Sloped termination in molybdenum layer and method for making the same

Abstract

A method (800) for forming a sloped termination of a molybdenum layer comprises providing a molybdenum layer (802) and applying a photoresist material to the molybdenum layer (804). The photoresist material is exposed to light under defocus conditions to form a mask (806), wherein edges of the exposed photoresist material correspond to the sloped termination. The molybdenum layer is etched with an etching material, wherein the etching material at least partially etches the photoresist material exposed under defocus conditions, and wherein the etching produces the sloped termination (808).

Description

Background Art

[0001] Some components in integrated circuits are made from layers of molybdenum (Mo), titanium tungsten (TiW), and aluminum nitride (AlN). One such device is a bulk acoustic wave (BAW) device, which has a stack of Mo, TiW, and AlN layers. The AlN forms an acoustic resonator with the Mo layer, and the Mo layer serves as the electrode at each end of the acoustic resonator. Summary of the Invention

[0002] In the described example, a method for forming a sloped termination of a molybdenum layer includes providing a molybdenum layer and applying a photoresist material to the molybdenum layer. The photoresist material is exposed under defocused conditions to form a photoresist mask having an edge portion. The molybdenum layer is etched at least at the edge portion of the photoresist mask to form the sloped termination of the molybdenum layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Figure 1 A side cross-sectional view of an aluminum nitride piezoelectric matrix acoustic wave device.

[0004] Figure 2 for Figure 1 Expanded view of the molybdenum terminations within a bulk acoustic wave device.

[0005] Figure 3 for Figure 1 Expanded view of another molybdenum terminal within the BAW device.

[0006] Figure 4 Developed view of the joint formed near the molybdenum terminal.

[0007] Figure 5 For the preparation process Figure 1 Side cross-sectional view of a bulk acoustic wave device.

[0008] Figure 6 After the application of the photolithography step and the removal of the remaining portion of the photoresist material Figure 5 A side cross-sectional view of the device.

[0009] Figure 7 After etching Figure 6 A side cross-sectional view of the device.

[0010] Figure 8 A flow chart illustrating an embodiment of a method for producing a beveled termination in a molybdenum layer. DETAILED DESCRIPTION

[0011] In the described examples, components are fabricated on integrated circuits by stacking together multiple layers. These devices are susceptible to cracks and seams at or near the terminations of the molybdenum layers in the stack. For example, a device having molybdenum as one or more layers in a stack is susceptible to cracks or seams where the molybdenum layers terminate at sharp corners in the stack. Sharp corners weaken other materials in the stack and can cause cracks to form, which can propagate through the stack. In some cases, sharp corners cause seams or wrinkles to form, which also propagate through seams. Both cracks and seams can cause component failure at the location of the stack. The devices and methods described herein reduce the angle at the terminations of the molybdenum layers, which reduces the likelihood of cracks and seams forming in the stack.

[0012] One example of a component fabricated with a stack having multiple layers is a bulk acoustic wave (BAW) device. Figure 1 is an example of an aluminum nitride piezoelectric-based BAW device 100 fabricated using multiple layers. The BAW device 100 may be part of a larger integrated circuit (not shown). The layers form a stack 102, which results in a rise near the area where the BAW device 100 is located. An acoustic resonator 106 is formed within the stack 102. Figure 1 In the example of FIG, resonator 106 includes an aluminum nitride (AlN) layer 110 sandwiched between a top electrode 112 formed of molybdenum and a bottom electrode 114 also formed of molybdenum. Top electrode 110 and bottom electrode 112 are layers fabricated within stack 102. Layers described herein are sometimes referred to as thin films. Figure 1 In the example shown, the oxide layer 116 is located between the top electrode 112 and the AlN film 110 and serves as a temperature compensation layer for the resonator 106. In some embodiments, the BAW device 100 does not include the oxide layer 116. The acoustic wave bounces between the top electrode 112 and the bottom electrode 114 to resonate within the resonator 106.

[0013] The stack 102 includes an upper Bragg mirror 120 and a lower Bragg mirror 122. The Bragg mirrors 120 and 122 prevent energy from escaping the resonator 106 and are used to achieve conventional resonator specifications. The lower Bragg mirror 122 is formed of a plurality of alternating layers or thin films. Figure 1 The example is alternating layers of titanium, tungsten and oxide. Figure 1The lower Bragg mirror 122 has a first oxide layer 130 adjacent to the bottom electrode 114 and extending the length of the BAW device 100. A first titanium tungsten layer 132 is located between the first oxide layer 130 and the second oxide layer 134. The second oxide layer 134 extends the length of the BAW device 100 like the first oxide layer 130. A second titanium tungsten layer 136 is located between the second oxide layer 134 and the third oxide layer 138. A substrate (not shown) is located below the third oxide layer 138 and is a material such as silicon on which the BAW device 100 is fabricated. Both the first titanium tungsten layer 132 and the second titanium tungsten layer 136 are located below the resonator 106 and terminate near the edge of the resonator 106. In some embodiments, the seed layer ( Figure 1 ) is located between the first oxide layer 130 and the bottom electrode 114.

[0014] The upper Bragg mirror 120 is formed of a plurality of alternating layers that are similar or identical to the layers forming the lower Bragg mirror 122. Figure 1 In the example of FIG. 1 , the upper Bragg mirror 120 includes a first oxide layer 140 located adjacent to the upper electrode 112 and extending the length of the BAW device 100. The BAW device 100 has two Figure 1 14 and 144. The connectors 142 and 144 are shown in cross-section, and they may be relatively small. Therefore, portions of the oxide layer in the upper Bragg mirror 120 extend around the connectors 142 and 144. A first titanium-tungsten layer 146 is sandwiched between the first oxide layer 140 and the second oxide layer 148. The second oxide layer 148 extends the length of the BAW device 100. A second titanium-tungsten layer 150 is sandwiched between the second oxide layer 148 and the third oxide layer 152. The third oxide layer 152 also extends the length of the BAW device 100. The first titanium-tungsten layer 146 and the second titanium-tungsten layer 150 terminate at the edge of the resonator 106.

[0015] The top electrode 112 terminates at a first terminal 160 and a second terminal 162, and the bottom electrode 114 terminates at a first terminal 164 and a second terminal 166. In conventional devices, the terminals 160-166 are fabricated using conventional fabrication techniques, which result in steep termination angles. In addition to being generally steep, the termination angles at the terminals 160-166 cannot be maintained generally within a predetermined range or below a predetermined angle. Figure 2 is an expanded view of terminal 160, Figure 3 is the expanded view of terminal 162. Angle α is the termination angle at terminal 160. Figure 2 As shown, the termination angle α is approximately 70 degrees, and may range between 40 and 90 degrees. Figure 3, the termination angle β at terminal 162 is also approximately 70 degrees and may be in the range of 40 to 90 degrees. The termination angles of terminals 164 and 166 are similar to the termination angle α at terminal 160 and the termination angle β at terminal 162. The fabrication method described herein reduces the termination angles α and β to less than a predetermined angle, such as 30 degrees, thereby preventing the formation of cracks and seams within the layers of BAW device 100. In some embodiments, the termination angles α and β are between 12 and 30 degrees. It has been found that large termination angles in the molybdenum layer cause seams and / or cracks to form in the layers of BAW device 100.

[0016] Reference Figure 1 The effect of the steep termination angle at the terminals 160-166 is shown. A crack 170 is formed from the terminal 164 and extends through the AlN layer 110 into the upper Bragg mirror 120. In addition, a seam 172 is formed on the terminal 166, as shown in FIG. Figure 4 170 and the seam 172 interfere with the resonator 106 and the Bragg reflectors 120 and 122, which may change the resonance and quality of the acoustic waves resonating in the resonator 106. Moreover, the crack 170 and the seam 172 can cause a discontinuity in the top molybdenum electrode 112 between the resonator 106 and the electrical contact 142. Therefore, the crack 170 and / or the seam 172 can cause the BAW device 100 to malfunction, depending on the location of the crack 170 and / or the crack 172. The crack and the seam may be near any molybdenum termination in the BAW device 100 or on other devices where the molybdenum layer ends at a steep angle.

[0017] The devices described herein are not subject to the formation of cracks and seams like conventional devices because the fabrication technique maintains the termination angle in the molybdenum layer below a predetermined value, which may be 12 to 30 degrees. It has been found that a termination angle of 30 degrees or less at the termination of the molybdenum layer reduces the likelihood of cracks and seams occurring in the stack. Conventional molybdenum layer terminations are made using metal dry or wet etching processes that result in terminations with steep angles, such as greater than 70 degrees. Some processes use tilted metal wet etching techniques, but this technique produces inconsistent termination angles, which may be greater than 30 degrees. The process described herein results in termination angles in the molybdenum layer that are consistently less than 30 degrees.

[0018] Figure 5 is a side view of the device 500 during the fabrication process. The fabrication process described here is mainly about the fabrication of the molybdenum layer, so the process starts with the fabrication of the molybdenum layer 502 on the oxide layer 504. In some embodiments, the molybdenum layer 502 is formed on a very thin aluminum nitride seed layer 503, which may extend to cover the oxide layer 504. Figure 1In the BAW device 100, the molybdenum layer 502 corresponds to the bottom electrode 114, and the oxide layer 504 corresponds to the first oxide layer 130. The thickness of the molybdenum layer 502 varies depending on the type of device in which it is located. In some examples, the molybdenum layer 502 is approximately 400 nm.

[0019] A photoresist layer 510 is prepared on the molybdenum layer 502. The etching process etches the photoresist layer 510 faster than the molybdenum layer 502. For example, when the photoresist layer 510 and the molybdenum layer 502 are exposed to the etching material, the former etches about 3 times faster than the latter, as described below. A photolithography step is performed on the device 500, wherein the photoresist layer 510 is exposed under defocus conditions to produce the photoresist beveled edges 514 and 516. In some embodiments, the defocus is approximately +17um or -17um. In other examples, in order to achieve the desired termination angle in the finished molybdenum layer 502, the defocus is between + / -11um and + / -22um. Reference Figure 6 , which is a view of the device 500 after application of a photolithography step and removal of the remaining portions of the photoresist layer 510. The photolithography step generates a photoresist mask 600 from the photoresist layer 510, wherein, during a subsequent etching step, the portions of the molybdenum layer 502 covered by the photoresist mask 600 are not etched or are etched in proportion to the thickness of the photoresist mask 600 covering the molybdenum layer 502.

[0020] Figure 6 The photoresist layer 510 is processed during the photolithography step to remove the excess photoresist material in the photoresist layer 510 that is not exposed. The resulting photoresist layer 510 is a photoresist mask 600. Figure 6 The photoresist mask 600 is shown having beveled edges 514 and 516 located above the portion of the molybdenum layer 502 that will become the beveled termination. The defocus applied to the photoresist layer 510 during photolithography establishes the slope of the beveled edges 514 and 516, which determines the termination angle in the final molybdenum layer 502. In conventional fabrication techniques, the photoresist is exposed under best focus conditions to provide a nearly vertical photoresist sidewall profile.

[0021] The defocus during the photolithography step generates the sloped edges 514 and 516. The etched material of the molybdenum layer 502 consumes some of the photoresist mask 600 during the etching process. However, the sloped edges 514 and 516 of the photoresist mask 600 are thinner than other parts of the photoresist mask 600 and are less resistant to the etched material, so they are consumed faster. Figure 7 , which is after etching Figure 66. The thicker inner portions 604 and 606 of the beveled edges 514 and 516 are consumed more slowly than the thinner outer edges 608 and 610 of the beveled edges 514 and 516. As a result, the molybdenum layer 502 is etched more near the outer portions 608 and 610 of the beveled edges 514 and 516 than the inner portions 604 and 606, which results in the terminations 700 and 702 of the molybdenum layer 502 having desired termination angles 710 and 712. In some embodiments, a defocus of plus or minus 17 μm on the 3.5 μm photoresist layer 510 produces a termination angle of 28 degrees. In some embodiments, the angles 710 and 712 are less than 30 degrees, which reduces the likelihood of cracks and seams in the device where the molybdenum layer 502 is located.

[0022] In some embodiments, the etching of device 500 includes placing the device 500 having a photoresist mask 600 prepared thereon into a plasma etching chamber, wherein the portion of the molybdenum layer 502 not covered by the photoresist mask 600 is etched. As described above, the portion of the molybdenum layer 502 covered by the photoresist mask 600 will not be completely etched. However, as described above, the inclined edges 514 and 516 will be partially etched to produce predetermined termination angles 710 and 712. In certain embodiments, an etching process is performed in which a flowing gas mixture containing chlorine and oxygen enters the etching chamber. It has been found that a ratio of oxygen to chlorine of 4:1 further causes the termination angles 712 and 714 described above to be formed in the molybdenum layer 502. More specifically, the chlorine and oxygen mixture etches the photoresist layer 510 so that the termination angles 710 and 712 are formed adjacent to the inclined edges 514 and 516. In certain embodiments, the ratio of oxygen to chlorine is between 3:1 and 5:1. This process has been found to provide a high etch selectivity of molybdenum to aluminum nitride or oxide at greater than 100: 1. This process also produces generally straight terminals 700 and 702 rather than curved terminals.

[0023] The etching may further use a low conversion coupled plasma (TCP RF) of about 300 W. In some embodiments, the power is between 275 W and 400 W. During etching, a bias RF having a peak voltage of about -150 V or a peak voltage between -125 V and -175 V is applied to the device 500. The combination of defocus, gas ratio, and RF causes the molybdenum layer 502 to consistently have termination angles 710 and 712 of 30 degrees or less.

[0024] At the above-mentioned gas ratios and / or RF powers, the etching step is terminated at the endpoint. This may be followed by a timed over-etching step using the same process for cleaning the molybdenum residue. After etching the molybdenum layer 502, the device 500 may be placed in a plasma ash chamber, where an ashing process is applied to the photoresist mask 600 using this chamber. In some processes, the device 500 is heated to a temperature of approximately 175° C. to perform the plasma ashing process, where oxygen is flowed into the ash chamber. Ashing is controlled, and this process is followed by a timed over-ashing to remove any photoresist residue.

[0025] Figure 7 The resulting termination angles 710 and 712 are shown. Figure 7 The end angle 710 in Figure 2 The angle α in the figure corresponds to the angle α in the figure, and the terminal angle 712 corresponds to the angle α Figure 3 The termination angles 710 and 712 are within predetermined values, which in the example described herein are less than 30 degrees. The low termination angles 710 and 712 prevent the aforementioned cracks and seams from forming near the terminals 700 and 702. All molybdenum layers in device 500 can be prepared as described above to prevent the aforementioned steep termination angles.

[0026] Different variations of the above method can be implemented in the manufacturing process. For example, the photoresist material 510 can be between 3.5 μm and 5.5 μm. In the case of a 5.5 μm photoresist material 510 and a defocus of + / - 17 μm, the stop angles 710 and 712 can be approximately 11 degrees. The etched material can have a high selectivity, such as a selectivity of molybdenum to aluminum nitride greater than 100:1, and a selectivity of molybdenum to oxide greater than 100:1. In some embodiments, the stop angles 710 and 712 are maintained within 1% of the device on which the molybdenum layer 502 is positioned.

[0027] Figure 8 Flowchart 800 illustrates an embodiment of the above-described fabrication process. The process begins at step 802 by providing a molybdenum layer. Figure 5 The provided molybdenum layer may be the molybdenum layer 502 formed on the oxide layer 504 or a seed layer (not shown) formed on the oxide layer 504. At step 804, a photoresist material is applied to the molybdenum layer. The photoresist material may be, for example, Figure 5 . In step 806, the photoresist material is exposed under defocus conditions to produce a photoresist mask with an edge portion. The photoresist mask can correspond to mask 600, and the edges of the exposed photoresist material can correspond to any of the inclined edges 514 and 516. In step 808, the molybdenum layer is etched at the edge portion of the photoresist mask to produce an inclined terminal end of the molybdenum layer.

[0028] Modifications are possible in the embodiments described above, and other embodiments are possible within the scope of the claims.

Claims

1. A method for preparing a tilted terminal of a molybdenum layer, the method comprising: providing the molybdenum layer; applying a photoresist material to the molybdenum layer; exposing the photoresist material under defocus conditions to generate a photoresist mask, wherein an edge of the exposed photoresist material corresponds to the inclined terminal; as well as etching the molybdenum layer with an etching material, wherein the etching material at least partially etches the photoresist material exposed under the defocused condition, the etching producing the angled termination; wherein the etching comprises exposing the molybdenum layer and the photoresist material to a gas comprising chlorine and oxygen, and the ratio of oxygen to chlorine is between 3:1 and 5:1; wherein the etching comprises applying a low conversion coupled plasma between 275 watts and 400 watts; wherein said etching comprises applying a bias RF between -125 peak volts and -175 peak volts; wherein the defocus condition is between + / - 11 μm and + / - 22 μm; wherein the etching material has a selectivity of molybdenum to aluminum nitride greater than 100:1; and The etching material has a selectivity of molybdenum to oxide greater than 100:

1. 2 . The method of claim 1 , wherein providing the molybdenum layer comprises providing a molybdenum layer on an oxide layer.

3. The method of claim 1 , wherein the ratio of oxygen to chlorine is 4:

1.

4. The method of claim 1, wherein the etching comprises applying a 300 watt low-conversion coupled plasma. The method of claim 1 , wherein the etching comprises applying an RF bias of −150 volts. The method according to claim 1 , wherein the defocus condition is + / - 17 μm.

7. A method for preparing a Bragg mirror having a tilted termination of a molybdenum layer, the method comprising: providing the molybdenum layer on the seed layer; applying a photoresist material to the molybdenum layer; exposing the photoresist material under defocus conditions to generate a photoresist mask, wherein an edge of the exposed photoresist material corresponds to the inclined terminal; as well as etching the molybdenum layer with an etching material, wherein the etching material at least partially etches the photoresist material exposed under the defocused condition, the etching producing the angled termination, wherein the etching comprises exposing the molybdenum layer and the photoresist material to a gas comprising oxygen and chlorine, the ratio of oxygen to chlorine being between 3:1 and 5:1; wherein the etching comprises a low conversion coupled plasma applied at between 275 watts and 400 watts; wherein said etching comprises applying a bias RF between -125 peak volts and -175 peak volts; wherein the defocus condition is between + / - 11 μm and + / - 22 μm; and The etching material has a selectivity of molybdenum to aluminum nitride greater than 100:1, and a selectivity of molybdenum to oxide greater than 100:

1.

8. The method of claim 7, wherein the etching comprises applying a 300 watt low-conversion coupled plasma.

9. The method of claim 7, wherein the etching comprises applying an RF bias of -150 volts.

10. The method of claim 7, wherein the defocus condition is + / - 17 μm.

11. The method of claim 7, wherein the ratio of oxygen to chlorine is 4:

1.

12. A method for preparing an inclined terminal of a molybdenum layer, the method comprising: providing the molybdenum layer; applying a photoresist material to the molybdenum layer; exposing the photoresist material under a defocus condition between + / - 11 μm and + / - 22 μm to generate a photoresist mask, an edge of the exposed photoresist material corresponding to the tilted termination; as well as etching the molybdenum layer with an etching material, wherein the etching material has a selectivity of molybdenum to aluminum nitride greater than 100:1 and a selectivity of molybdenum to oxide greater than 100:1, the etching material at least partially etches the photoresist material exposed under the defocused condition, the etching producing the angled termination, wherein the etching comprises exposing the molybdenum layer and the photoresist material to a gas comprising oxygen and chlorine, the ratio of oxygen to chlorine being between 3:1 and 5:1; wherein the etching comprises a low conversion coupled plasma applied at between 275 watts and 400 watts; Wherein said etching comprises applying an RF bias voltage between -125 peak volts and -175 peak volts.

13. The method of claim 12, wherein the ratio of oxygen to chlorine is 4:

1.

14. The method of claim 12, wherein the etching comprises applying a 300 watt low-conversion coupled plasma. The method of claim 12 , wherein the etching comprises applying an RF bias of −150 volts. The method of claim 12 , wherein the defocus condition is + / - 17 μm.

Images