Plasma etching method

Abstract

To provide a method of plasma-etching an additive-containing aluminum nitride film containing an additive element selected from scandium, yttrium and erbium.SOLUTION: A method comprises placing a workpiece 11 upon a platen within a plasma chamber 12, where the workpiece 11 comprises: a substrate having an additive-containing aluminum nitride film deposited thereon; and a mask disposed upon the additive-containing aluminum nitride film and defining at least one trench. The method also comprises: introducing a first etching gas into the chamber 12 with a first flow rate; introducing a second etching gas into the chamber 12 with a second flow rate; and establishing plasma within the chamber 12 to etch the additive-containing aluminum nitride film exposed within the trench; where the first etching gas comprises boron trichloride, and the second etching gas comprises chlorine, with a flow rate ratio being greater than or equal to 1:1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a plasma etching process, and more particularly to a method for plasma etching an additive-containing aluminum nitride film in which the additive element is selected from scandium (Sc), yttrium (Y), or erbium (Er). [Background technology]

[0002] Aluminum nitride (AlN) and aluminum scandium nitride (AlScN) piezoelectric devices are widely used, for example, in bulk acoustic wave (BAW) devices and piezoelectric micromachine ultrasonic transducers (PMUTs). Mobile phones typically incorporate several AlN and AlScN BAW devices, and generating higher operating frequencies requires the use of thinner BAW devices. Improving piezoelectric performance for thinner devices is a major challenge as tolerances become tighter and the integration of these devices onto circuit boards becomes more complex. While the addition of Sc is known to improve the piezoelectric properties of BAW devices, the incorporation of Sc into the AlN lattice can lead to increased defect rates in the deposited piezoelectric film, and the non-volatility of Sc can generate residues that need to be removed during the plasma etching process.

[0003] As the percentage of Sc in doped AlN increases, the etching rate typically decreases when using standard chlorine (Cl2) / argon (Ar) based chemicals. This decrease results in lower AlScN selectivity for the mask (such as a photoresist or SiO2 mask), which reduces the critical dimension (CD) and consequently leads to shallower sidewall angles within the AlScN trench. Common methods for controlling the sidewall profile include adjusting the pre-etching mask slope, and changing the platen bias, etching gas flow, or process pressure. These methods are usually effective for AlScN with lower Sc content, but at higher Sc percentages, etching becomes increasingly physical, reducing the overall effectiveness of these methods. Similar effects are observed in AlYN and AlErN films.

[0004] A decrease in the AlScN etching rate also reduces selectivity for the metal underlayer, leading to increased underlayer losses that can impair the performance of some devices, such as BAW filters. Lower electrical contacts for BAW devices are typically molybdenum (Mo), tungsten (W), or platinum (Pt), and as excess amounts of metal are removed, the decrease in the AlScN etching rate increases the electrical resistance of the contacts, resulting in a degradation of device performance. Typical changes to increase the AlScN etching rate, such as increasing the platen bias or increasing the Cl2 flow rate, may ultimately have little or no effect on the sidewall angle or underlayer selectivity, and in some cases may even worsen the problem. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] IEEE Electron Device Letters, 42(9), September 2021, Shao et.al., “High Figure-of-Merit Lamb Wave Resonators Based on Al0.75Sc0.3N Thin Film”. 1378-1381. [Non-Patent Document 2] IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control, 66(1), January 2019, Lozzi et al., “Al0.83Sc0.17N Contour Mode Resonators With Electromechanical Coupling in Excess of 4.5%”, 146-153 [Overview of the project] [Problems that the invention aims to solve]

[0006] This invention aims to address at least some of the problems described above. [Means for solving the problem]

[0007] According to one aspect of the present invention, a method for plasma etching an additive-containing aluminum nitride film containing an additive element selected from scandium (Sc), yttrium (Y), or erbium (Er) is provided. The method is as follows: A step of placing a workpiece on a platen in a plasma chamber, wherein the workpiece comprises a substrate having an additive-containing aluminum nitride film deposited thereon, and a mask disposed on the additive-containing aluminum nitride film and defining at least one trench, A step of introducing a first etching gas into the chamber at a first flow rate, The steps include introducing a second etching gas into the chamber at a second flow rate, The steps include establishing a plasma in the chamber and etching the additive-containing aluminum nitride film exposed in the trench, The first etching gas contains boron trichloride, and the second etching gas contains chlorine.

[0008] The ratio of the first flow rate to the second flow rate is 1:1 or greater.

[0009] This invention enables the generation of steeper sidewall profiles by using a flow rate ratio of boron trichloride (BCl3) to chlorine (Cl2) of ≥ 1:1 when etching an additive-containing aluminum nitride film. Furthermore, by setting the flow rate ratio of BCl3 to Cl2 to 1:1 or higher, the selectivity of the additive-containing aluminum nitride film for the lower electrode can be increased, and the substrate loss when etching one of these underlying electrode films can be reduced.

[0010] In one embodiment, the method further includes introducing a third etching gas, such as argon, into the chamber. The third etching gas is introduced into the chamber at a third flow rate, and the ratio of the first flow rate to the third flow rate is greater than 2:1.

[0011] In one embodiment, the plasma etching method includes a primary etching step and a secondary etching step. The secondary etching step can be performed immediately after the primary etching step. The first flow rate of the first etching gas into the chamber is substantially the same in the primary and secondary etching steps, and the second flow rate of the second etching gas into the chamber is substantially the same in the primary and secondary etching steps.

[0012] In one embodiment, the first flow rate includes substantially 60 sccm, and the second flow rate includes substantially 40 sccm. A third etching gas is introduced into the chamber at a third flow rate in the range of 20 to 25 sccm.

[0013] In one embodiment, the gas pressure in the chamber during the primary etching stage includes about 50% of the gas pressure during the secondary etching stage. The gas pressure in the chamber is maintained substantially at 2 - 4 mTorr during the primary etching stage and substantially at 5 mTorr during the secondary etching stage.

[0014] In one embodiment, the plasma is established in the chamber using an inductively coupled plasma generating device. The plasma generating device is supplied with power at about 50% of the power during the secondary etching stage as compared to the primary etching stage.

[0015] In one embodiment, the platen is supplied with power during the secondary etching stage at a power less than 50% of the power used during the primary etching stage. The secondary etching stage is used when etching trenches through the additive-containing aluminum nitride film toward the side of the substrate opposite to the side on which the additive-containing aluminum nitride film is deposited. In one embodiment, the secondary etching stage is started before the trenches extend from the substrate.

[0016] In one embodiment, the workpiece includes a metal film such as molybdenum disposed on the side of the substrate opposite to the side on which the additive-containing aluminum nitride film is deposited.

[0017] In one embodiment, the mask includes a photoresist mask.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic diagram of a plasma etching apparatus for etching an additive-containing aluminum nitride film. [Figure 2] It is a flowchart showing an overview of steps related to a method for plasma etching an additive-containing aluminum nitride film according to an embodiment of the present invention. [Figure 3] It is a graph showing an increase in the AlScN to photoresist selectivity in a 5 μm CD trench as the flow rate ratio of BCl3 to Cl2 increases. [Figure 4] This graph shows the decrease in CD at the top and bottom of a 5 μm trench as the flow rate ratio of BCl3 to Cl2 increases. [Figure 5] This graph shows the increase in sidewall angles in 5 μm and 100 μm CD trenches as the flow rate ratio of BCl3 to Cl2 increases. [Figure 6] Scanning electron microscope images of reflowed photoresist on a 500 nm thick Al0.7Sc0.3N film on a silicon substrate, where (a) shows the sidewall angle of a 5 μm trench in the absence of BCl3, and (b) shows the use of BCl3. [Figure 7] Scanning electron microscope images of a reflowed photoresist mask on a 500 nm thick Al0.7Sc0.3N film on a Si substrate. (a) shows the sidewall angle in a 100 μm CD trench without BCl3, and (b) shows the sidewall angle with BCl3. The 100 μm CD trench has a shallower pre-etching profile than a 5 μm trench. [Figure 8] This graph shows the increasing selectivity of Al0.7Sc0.3N versus Mo (lower electrode) as the flow rate ratio of BCl3 to Cl2 increases, and the Mo etching rate is calculated for the photoresist patterning blanket Mo on the Si wafer. [Modes for carrying out the invention]

[0019] Referring to Figure 1 of the drawings, a schematic diagram of an apparatus 10 for plasma etching a workpiece 11 is provided, comprising a process chamber 12 in which the plasma etching of the workpiece 11 is performed.

[0020] The apparatus 10 may be made of a metal such as aluminum and placed within the chamber 12, but further comprises a platen assembly 13 electrically isolated from the chamber wall 12a by conventional means such as a ceramic break 14. The platen assembly 13 comprises a body 13a having a support surface 13b for receiving a workpiece 11 and is electrically biased using a radio frequency (RF) voltage generator. Providing a negative bias voltage to the platen assembly 13 can help control, for example, positively charged ion bombardments on the surface of the workpiece 11 from the plasma.

[0021] The processing chamber 12 can be made of a metal such as aluminum and typically includes an electrically grounded chamber wall 12a. The chamber 12 further includes first, second, and third gas inlets 15a, 15b, and 15c, through which first, second, and third etching gas sources (not shown) can be fluidically connected to introduce gas into the chamber 12. The chamber 12 further includes an outlet 16 through which the gas and any by-products of the etching process can exit the chamber 12.

[0022] In one embodiment, plasma is generated by applying an RF voltage from an RF voltage generator 17 to one or more antennas 18 positioned around the chamber 12 and adjacent to each dielectric window section 12b formed in the chamber wall 12a.

[0023] One or more antennas 18 may, for example, have a substantially planar helical configuration, a helical coil configuration, or a toroidal configuration, and impedance matching of the RF signal from the generator 17 with the antennas 18 is performed in accordance with standard practice to minimize the reflection of power from the antennas 18. The antennas 18 are positioned around the chamber 12, and the power is inductively coupled into the chamber 12 through the dielectric window section 12b.

[0024] Plasma is generated in a region 19 of a chamber 12 positioned above the workpiece 11 so that the workpiece 11 is exposed to the plasma. The gas is introduced into the chamber 12 through flow regulators 20a, 20b, and 20c, respectively, which are coupled to inlets 15a, 15b, and 15c, respectively, and the inlets 15a, 15b, 15c and outlet 16 of the chamber 12 are positioned on either side of the plasma region 19 so that the etching gas must pass through the chamber 12, through the region 19, and over the workpiece 11.

[0025] Referring to Figure 2 of the drawings, a flowchart outlining the steps related to Method 100 for plasma etching an additive-containing aluminum nitride film according to one embodiment of the present invention is shown. The method is demonstrated with reference to an AlScN film, but those skilled in the art will recognize that the method is equally applicable to AlYN and AlErN films.

[0026] This method includes, in step 101, placing a workpiece 11 on a platen 13 in a plasma chamber 12. The workpiece 11 includes a substrate 11a such as a silicon wafer substrate, on which a piezoelectric AlScN film 11b is deposited, for example, using pulsed DC sputtering technique. In one embodiment, the film is Al 0.7 Sc 0.3 The film contains N, i.e., a film comprising 70% aluminum and 30% scandium. The determination of the film composition is typically achieved using X-ray energy dispersion analysis (EDAX). The workpiece 11 further comprises a mask 11b on which trenches 5 μm to 100 μm in size 11c are formed on the film 11a using a 4 to 4.4 μm photoresist before reflow.

[0027] With the workpiece 11 positioned on the platen 13 in the chamber 12, the first, second, and third etching gases are introduced into the chamber 12 in step 102 through their respective inlets 15a to c using their respective flow regulators 20a to c, and the pressure in the chamber 12 is maintained at approximately 2 to 5 mTorr or substantially 3 mTorr by a pressure regulator (not shown). Once the chamber 12 is properly conditioned with the etching gases, in step 103, an RF potential is applied to the antenna 18 via the generator 17, inductively coupling the power to the etching gases and thus generating a plasma, Al 0.7 Sc 0.3 Etching of the N film is initiated. In step 104, a bias voltage is also applied to the platen assembly 13 through the use of a voltage generator 21, typically operating at 13.56 MHz, providing etching of the AlScN film 11b in step 105.

[0028] Antenna 18 is powered with approximately 1000W of power, and platen 13 is powered with approximately 1025W of power. The first etching gas contains boron trichloride (BCl3), and flow regulator 20a is configured to supply BCl3 into chamber 12 at a flow rate of substantially 60 sccm. The second etching gas contains chlorine (Cl2), and this Cl2 gas is delivered into chamber 12 at a rate of substantially 40 sccm, as determined by flow regulator 20b. The third etching gas contains argon (Ar), and this Ar gas is delivered into chamber 12 at a rate of substantially 25 sccm, as determined by flow regulator 20c. The flow rate ratio of BCl3 gas to Cl2 gas into chamber 12 is approximately 3:2, i.e., greater than 1:1, and the flow rate ratio of BCl3 gas to Ar gas into chamber 12 is approximately 3:1, i.e., greater than 2:1.

[0029] Increasing the BCl3:Cl2 flow rate ratio was found to increase the peak-to-peak voltage between the platen 13 and the chamber 12, which is known to increase the etching rate of AlScN. To offset this increase and obtain more accurate values ​​of the chemical and sputtered components of BCl3, the platen bias was adjusted using the generator 21 to ensure that the peak-to-peak voltage was nearly consistent across all data points. However, even when the bias was adjusted to compensate for this effect, an increase in the AlScN etching rate was still observed, suggesting that this is due to the increased sputtered component of BCl3.

[0030] The addition of BCl3 during Cl2-based AlScN etching is thought to promote the formation of boron-nitrogen-based polymers. These polymers condense on the mask 11b (photoresist, SiO2, etc.), reducing the mask etching rate (see Figure 3), while the presence of additional boron may increase the sputtering component of the etching, leading to an increase in the AlScN etching rate. In addition, BCl3 is thought to be less readily fractionated into reactive Cl- ions and neutral substances compared to Cl2, thereby reducing the chemical composition of the etching as the BCl3:Cl2 ratio increases. Typically, the etching rates of both the AlScN film 11a and the photoresist decrease by a similar percentage as the Cl2 flow decreases, resulting in invariant selectivity. However, as the presence of Cl decreases from an increasing BCl3:Cl2 flow ratio, the decrease in the AlScN etching rate is offset by the increase in the sputtering component of BCl3, resulting in increased selectivity for the mask 11b.

[0031] The combined effect of the BN-based polymer and the different fractionation properties of BCl3 results in increased AlScN selectivity for mask 11b. The reduced photoresist etching rate and increased sputtering component also contribute to the observed Al 0.7 Sc 0.3It is also applicable to scanadium percentages lower and higher than the N data, which means that in the range of scanadium percentages, a steeper sidewall trench profile and increased selectivity can be obtained. Similar results are obtained for Al 1-x Y x N and Al 1-x Er x and are also expected for the N film. The boiling points of erbium chloride and yttrium chloride are about 1500 °C, while that of scandium chloride is about 960 °C. The etching front contains residues that require sputtering / physical removal. Thus, Al 1-x Y x N and Al 1-x Er x N films are also expected to require similar physical / chemical process conditions to achieve results similar to those of AlScN, which also has very high-temperature Cl compounds.

[0032] Referring to FIG. 4, an increase in selectivity for a 5 μm CD mask 11b has been shown to reduce the lateral etching rate of the mask, thereby reducing the loss of critical dimension (CD). The reduction in CD loss enables a steeper sidewall profile, as shown in FIGS. 5 and 6(a), (b), and FIGS. 7(a), (b). A 100 μm CD trench has a shallower pre-etch mask profile than that of a 5 μm trench, demonstrating that an increased BCl3 flow rate compared to the Cl2 flow rate can affect the CD of the profile range and characteristics.

[0033] A common way to control the selectivity for the underlying layer of the workpiece 11 is by using a "soft-landing" step, which typically utilizes a lower platen bias to reduce the underlying layer etching rate and subsequent loss. BCl3 enhances the efficiency of AlScN etching, but assuming that its use also reduces the etching rate of the underlying layer and increases the selectivity for these films as a result of the addition of the sputtering component of boron is counterintuitive.

[0034] Therefore, in further embodiments, a similar workpiece was formed using a Si wafer substrate, but in this embodiment, Al 0.7 Sc 0.3 The effect of the BCl3:Cl2 flow rate ratio on the etching rate of the Mo film 11a' was demonstrated by replacing the N piezoelectric film 11a with a sputtered Mo film 11a' containing the same mask 11b. Next, the substrate 11a' with the Mo film was placed in the processing chamber and the Mo film was etched. For comparison, the process conditions for Mo etching are shown below along with the process conditions for piezoelectric film etching. [Table 1]

[0035] From the table above, it is clear that the plasma generator 17 is powered at approximately twice the power during primary (main) etching compared to secondary (soft-landing) etching. Similarly, the platen 13 is powered at approximately 50% of the power during secondary etching compared to primary etching. However, the pressure inside the chamber during secondary etching is almost twice that during primary etching.

[0036] A BCl3:Cl2 flow rate ratio ≥ 1:1 was found to be optimal for reducing the etching rate of the Mo film 11a' and similarly increasing the AlScN selectivity for the Mo film 11a'. For the electrode, i.e., the underlying layer, which is formed on the opposite side of the piezoelectric film 11a of the substrate 11 and is readily etched in chlorine-based chemicals such as Mo, reducing the presence of Cl2 reduces the underlying layer etching rate. Similar to AlScN:PR selectivity, as the Cl2 flow rate decreases, the AlScN etching rate also decreases, resulting in minimal change to the underlying layer selectivity. However, AlScN:Mo selectivity can be increased by the same mechanism described above for photoresist. An increased BCl3:Cl2 flow rate ratio reduces the presence of Cl2 and decreases the Mo etching rate, but the increased sputtering component increases the AlScN etching rate, resulting in increased selectivity for the electrode. Ultimately, this reduces electrode loss when etching AlScN down to this metal film, as shown in Figure 8. In the BCl3:Cl2 flow ratio tests at 1.5 and 2.125 (the two data points on the right), the platen output was reduced to minimize the impact of the increased peak-to-peak voltage on the results.

[0037] The process for etching a workpiece 11 having an AlScN film 11a on one side and a Mo film 11a' on the underside includes a primary etching stage 100 for etching trenches that penetrate most of the AlScN film 11a and the Si substrate 11, and a secondary etching stage 200 performed immediately after the primary etching stage 100. This is started before the trenches extend from the Si substrate. The secondary etching stage 200 includes increasing the pressure in the chamber 12 in step 201, reducing the power supplied to the antenna in step 202, and reducing the electrical bias to the platen in step 203. Once specific process conditions for the secondary etching stage (as shown in Table 1) are achieved, the Mo film 11a' is then etched in step 204 to effectively extend the trenches through the workpiece 11.

[0038] It has been demonstrated that a BCl3:Cl2 flow rate ratio of ≥1:1 ​​can result in high AlScN (and AlYN and AlErN) etching rates (>170 nm / min for 100 μm trenches, >250 nm / min for 5 μm trenches) with improved selectivity for the photoresist and lower electrode metal. While a flow rate ratio of ≥1:1 ​​is applicable to all Al:Sc ratios, this method becomes increasingly beneficial as etching rates decrease and etching of AlN with a high proportion of doped scandium becomes difficult. [Explanation of Symbols]

[0039] 10 Apparatus, 11 Workpiece, 12 Chamber, 13 Platen assembly, 15a, 15b, 15c Gas inlet, 16 Gas outlet, 17 RF voltage generator.

Claims

1. A method for plasma etching an additive-containing aluminum nitride film, wherein the additive-containing aluminum nitride film comprises an additive element selected from scandium (Sc), yttrium (Y), or erbium (Er), A step of placing a workpiece on a platen in a plasma chamber, wherein the workpiece comprises a substrate having the additive-containing aluminum nitride film deposited thereon, a metal film disposed in a layer below the additive-containing aluminum nitride film, and a mask disposed on the additive-containing aluminum nitride film defining at least one trench; The steps include introducing a first etching gas into the chamber at a first flow rate, The steps include introducing a second etching gas into the chamber at a second flow rate, The steps include introducing a third etching gas into the chamber at a third flow rate, A step comprising: a primary etching step of establishing plasma in the chamber to etch the additive-containing aluminum nitride film exposed in the trench; and a secondary etching step performed immediately after the primary etching step. The first etching gas contains boron trichloride, the second etching gas contains chlorine, and the third etching gas contains argon. The ratio of the first flow rate to the second flow rate is 1:1 or greater. The ratio of the first flow rate to the third flow rate is greater than 2:

1. The gas pressure in the chamber during the primary etching stage is 50% of the gas pressure during the secondary etching stage. Plasma etching method.

2. The plasma etching method according to claim 1, wherein the flow rate of the first etching gas into the chamber is substantially the same in the primary etching step and the secondary etching step.

3. The plasma etching method according to claim 1, wherein the flow rate of the second etching gas into the chamber is substantially the same in the primary etching step and the secondary etching step.

4. The plasma etching method according to claim 1, wherein the plasma generator is supplied with 50% of the power during the secondary etching stage compared to the primary etching stage.

5. The plasma etching method according to claim 1, wherein the platen is powered during the secondary etching step at less than 50% of the power used during the primary etching step.

6. The plasma etching method according to claim 1, wherein the secondary etching step is used to etch a trench through the additive-containing aluminum nitride film toward the substrate side opposite to the side on which the additive-containing aluminum nitride film is deposited.

7. The plasma etching method according to claim 6, wherein the secondary etching step is started before the trench extends from the substrate.

8. The plasma etching method according to claim 1, wherein the metal film contains molybdenum.

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