**4. Applications on MEMS**

142 Microelectromechanical Systems and Devices

Fig. 10. Effect of pre-adsorbed surfactant layer (1 vol% Triton in DI water) on the surface

For room temperature, the dramatic transformation in the surface morphology correlates directly with a strong reduction in the measured surface roughness. For {110}, it is found that typical zigzag structures emerge in pure TMAH while short pre-treatment in Triton (producing a layer thickness of 12 Å) drastically smoothens the surface, in spite of the reduced thickness of the surfactant layer. The saturated layer thickness for {110} (≈ 16 Å) produces a very smooth silicon surface (Ra ≈ 20 nm). For {100}, the surfactant pre-treatment provides some improvement in the morphology, even when the initial surface is already very smooth. Similar experiments carried out at 60 ºC are shown in Fig. 10(b) (etch depth ~ 35 ± 3 µm). Although {110} shows similar surface morphologies at 60 ºC and RT, the surface morphology of {100} is very different, characterized by the formation of pyramidal hillocks. Nevertheless, the roughness of both {110} and {100} is improved by the use of the surfactant

roughness (Ra) and etch rate for Si {110} and Si {100} in 10 wt% TMAH: (a) room

temperature; and (b) 60 ºC.

layer.

Significantly different etching behaviour of TMAH + Triton from that of traditionally used anisotropic etchants is very useful for MEMS applications in order to extend the range of 3D structures fabricated by wet etching because the surfactant is adsorbed at the silicon-etchant interface as a thin layer to act as a filter moderating the etching behaviour. In this chapter, we present three applications using surfactant-modified etchants and point out its great potential on advanced MEMS structures.

#### **4.1 Conformal structures**

The corner compensation method is the most widely used method for fabricating the sharp edge convex corners. The design and dimensions of the compensation structures require the knowledge of the undercutting ratio and its dependence on the etchant. If the design of MEMS structures does not include any rounded concave and/or sharp convex corners but a smooth etched surface is necessarily required, then the high concentration (20–25 wt% TMAH) should be selected for anisotropic etching. If the structures comprise rounded concave and sharp edge convex corners, the pure TMAH cannot be used due to severe undercutting.

Fig. 11. Conformal mesa shape fabricated in surfactant-modified solutions.

Advanced Surfactant-Modified Wet Anisotropic Etching 145

at different locations are shown in Fig. 13. The qualitative analysis (SEM image) reveals that the portion between two {111} planes centered by <110> orientation (marked by F), where corrugation patterned is disappeared, is sufficiently smooth for optical applications. This is because that the planes appearing in this portion exhibit very slow and almost same etch rates for 61 ºC. Due to symmetry, the portion Q also has the smooth surface. This property is very useful for the fabrication of cylindrical lens, which can be easily integrated with other

Fig. 13. SEM pictures of the ring structure and the morphologies on its different side walls

The details of cylindrical lens portion are schematically illustrated in Fig. 14(a). The total range of the portion within two {111} planes on {110} surface is 70.54°. Since the {111} planes are the slowest etch rate planes in wet anisotropic etchants, in order to maintain the roundness of the curved profile, these planes should not be included in the design. Hence, the curved portion making an angle of 70º, is only useful portion for achieving smooth rounded vertical walls using DRIE followed by TMAH + Triton treatment. In this work, in order to demonstrate the application of surfactant-added TMAH for the removal microcorrugation at rounded surface and the fabrication of cylindrical lens with smooth surface,

The etching time for scalloping removal should be controlled accurately in order to achieve highly smooth etched surface finish while maintaining desired shape profile. In this work, the mechanism of scalloping removal is that the top area of the corrugation is etched with highest etch rate. Firstly, the left equation corresponds to the calculation of wet etching time in order to remove micro-corrugation at {110} side wall (Fig. 14(b) A-A'), where T is the etching time, R{170} is the highest etching rate at 61 °C, α is the angle between {110} and {170}, and H1 is the height of corrugation. At 61 °C, the etching rates for {170} and {110} are the largest and smallest respectively. {170} project the corrugation, and the projected region is etched with the highest etching rate. On the other hand, {110} appears at the bottom of the

optical elements on a silicon substrate.

after wet anisotropic etching at 61 °C.

the curved portion with hatched lines is selected.

The sharp convex corners and rounded concave corners etched in surfactant-modified TMAH solutions are shown in Fig. 11. Various kinds of etched patterns (etch depth about 35 μm) using 25 wt% TMAH without and with the surfactant are shown in Fig. 12. The undercutting at the convex corners is considerably reduced because of the changed etching anisotropy by the adsorption of surfactant molecules. This solution exhibits minimum undercutting and provides very smooth surfaces while keeping a reasonable etch rate.

Fig. 12. Two kinds of etched patterns in pure and surfactant added 25 wt% TMAH at 60 ºC (etch depth = 35 μm).

#### **4.2 Scalloping removal for vertical micro-lens**

Recently, the optical elements perpendicular to the substrate, for instance the cylindrical lens for micro-laser-scanning module applications, have been required as they can be integrated to a desired position on the same substrate using standard semiconductor processes. In order to fabricate some class of devices such as a micro-lens for optical MEMS, a highly smooth rounded column surface is required. Basically commonly used wet etchants (e.g. pure KOH and TMAH) provide very high etch rates at a rounded profile and exhibit a quite rough surface as the many faceted orientations appear on such kinds of surfaces. Hence, the removal of scalloping on rounded column surfaces is a challenging issue.

Due to the selective adsorption of surfactant molecules, the etch rate of {110} in TMAH + Triton solutions is significantly reduced. The etch rates of the planes lying between {111} and {110} at 61ºC are almost same with values less than 0.05 µm. This property has been studied to remove the scalloping at the sidewalls of DRIE etched patterns on {110} silicon wafers. The study of scalloping removal is performed by short time dipping of DRIE etched patterns in TMAH + Triton. In order to investigate the effect of crystallographic directions on the surface roughness and the final shape of etched profile, a ring shape structure with internal and external diameters 200 µm and 300 µm respectively is fabricated by DRIE of about 40 µm deep. Thereafter, wet anisotropic etching is employed for 10 min in 25 wt% TMAH + 0.1 vol% Triton at 61ºC. This time is long enough to clearly observe the change in profile. The SEM pictures of resultant profile and the surface morphologies of its sidewalls

The sharp convex corners and rounded concave corners etched in surfactant-modified TMAH solutions are shown in Fig. 11. Various kinds of etched patterns (etch depth about 35 μm) using 25 wt% TMAH without and with the surfactant are shown in Fig. 12. The undercutting at the convex corners is considerably reduced because of the changed etching anisotropy by the adsorption of surfactant molecules. This solution exhibits minimum undercutting and provides very smooth surfaces while keeping a reasonable etch rate.

Fig. 12. Two kinds of etched patterns in pure and surfactant added 25 wt% TMAH at 60 ºC

Recently, the optical elements perpendicular to the substrate, for instance the cylindrical lens for micro-laser-scanning module applications, have been required as they can be integrated to a desired position on the same substrate using standard semiconductor processes. In order to fabricate some class of devices such as a micro-lens for optical MEMS, a highly smooth rounded column surface is required. Basically commonly used wet etchants (e.g. pure KOH and TMAH) provide very high etch rates at a rounded profile and exhibit a quite rough surface as the many faceted orientations appear on such kinds of surfaces.

Due to the selective adsorption of surfactant molecules, the etch rate of {110} in TMAH + Triton solutions is significantly reduced. The etch rates of the planes lying between {111} and {110} at 61ºC are almost same with values less than 0.05 µm. This property has been studied to remove the scalloping at the sidewalls of DRIE etched patterns on {110} silicon wafers. The study of scalloping removal is performed by short time dipping of DRIE etched patterns in TMAH + Triton. In order to investigate the effect of crystallographic directions on the surface roughness and the final shape of etched profile, a ring shape structure with internal and external diameters 200 µm and 300 µm respectively is fabricated by DRIE of about 40 µm deep. Thereafter, wet anisotropic etching is employed for 10 min in 25 wt% TMAH + 0.1 vol% Triton at 61ºC. This time is long enough to clearly observe the change in profile. The SEM pictures of resultant profile and the surface morphologies of its sidewalls

Hence, the removal of scalloping on rounded column surfaces is a challenging issue.

(etch depth = 35 μm).

**4.2 Scalloping removal for vertical micro-lens** 

at different locations are shown in Fig. 13. The qualitative analysis (SEM image) reveals that the portion between two {111} planes centered by <110> orientation (marked by F), where corrugation patterned is disappeared, is sufficiently smooth for optical applications. This is because that the planes appearing in this portion exhibit very slow and almost same etch rates for 61 ºC. Due to symmetry, the portion Q also has the smooth surface. This property is very useful for the fabrication of cylindrical lens, which can be easily integrated with other optical elements on a silicon substrate.

Fig. 13. SEM pictures of the ring structure and the morphologies on its different side walls after wet anisotropic etching at 61 °C.

The details of cylindrical lens portion are schematically illustrated in Fig. 14(a). The total range of the portion within two {111} planes on {110} surface is 70.54°. Since the {111} planes are the slowest etch rate planes in wet anisotropic etchants, in order to maintain the roundness of the curved profile, these planes should not be included in the design. Hence, the curved portion making an angle of 70º, is only useful portion for achieving smooth rounded vertical walls using DRIE followed by TMAH + Triton treatment. In this work, in order to demonstrate the application of surfactant-added TMAH for the removal microcorrugation at rounded surface and the fabrication of cylindrical lens with smooth surface, the curved portion with hatched lines is selected.

The etching time for scalloping removal should be controlled accurately in order to achieve highly smooth etched surface finish while maintaining desired shape profile. In this work, the mechanism of scalloping removal is that the top area of the corrugation is etched with highest etch rate. Firstly, the left equation corresponds to the calculation of wet etching time in order to remove micro-corrugation at {110} side wall (Fig. 14(b) A-A'), where T is the etching time, R{170} is the highest etching rate at 61 °C, α is the angle between {110} and {170}, and H1 is the height of corrugation. At 61 °C, the etching rates for {170} and {110} are the largest and smallest respectively. {170} project the corrugation, and the projected region is etched with the highest etching rate. On the other hand, {110} appears at the bottom of the

Advanced Surfactant-Modified Wet Anisotropic Etching 147

This surface roughness is sufficiently enough for optical MEMS (MOEMS) applications. Thus, the small time etching in 25 wt% TMAH + 0.1 vol% Triton at 61 ºC successfully remove the micro-corrugation without altering the desired shape of the DRIE etched profile

Fig. 15. SEM pictures of cylindrical lens after (a) DRIE and (b) DRIE followed by short time etching treatment in 25 wt% TMAH + 0.1 vol% Triton at 61 ºC. The AFM results reveal surface roughness at different locations of vertical sidewalls. Dimension of each zoomed

(111) planes are often employed as self-stop sides in the development of MEMS structures. However, on (111) silicon wafers, if certain designated planes are exposed by dry etching, novel structures dominated by fast planes might be formed in the subsequent wet anisotropic etching. In the etchant of TMAH + Triton X-100, although (110) and its vicinal

figure is 9 µm x 9 µm (Reproduced with permission from IOP).

**4.3 Sharp tips with high aspect ratio** 

and provides almost homogeneous surface finish.

corrugation, minimizing the over-etching of the sidewall. The projected region of the corrugation is becomes small as the etching process, and it finally flattens out. In our case, α is measured as 37° and H1 is 69.428 nm. The etching time of 42 seconds is obtained. Secondly, removing of micro-corrugation on other planes {ijk} is shown in Fig. 14(b) B-B'. Here micro-corrugation caused by DRIE is considered as the same scalloping shape. Therefore, the angle β between {ijk} and {xyz}, where {xyz} has the highest etch rate between {110} and {ijk}, is β = α = 37°. Also, the height H2 = H1 = 69.428 nm. R{ijk} is the etching rate of those planes located between {110} and {111}. Now the etching formula is described as the right equation. As noted before, etching rate of planes lying between {110} and {111} at 61 °C exhibit little variation. For this reason, we consider R{ijk} = R{110}. Another important parameter is the etch rate of highest etch rate planes in corrugated structures. Luckily, the distributions of etching rates on the {100} vicinal planes exhibit nearly identical property, which implies R{xyz} = R{170}. Thanks to the wet etching characterization in this intriguing etchants. It is feasible for the fabrication of cylindrical lens.

Fig. 14. (a) Schematic top view of cylindrical lens and (b) Mechanism of micro-corrugation removal by wet anisotropic etching at the sidewalls of cylindrical lens fabricated by DRIE.

Fig. 15 shows the SEM pictures and AFM measurement results of silicon surface roughness before and after wet etching. The roughness of sidewalls has been improved to about 1 nm.

corrugation, minimizing the over-etching of the sidewall. The projected region of the corrugation is becomes small as the etching process, and it finally flattens out. In our case, α is measured as 37° and H1 is 69.428 nm. The etching time of 42 seconds is obtained. Secondly, removing of micro-corrugation on other planes {ijk} is shown in Fig. 14(b) B-B'. Here micro-corrugation caused by DRIE is considered as the same scalloping shape. Therefore, the angle β between {ijk} and {xyz}, where {xyz} has the highest etch rate between {110} and {ijk}, is β = α = 37°. Also, the height H2 = H1 = 69.428 nm. R{ijk} is the etching rate of those planes located between {110} and {111}. Now the etching formula is described as the right equation. As noted before, etching rate of planes lying between {110} and {111} at 61 °C exhibit little variation. For this reason, we consider R{ijk} = R{110}. Another important parameter is the etch rate of highest etch rate planes in corrugated structures. Luckily, the distributions of etching rates on the {100} vicinal planes exhibit nearly identical property, which implies R{xyz} = R{170}. Thanks to the wet etching characterization in this intriguing

Fig. 14. (a) Schematic top view of cylindrical lens and (b) Mechanism of micro-corrugation removal by wet anisotropic etching at the sidewalls of cylindrical lens fabricated by DRIE. Fig. 15 shows the SEM pictures and AFM measurement results of silicon surface roughness before and after wet etching. The roughness of sidewalls has been improved to about 1 nm.

etchants. It is feasible for the fabrication of cylindrical lens.

This surface roughness is sufficiently enough for optical MEMS (MOEMS) applications. Thus, the small time etching in 25 wt% TMAH + 0.1 vol% Triton at 61 ºC successfully remove the micro-corrugation without altering the desired shape of the DRIE etched profile and provides almost homogeneous surface finish.

Fig. 15. SEM pictures of cylindrical lens after (a) DRIE and (b) DRIE followed by short time etching treatment in 25 wt% TMAH + 0.1 vol% Triton at 61 ºC. The AFM results reveal surface roughness at different locations of vertical sidewalls. Dimension of each zoomed figure is 9 µm x 9 µm (Reproduced with permission from IOP).

#### **4.3 Sharp tips with high aspect ratio**

(111) planes are often employed as self-stop sides in the development of MEMS structures. However, on (111) silicon wafers, if certain designated planes are exposed by dry etching, novel structures dominated by fast planes might be formed in the subsequent wet anisotropic etching. In the etchant of TMAH + Triton X-100, although (110) and its vicinal

Advanced Surfactant-Modified Wet Anisotropic Etching 149

Fig. 17. A circularly graphic net of (111)-centered crystal orientations, illustrating the etched

Fig. 18. One photo from optical-microscope reflecting the intermediate status during wet etching with a triangular mask on Si (111) in 25 wt% TMAH + 0.1 vol% Triton at 80 ˚C.

Fig. 19 shows an SEM image of the etched tip after 33 min, on which there is still a mask cap left. Furthermore, the completed tip is exhibited in Fig. 20. It is clear that the tip has a height of 30 μm, leading to the high aspect ratio of 6:1, and the front angle is measured as about

plane of (221).

planes are strongly affected by the adsorption of surfactant molecules, having lower etch rates than other planes, there is still a local maximum plane (221) located between (110) and (111) planes at 80˚C, as shown in Fig. 16. That means (221) planes are capable of being applied to tips as the undercutting sides. The data are obtained from hemispherical samples. As we mentioned previously, there is some difference of etch rates on stressed and flat surfaces, but influence from hemispheres is limited and the relative value among the planes is sufficiently small to be ignored.

Fig. 16. Etch rate distribution between (111) and (111) in 25 wt% TMAH + 0.1 vol% Triton X-100 at 80 ˚C.

Here, we use a mask of equilateral triangle on (111) silicon with each side length of 60 μmin the formation of silicon tips. Wafers are firstly deep dry etched of 50 μm and then dipped into TMAH + Triton X-100 solutions. Fig. 17 shows a circularly graphic net of (111)-centered crystal orientations analyzing the wet etched shapes. Once dry etched, six fast etching (221) planes become dominant until they meet together. The angle from periphery to (221) is about 10˚, leading to a much more oblique slope than conventional ones surrounded by (311) or (411) on (100) silicon wafers etched in pure KOH or TMAH. Therefore, it is allowed to fabricate silicon tips with very high aspect ratio in surfactant-modified etchants. The picture of etched sample, not over-etched, permits the examination of etched planes. The top view of one etched tip after 18 min in the TMAH + Triton X-100 at 80 ˚C, as shown in Fig. 18, exhibits that the etched corner is mainly composed of two planes which have fast and same etch rates. The included angle between two adjacent lines is measured as 150±2˚. Note that it is on (111) silicon and the vertex angles are directed at <112>. Making the crystalline projection as a reference, this angle indicates those orientations are located at the red line. Moreover, as mentioned above, planes with the local maximum etch rates would control the control the final. Here, in the solution of TMAH + Triton, this point lies in between (111) and (110), i.e., in blue line. The junction near (221) means those planes constituting the structure which also are in the same crystal class. This result is in good persistence with previous design, indicating the truth of a new tip with very high aspect ratio.

planes are strongly affected by the adsorption of surfactant molecules, having lower etch rates than other planes, there is still a local maximum plane (221) located between (110) and (111) planes at 80˚C, as shown in Fig. 16. That means (221) planes are capable of being applied to tips as the undercutting sides. The data are obtained from hemispherical samples. As we mentioned previously, there is some difference of etch rates on stressed and flat surfaces, but influence from hemispheres is limited and the relative value among the planes

Fig. 16. Etch rate distribution between (111) and (111) in 25 wt% TMAH + 0.1 vol% Triton X-

Here, we use a mask of equilateral triangle on (111) silicon with each side length of 60 μmin the formation of silicon tips. Wafers are firstly deep dry etched of 50 μm and then dipped into TMAH + Triton X-100 solutions. Fig. 17 shows a circularly graphic net of (111)-centered crystal orientations analyzing the wet etched shapes. Once dry etched, six fast etching (221) planes become dominant until they meet together. The angle from periphery to (221) is about 10˚, leading to a much more oblique slope than conventional ones surrounded by (311) or (411) on (100) silicon wafers etched in pure KOH or TMAH. Therefore, it is allowed to fabricate silicon tips with very high aspect ratio in surfactant-modified etchants. The picture of etched sample, not over-etched, permits the examination of etched planes. The top view of one etched tip after 18 min in the TMAH + Triton X-100 at 80 ˚C, as shown in Fig. 18, exhibits that the etched corner is mainly composed of two planes which have fast and same etch rates. The included angle between two adjacent lines is measured as 150±2˚. Note that it is on (111) silicon and the vertex angles are directed at <112>. Making the crystalline projection as a reference, this angle indicates those orientations are located at the red line. Moreover, as mentioned above, planes with the local maximum etch rates would control the control the final. Here, in the solution of TMAH + Triton, this point lies in between (111) and (110), i.e., in blue line. The junction near (221) means those planes constituting the structure which also are in the same crystal class. This result is in good persistence with previous

design, indicating the truth of a new tip with very high aspect ratio.

is sufficiently small to be ignored.

100 at 80 ˚C.

Fig. 17. A circularly graphic net of (111)-centered crystal orientations, illustrating the etched plane of (221).

Fig. 18. One photo from optical-microscope reflecting the intermediate status during wet etching with a triangular mask on Si (111) in 25 wt% TMAH + 0.1 vol% Triton at 80 ˚C.

Fig. 19 shows an SEM image of the etched tip after 33 min, on which there is still a mask cap left. Furthermore, the completed tip is exhibited in Fig. 20. It is clear that the tip has a height of 30 μm, leading to the high aspect ratio of 6:1, and the front angle is measured as about

Advanced Surfactant-Modified Wet Anisotropic Etching 151

In this chapter, firstly the etch rate anisotropy in surfactant-modified etch solution is investigated, showing intriguing properties that are different from that of pure alkaline solutions. The etch rates of exact and vicinal {100} planes are almost unaffected when the surfactant is added, while the etch rates of exact and vicinal {110} planes are reduced significantly. The improved anisotropy ({mnl}/{100}) at high temperature provides better conformity to the mask profile for the formation of a micro cavity. The activation energy of TMAH + Triton (0.1-0.2 eV) is lower than that of pure TMAH (or KOH) solution (0.5-0.7 eV),

Secondly, the underlying effect of the surfactant in etching is understood microscopically and proved macroscopically that enables manufacturing of advanced and exciting structures for MEMS. Thicknesses of surfactant layers are investigated depending on the variation of orientation, temperature et al. The pre-adsorbed surfactant layers are formed and their effects on etch rate, surface roughness and corner undercutting indicate that the

Finally three applications by using surfactant-modified etching process are exhibited, involving the fabrication of conformal structures, scalloping removing for vertical microlens, and sharp tips with high aspect ratio. Much effort will be dedicated on other potential aspects in MEMS, and more advanced devices made by this etching technique could be

We acknowledge support by MEXT (Micro/Nano Mechatronics G-COE and grant-in-aid for scientific research (A) 19201026 and 70008053) and the Chinese High-level University

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**5. Conclusion** 

anticipated in future.

**6. Acknowledgment** 

16-19, 1997

Program.

**7. References** 

18°, which conforms with the included angle of two (221) planes. Moreover, the curvature radius of less than 10 nm is achieved without another oxidation-sharpening treatment.

Fig. 19. An SEM image of the etched tip with a cap.

Fig. 20. A completely etched sharp tip of high aspect ratio.
