as FL1/Ru/FL2 where FL1 and FL2 are two magnetic layers that are antiferromagnetically coupled, or is a laminated stack with inherent PMA described previously with respect to the reference layer composition.

[0030] The hard mask 15 is also referred to as a capping layer and is typically comprised of one or more of Ta, Ru, TaN, Ti, TiN, and W. It should be understood that other hard mask materials including MnPt may be selected in order to provide high etch selectivity relative to underlying MTJ layers during an etch process that forms MTJ nanopillars with sidewalls that stop on the bottom electrode. All layers in the MTJ stack may be deposited in a DC sputtering chamber of a sputtering system such as an Anelva C-7100 sputter deposition system that includes ultra high vacuum DC magnetron sputter chambers with multiple targets and at least one oxidation chamber. Usually, the sputter deposition process involves an argon sputter gas and a base pressure between 510.sup.8 and 510.sup.9 torr.

[0031] Once all of the layers 11-15 are laid down, the MTJ stack 1 may be annealed by heating to a temperature between about 360 C. to 400 C. for a period of up to a plurality of hours to grow a bcc structure in the reference layer, free layer, and tunnel barrier layer thereby enhancing PMA in the reference layer and free layer. The matching crystal structure in the aforementioned layers is also believed to improve the magnetoresistive ratio in the resulting MTJ nanopillars.

[0032] As a first step in the MTJ patterning process according to the present disclosure, a BARC or DARC layer 16, and a photoresist layer 17 are sequentially coated on the top surface 15t of the hard mask. BARC or DARC with top surface 16t has a refractive index that minimizes reflection of light during the subsequent patternwise exposure thereby enabling more uniform island shapes with less CD variation to be formed in the photoresist layer. Next, a conventional patternwise exposure and developer sequence is employed to form a pattern in the photoresist layer that comprises a plurality of islands with sidewall 20. As indicated later by a top-down view in FIG. 8, the islands are in an array with a plurality of rows and columns. However, only one island is shown in FIG. 1 in order to simplify the drawing. Each island has a critical dimension w1 that in some embodiments is between 60 nm and 100 nm, and in other embodiments is from about 10 nm to 60 nm that corresponds to the CD required in state of the art memory devices. Note that some devices are circular such that w1 is formed in both of the x-axis and z-axis directions. However, the top-down shape of island 17 may be an ellipse or a polygon such that the z-axis dimension is different from the x-axis dimension.

[0033] In the initial etch step 30 that may be an IBE with one or more of Ar, Kr, Xe, or Ne, or may comprise RIE with a fluorocarbon or chlorocarbon gas, the island shape in photoresist layer 17 is transferred through the BARC or DARC layer 16. Accordingly, sidewall 20 now extends from a top surface of the photoresist layer to a top surface 15t of the hard mask 15, and CD w1 is duplicated in the DARC or BARC layer. The photoresist layer may then be removed with a well known method, or is etched away during subsequent etch processes.

[0034] Referring to FIG. 2, a second etch process 31 is performed to transfer the island shape with sidewall 20 and CD w1 through the hard mask 15. In some embodiments, a second IBE with inert gas, or a second fluorocarbon or chlorocarbon based RIE may be used. Moreover, the second IBE or RIE may include oxygen. However, in other embodiments, the present disclosure anticipates that the conditions employed for etch process 31 are essentially the same as applied in etch process 30 such that the etch transfer through the hard mask may be a continuation of the etch transfer through DARC or BARC layer 16. In other words, etch 30 in FIG. 1 may be continued until stopping on top surface 14t of the free layer. As mentioned earlier, the etch transfer process through the hard mask is likely to remove any remaining photoresist layer 17 because the etch rate through the latter is generally high relative to the hard mask, and the hard mask may be substantially thicker than the remaining photoresist layer once etch process 30 clears the exposed DARC or BARC layer 16. In some embodiments, a passivation step comprised of applying oxygen plasma or flowing oxygen into the process chamber immediately after the second RIE or IBE is completed, and without breaking a vacuum, is performed to generate a smoother sidewall 20.

[0035] In the embodiments described herein, it should be understood that IBE typically comprises rotating the work piece (wafer) on which the MTJ stack of layers is formed. Moreover, the incident or penetration angle of noble gas ions directed at the wafer surface may be between 0 and 90. IBE may be employed in one or more of hard mask etching, MTJ etching, cleaning, and volatilization steps described in later sections. On the other hand, RIE is used only for hard mask or MTJ etching, involves a chemical reactant and stationary wafer, and the resulting plasma is limited to a 90 direction or orthogonal to the wafer surface. According to the present disclosure, a plasma sputter etch is employed only for volatilization or cleaning steps, comprises a noble gas, and is also limited to a 90 direction (orthogonal to wafer surface).

[0036] Referring to FIG. 3, a key feature according to a first embodiment of the present disclosure is a single RIE step 32m that effectively transfers the island shape in the hard mask through all of the underlying MTJ layers 11-14 thereby forming MTJ nanopillar 1a. Etch step 32m comprises a physical component represented by inert gas ions or plasma, and a chemical component comprising ions or plasma of a chemical species that are generated with reactive ion etch conditions. Here, inert gas is defined as a noble gas that is one of Ar, Kr, Ne, and Xe. Thus, both of an inert gas and one or more chemicals including methanol, ethanol, H.sub.2O.sub.2, H.sub.2O, N.sub.2O, NH.sub.3, and CO are fed into an etch chamber while a plasma is induced with a RF power between 600 Watts and 3000 Watts at a temperature proximate to room temperature. It should be understood that the RF power applied to a top electrode may be different than the RF power applied to a bottom electrode in a RIE chamber. Typically, one or more wafers are held on the bottom electrode during RIE processing. The resulting ions and plasma in a preferred embodiment are directed orthogonal to the top surface of the substrate along the y-axis direction. As a result, sidewall 20 is a continuous surface extending from a top surface 15t of hard mask 15 to top surface 10t of the bottom electrode.

[0037] In a preferred embodiment, the sidewall 20 is substantially vertical such that CD w1 is substantially maintained in all MTJ layers 11-15. Note that sidewall angle tends to become more vertical as the methanol (or chemical) content increases in the noble gas/chemical mixture of step 32m. For example, angle may be proximate to 75 when noble gas content is 100% but becomes substantially equal to 90 with a chemical content around 50% or greater. Here, the term content refers to flow rate ratio. Therefore, a 50:50 flow rate ratio of noble gas:chemical indicates a 50% chemical content and 50% noble gas content in the RIE gas mixture. Moreover, we have surprisingly found the combined physical and chemical etching provides for a sidewall that has substantially reduced sidewall damage compared with conventional methanol based RIE, and minimal residue.

[0038] In the exemplary embodiment, a thickness t of the DARC or BARC layer remains after the etch transfer. However, depending on the initial thickness and composition of layer 16, and the etch conditions, the DARC or BARC layer may be completely removed during etch process 32m such that hard mask top surface 15t is exposed. Therefore, a hard mask 15 is advantageously selected that has a high etch rate selectivity to the underlying MTJ layers so that a substantial thickness of hard mask remains after etch process 32m.

[0039] An optional volatilization step 34v illustrated in FIG. 6 may be employed to remove any residue that has accumulated on sidewall 20 at the end of etch step 32m. The volatilization preferably comprises an IBE or plasma sputter etch with inert gas wherein Ar+ ions or ions of Kr, He, or Ne that are generated with a RF or DC power are directed orthogonal or with a certain penetration angle toward a top surface 10t of the substrate. The wafer (not shown) on which the MTJ nanopillar 1 is formed is typically rotated during IBE or is static for a plasma sputter etch during step 34v. In another embodiment, the volatilization is a thermal treatment in an inert or oxidant atmosphere at a temperature from 50 C. to 450 C.

[0040] According to a second embodiment shown in FIGS. 4a-4b, FIG. 5, and FIG. 6, the present disclosure encompasses a process flow where the chemical component in the combined etch of the first embodiment is separated from the physical etch component. One or two optional steps related to a chemical treatment (FIG. 5) and a volatilization (FIG. 6) may be included to realize a MTJ nanopillar 1a with minimal residue and with substantially reduced sidewall damage compared with conventional methanol based RIE in the prior art.

[0041] In FIG. 4a, an IBE 32i with inert gas is employed to remove MTJ layers 11-14 that are not protected by the DARC or BARC layer 16, and by hard mask 15. As a result, MTJ nanopillar 1a is generated but has substantial non-volatile residues 19 on sidewall 20 and on substrate top surface 10t surrounding the MTJ nanopillar. Thereafter, in FIG. 5, a chemical treatment 33 may be performed to convert the non-volatile residues to volatile residues 19x on the sidewall. Although not bound by theory, it is believed the chemical treatment converts metal residue such as Ta to an oxide thereof wherein the oxide is more volatile than the metal residue. The chemical treatment comprises one or more of methanol, ethanol, and ammonia, and a temperature between room temperature and 150 C. The chemical treatment is performed in a chamber within an Anelva mainframe or the like, or may be conducted in a stand-alone process tool outside a mainframe although the latter option slows throughput. In some cases, the one or more chemicals are injected into the chamber with the substrate. Moreover, oxygen may be added to the chemical treatment chamber to enhance the rate of oxidation of the non-volatile residue 19. Alternatively, a plasma may be generated from the one or more chemicals with RIE conditions in an etch chamber. Preferably, the applied RF power is sufficiently low so that no significant damage occurs to MTJ sidewall 20. For example, the RF power may be maintained between 100 Watts and 800 Watts.

[0042] Referring to FIG. 6, volatilization step 34v may be performed to remove the volatile residue 19x and comprises an IBE or a plasma sputter etch with Ar or another inert gas flow, or a thermal treatment in an etch chamber at a temperature between 50 C. and 450 C. for a period up to 5 minutes. In either of the IBE, plasma sputter etch, or thermal treatment embodiments, an oxygen flow may be added to the inert gas flow during the volatilization step. Step 34v may have a preset endpoint time of duration based on a separate experimental study that establishes a volatilization time for a typical thickness of residue 19x. When Ar plasma sputter etching is selected, RF power is preferably kept at 100 Watts or below to avoid damage to MTJ sidewalls. Preferably, the plasma sputter etch is followed immediately in the same process chamber by an encapsulation process described later.

[0043] The present disclosure also encompasses an embodiment wherein steps 33 and 34v are performed simultaneously following etch process 32i. In particular, one or more of methanol, ethanol, NH.sub.3, and CO may be introduced into an etch chamber along with an inert gas flow. A thermal treatment may be applied at a temperature between 50 C. and 450 C. while the gas mixture is in the etch chamber. In some embodiments, a plasma is generated with a RF power while the gas mixture is in the chamber and with a temperature in the range of 50 C. to 150 C. Alternatively, a plasma sputter etch may be performed at a temperature proximate to room temperature.

[0044] In a third embodiment that represents a modification of the process flow in the second embodiment, a reactive ion etch 32r shown in FIG. 4b and comprising one or more chemicals including methanol, ethanol, NH.sub.3, and CO is inserted either before or after step 32i in FIG. 4a. In this case, chemical treatment 33 may not be necessary since step 32r may oxidize non-volatile residues 19 generated by a preceding step 32i to yield volatile residue 19x on sidewall 20. Volatilization step 34v may be employed after steps 32i and 32r to remove any volatile residue that remains on sidewall 20 of MTJ nanopillar 1a.

[0045] In a fourth embodiment, the process flow in the second embodiment is modified such that etch step 32i is replaced by etch step 32r. Chemical treatment step 33 is usually not necessary since step 32r has a tendency to serve the same purpose of oxidizing any non-volatile residue formed on sidewall 20. An optional volatilization step 34v may follow step 32r to remove any volatile residue formed on sidewall 20 of MTJ nanopillar 1a.

[0046] Referring to FIG. 7, an encapsulation layer 25 that is comprised of a dielectric material is deposited over the MTJ array after clean sidewalls 20 are formed according to one of the preceding embodiments. Preferably, the encapsulation layer has a thickness of 5-250 nm and is one or more of SiN, SiO.sub.2, MgO, Al.sub.2O.sub.3, AlN, BN, or the like that is deposited immediately after volatilization (in-situ) by physical vapor deposition (PVD), chemical vapor deposition (CVD), ion beam deposition (IBD), or atomic layer deposition (ALD) without breaking the vacuum in the process chamber previously employed for plasma sputter etching in step 34v. In embodiments where a thermal treatment in a stand-alone tool is employed for step 34v, then the wafer must be exposed to atmosphere and moved to the encapsulation process chamber.

[0047] Thereafter, a chemical mechanical polish (CMP) process is performed to form a top surface 25t on the encapsulation layer that is coplanar with top surface 15t on hard mask 15. In some embodiments, the CMP process removes any DARC or BARC layer 16 remaining after etch transfer step 32m, 321, or 32r in the previously described embodiments.

[0048] Referring to FIG. 8, the plurality of island shapes of the MTJ nanopillars formed by an etch process or process flow of the present disclosure is depicted from a top-down view after removing overlying layers in the memory structure. MTJ nanopillar 1a is shown in the same row as MTJ nanopillar 1b, and MTJ nanopillars 1c, 1d are in the same column as MTJ nanopillars 1a and 1b, respectively. As explained previously, the MTJ nanopillars are depicted with a circular shape but may have elliptical shapes in other embodiments. Generally, millions of MTJ nanopillars are formed in an array but only four are illustrated here to simplify the drawing.

[0049] Thereafter, a top electrode layer comprised of a plurality of parallel conductive lines (not shown) is formed by a conventional method on the MTJ nanopillars and encapsulation layer 25 as appreciated by those skilled in the art. A first top electrode line may contact a top surface 15t of MTJ nanopillars 1a, 1c while a second top electrode line contacts top surface 15t in MTJ nanopillars 1b, 1d. Conductive lines in the top electrode layer are preferably formed along the z-axis direction that is orthogonal to the conductive lines along the x-axis direction in the bottom electrode layer. Therefore, bottom electrode line 10 may contact a bottom surface of both MTJ nanopillars 1a and 1b while a second bottom electrode line 10-1 contacts the bottom surfaces of MTJ nanopillars 1c and 1d.

[0050] Referring to FIG. 9, a flow diagram is provided for the process flow of the first embodiment where a pattern with a CD is formed in the MTJ hard mask layer by an etch process 100 that comprises step 30 or both steps 30, 31 described previously. In step 110, a combined physical/chemical etch 32m is applied to transfer the pattern through the remaining MTJ layers and thereby form a plurality of MTJ nanopillars. An optional volatilization step 114 comprising IBE, plasma etching, or a thermal treatment is performed after step 110. Finally, an encapsulation layer is formed around the MTJ nanopillars in step 120 to electrically isolate the MTJ nanopillars from each other.

[0051] In FIG. 10, a flow diagram is depicted for the process flow of the second embodiment of the present disclosure where step 100 is followed by a physical (IBE) etch 32i in step 111, a chemical treatment 33 in step 113, volatilization process 34v in step 114, and then encapsulation with step 120.

[0052] FIG. 11 shows a flow diagram for the process flow of the third embodiment of the present disclosure where step 100 is followed by a chemical (RIE) etch 32r in step 112, an IBE step 111, optional volatilization step 114, and finally encapsulation in step 120. In alternative embodiment described previously, IBE step 111 may precede RIE step 112, followed by optional step 114, and step 120.

[0053] We have demonstrated the benefits of the combined physical/chemical etch process of the present disclosure with results from an experiment where a series of MTJ nanopillars with various diameters (w1 in FIG. 8), and reference MTJ nanopillars with the same range of device sizes were fabricated. All process flows described below included etching a DARC layer/Ta hard mask stack with conditions comprising CF.sub.4 RIE, 500 Watts (top)/52 W (bottom), 50 sccm CF.sub.4 only, and 4.5 mT of pressure followed by a passivation step comprised of flowing oxygen into the etching chamber. Each MTJ stack of layers in all devices included a MgO tunnel barrier layer between a CoFeB free layer and a CoFeB reference layer, a Ta hard mask, and a TaN/NiCr seed layer.

[0054] Following the etch through the hard mask, reference MTJ nanopillars were fabricated by a conventional method involving an Ar IBE comprised of 450 mm IBS (Ion Beam Source) and 800 W of IBS RF power, 200V/950V of G1/G2 voltage, 400 mA of G1 current, 60 rpm rotation, 40 and 80 penetration angles with no subsequent volatilization process. The wafers were exposed to air between MTJ etching and encapsulation.

[0055] According to a process described in the first embodiment, a MTJ stack of layers 11-14 (FIG. 3) with an overlying patterned hard mask layer 15 was etched with a RIE step comprising a 50% Ar/50% methanol mixture and a RF power of 1500 Watts (top)/1100 Watts (bottom) for 60 seconds. The Ar flow rate was 7.5 standard cubic centimeters per minute (sccm) and the CH.sub.3OH flow rate was 7.5 sccm. Thereafter, a volatilization step comprised of Ar plasma sputter etching with a RF power of 75 Watts and a 60 sccm flow rate of Ar was employed for a period of 132 seconds at room temperature.

[0056] FIG. 12 is a plot of magnetoresistive ratio (DRR) vs. MTJ size (measured at 125 C.) for the reference MTJ nanopillars that were patterned by the IBE step. MTJ nanopillar size ranges from about 30 nm to 300 nm. Circled region 60 indicates a significant population of low tails corresponding to devices where redeposited metal residue bridges the MgO tunnel barrier layer and causes shorts.

[0057] In FIG. 13, results are plotted for MTJ nanopillars that were etched by the 50/50 Ar/CH.sub.3OH etch, and then subjected to a volatilization step with Ar plasma sputter etch There is clearly a substantial reduction in the number of devices with low DRR and thus a higher overall DRR, especially for MTJ nanopillar sizes between 30 and 100 nm. DRR is also more uniform for each MTJ size in FIG. 13 compared with FIG. 12, which indicates a more controlled process that is suitable for a manufacturing environment. Furthermore, the etch process embodiments disclosed herein may be readily implemented in existing manufacturing lines since no new tools or materials are required.

[0058] While this disclosure has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this disclosure.