57 Chapter 3 Fabrication of Accelerometer 3.1 Introduction There are basically two approaches for bulk micromachining of silicon, wet and dry. Wet bulk micromachining is usually carried out using anisotropic etchants like KOH (Potassium Hydroxide), TMAH (Tetra Methyl Ammonium Hydroxide) and EDP (Ethylene Diamine Pyrocatecol). Dry micromachining of silicon is done using Flourine based plasma chemistry. For the fabrication of accelerometer, the wet bulk micromachining approach is selected because of two major reasons (1) proof-mass and beams are having different thicknesses (2) beams need to be centralized w.r.t the proof-mass. Another critical process developed here is controlled wet oxidation and precisely controlled patterning of silicon dioxide using BHF (Buffered Hydro Fluoric acid). Silicon dioxide is used as a mask for etching in KOH. 3.2 Wet bulk micromachining Anisotropic chemical wet etching is a key technology in fabricating Micro Electro Mechanical Systems (MEMS). A substantial amount of research has been conducted to understand the mechanism and
58 eventually control the etched shape. Seidel et al. [40] Seidel [41] and Glembocki et al. [42] and Palik et al. [43] modeled the etching process and investigated the etching properties under a variety of KOH etching conditions. 3.2.1 Theory fig 3.1. Single-crystal silicon has a diamond lattice structure as shown in Fig 3.1 Silicon crystal structure Each silicon atom has four covalent bonds. Each bond connects a different pair of atoms. Silicon, with its four covalent bonds, coordinates itself tetrahedrally, and these tetrahedrons make up the diamond-cubic structure. This structure can also be represented as two interpenetrating Face-Centered Cubic (FCC) lattices, one displaced w.r.t the other. The lattice parameter a for Silicon is 5.4309 A and Silicon s diamond-cubic lattice has a packing density of 34%, compared to 74% for a regular FCC lattice. The {111} planes present the highest packing
59 density and the atoms are oriented such that three bonds are below the plane. When an atom is located on a surface, the bond belonging to the atom loses a neighboring atom. It is known as a dangling bond. The dangling bonds easily react with the etching agent. Though, in many cases, the dangling bonds do not remain free-ended, that is, the surface bonds are reconstructed by combining with each other in high vacuum, or the bonds are terminated with hydrogen atom in water, those bonds on the surface are still a source of surface reactions, i.e., etching. When the number of dangling bonds on three differently oriented surfaces, (100), (110), and (111) are compared, it is apparent that the (111) surface has the smallest number of bonds. There is only one dangling bond per surface atom for (111), whereas there are two for (100), and one dangling bond plus two surface bonds for (110). This is a conventional explanation for why (111) is stable against etching. Experimentally it is found that in pure KOH solutions, {110} planes exhibit highest etching rate. According Seidal et al, [10] the back bonds and the energy levels of the associated surface states is not necessarily the same for {110} and {111} planes, as that energy will also be influenced by the effect of the orientation of these bonds. Another argument in favor of high etching rates of {110} planes is the easier penetrability of {110} surfaces for water molecules along the channels in that plane [46].
60 3.2.2 Etching solutions Solutions showing orientation dependence on the etch rate of silicon are KOH, TMAH, EDP (Ethylenediamine and Pyrocatecol), N2H4 (hydrazine) and NaOH. All are used as water solutions. The chemical reaction for any of these etching solutions is described as follows: Si + 2OH + 2H 2 O SiO 2 (OH) 2 2 + 2H 2 Silicon reacts with water and an OH - ion and produces hydroxide ion and hydrogen gas bubbles. Etching masks are usually made of either SiO2 or Si3N4. KOH shows strong anisotropy, and shows large values of etch rate ratios among orientations of about (100). It means that high controllability can be expected in etched profiles, while suppressing mask undercut. KOH solutions are less toxic than other etchants, hence are easy to process. These are the main reasons why KOH is widely used for fabricating silicon microstructures in industry. A drawback with KOH is that, it etches SiO2 mask also significantly during long etching time. Selectivity of Si to SiO2 mask is about 150 under normal conditions. Another drawback is that KOH etching is incompatible to IC processes because contamination with potassium ion is strictly prohibited in IC processes.
61 3.2.3 Etching shapes Etching shapes are categorized into two, one is concave-etched profiles and the other is convex etched profile. Concave-etched profiles on silicon wafers have been fabricated so far for applications in pressure sensors and ink-jet printer head structures. In the case of concave profiles, orientations having an etch rate that is locally minimum appear and Si (111) is the only orientation that has an etch rate of a local minimum for any etching conditions. Incase of convex profiles, orientations having an etch rate that is locally maximum appear. It becomes far more difficult to predict and to control etch profiles having large etch rates. Again, characterization of anisotropy in etch rate is of great importance. The planes occurring at convex corners during anisotropic etching of (100) silicon in aqueous KOH were identified as {411} planes [40]. The etching rate of these planes in relation to the rate of the {100} planes declines with increasing Potassium Hydroxide concentration. In contrast, the temperature dependence of this etch rate ratio is negligible in the relevant range between 60 C and 100 C. 3.2.4 Silicon dioxide as masking material In the case of KOH etching, the etch rate of SiO2 is not negligible, even if it is a thermally grown oxide. With 40% KOH solution at a temperature of 60 C, it is experimentally verified that etching rate of SiO2
62 is about 80 nm/hr. Growth of thin Oxide films (<1 µm) and multiple oxidation steps are incorporated into the process to improve the quality of oxide and also to reduce the pinhole density. The disadvantage of higher etch rate of SiO2 when compared to other masking materials like Si3N4, is converted into an advantage in the process for fabrication of accelerometer structure by in-situ removal of oxide mask of required thickness during silicon etching itself. Other advantages of using SiO2 as mask are easy to grow by thermal oxidation and easy to remove using HF-based solutions. 3.2.5 Corner undercutting and compensation When etching rectangular corners, deformation of the edges occurs due to under cutting. This is an unwanted effect especially in the fabrication of acceleration sensors where total symmetry and perfect 90 o convex corners on the proof-mass are mandatory for good device prediction and specification. The undercutting is a function of etch time and thus directly related to the desired etch depth. An undercut ratio is defined as the ratio of undercut to etch depth. Saturating KOH solutions with isopropanol (IPA) reduces the convex corner undercutting. Unfortunately, this happens at the cost of the anisotropy of the etchant. Undercutting can also be reduced or even prevented by corner compensation structures which are added to the corners in the mask layout. Depending on the etching solutions,
63 different corner compensation schemes are used. Commonly used techniques are square compensation (EDP or KOH) and rotated rectangle corner compensation methods (KOH) [46]. The second method for corner compensation technique is used in this case. In the rotated rectangular corner compensation method, a properly scaled rectangle (Breadth B should be twice the etch depth De ) is added to each of the mask corners. The four sides of the mesa square (proof-mass) are still aligned along the <110> directions, but the compensation rectangles are rotated 45 o with their longer sides along the <100> directions. The rectangular bar along <100> direction is undercut by KOH along three preferential directions, namely <100> sidewalls, <410> sidewalls and <410> of the free end [44]. When the length of the <100> bar is kept at 1.6 B (for 33% KOH), the undercut of the latter two directions should stop first, and only the lateral (100) sidewalls etching determines the final undercut. Thus, the etching rate is the same as that of the (100) etching along with the depth, so if the width of the bar is twice the etching depth, complete convex corners can be obtained. Here, etch depth De = 172 µm is used, as etching takes place from both sides of the wafer (344 µm thick). Width of the rotated rectangle is B = 344 µm and Length L= 1.5 x 344 = 516 µm. The factor 1.5 comes from the fact that 40% KOH is used. This is explained further in section 3.3.2. But here, in the last KOH etching step, the mask above the proof-
64 mass is also removed for bringing down the proof-mass thickness to 300 µm. Therefore, the effective depth for which the corner compensation mask was retained is only ((344-44)/2 = 150 µm). Taking this into account, the new values are B = 300 µm and L = 450 µm. Considering process alignment tolerances, the final values are finalized as B = 310 µm and L = 460 µm as shown in fig 3.2. Fig 3.2 Corner compensation of the proof-mass (LHS full view and RHS Zoomed view) 3.3 Simulations in Intellisuite AnisE, an anisotropic etch process simulation tool from Intellisuite software is used for anisotropic etching simulation. The input to this is a two dimensional mask and the output is the three dimensional etched structure. The mask used looks as shown in fig. 3.3.
65 Fig 3.3 Mask without corner compensation The mask shown is used for etching the (100) plane of silicon wafer with 40% KOH concentration, at temperature 60 o C. Fig 3.4 Structure after etch simulation After etching, the silicon structure is as shown in fig.3.4. The structure consists of a proof -mass and four beams whose thickness is same as that of the proof-mass. The beams are attached to a frame of thickness 300 microns. It is apparent from the figure that the corners of the proof-mass and the beams are etched off. The deformation of the edges occurs due to undercutting and this necessitates the use of compensation structures.
66 3.3.1 Dimensions of corner compensation structure The experiments of Mayer et al showed that the undercutting of convex corners in pure KOH etches are determined exclusively by {411} planes. At the wafer surface, the sectional line of {411} and a {111} plane points in the <410> direction, forming an angle of 30.96 o with the <110> direction. From literature [44], it is clear that for a rotated rectangle compensation structure, the width B should be equal to twice the etch depth if etching is done from only one side. Meanwhile, the necessary length of the compensation bar depends on its width, hence the etching depth as well as the ratio of etching rates of <410> and <100> [45]. The length of the rectangle is given by L = 1.6 B for 33% KOH [44]. This is verified using Anis-E module of Intellisuite. Here one of the corners of the proof-mass is covered with compensation rectangle structure. The structures after etching are shown in fig 3.5. For 40% KOH solution, the length is even shorter and can fairly approximated as L = 1.5 B taking in to account of the fact that the lateral etching (Etch rate along <410> / Etch rate along <100>) decreases with increase in concentration of KOH [44].
67 L = B L = 1.2 B L = 1.4 B L = 1.6 B Fig 3.5 Proof-mass structures showing effect of length variation of compensation mask (33% KOH) 3.3.2 Centralization of beams with respect to proof-mass To realize the beams at the center of the proof-mass, a technique known as self aligned etching is used. The anisotropic etching of KOH
68 takes place in three phases. In the first phase, the silicon wafer is etched for 55µm by masking the proof-mass and beam areas. In the second phase of etching silicon, the beams are unmasked and the structure is realized with the proof-mass of whole wafer thickness and the beams are centralized with a thickness of 99 µm. In the final phase of etching (44µm), the oxide over the proof-mass is also removed and the complete structure is realized with a proof-mass thickness of 300 µm and beam thickness of 55µm. The etched structure looks as shown in fig 3.6 (a) and (b). Fig 3.6(a) Etch simulation result after first phase etching Fig 3.6(b) Etch simulation result after final phase etching
69 3.4 Process flow for fabrication 3.4.1 Silicon wafer processing for microstructure fabrication 1. (100) Oriented P-type silicon wafer 344 5 µm Thick, Resistivity of 0.1 -cm Silicon The processed wafer thickness measured is 344 µm. 2. Wet thermal oxidation for a final oxide thickness of 1.1µm SiO 2
70 3. Pattern oxide in the contact area. Oxide is etched using BHF to depth of 0.2µm.(Mask1) Photoresist MASK 1 The etch rate of silicon dioxide in BHF is 0.1µm / min. The above step is required since in the final step, a bulk BHF etching of oxide is to be carried out to open the contact area and also to reduce the oxide thickness in the frame area to 0.15µm-0.2µm. This step also has the added advantage of providing better aligning of the subsequent masks as the contact pad area lies at a fixed distance from the proof-mass beam area.
71 4. Oxide patterning with BHF in mass-beam area (Mask 2) MASK 2 5. Wet thermal oxidation in the mass-beam area for a thickness of 0.75µm
72 6. Oxide etching using BHF in the beam area (Mask 3) MASK 3 7. Wet thermal oxidation in the beam area for a thickness of 0.15µm
73 8. Oxide etching using BHF in the through etching area (Mask 4) MASK 4 9. First Phase KOH etching from both sides KOH concentration : 40% Temperature : 60 o C Etch depth from one side : 27.5 µm Total Silicon thickness removed from through etching area : 55 µ Etch rate Time of etch : 44 µm / hr : 37.5 mins
74 Masking material (SiO2) etch rate : 0.08 µm / hr Thickness of oxide removed from beam area : 0.05 µm Thickness of oxide remaining in beam area : 0.1 µm 10. Oxide etching using BHF in the beam area Etch rate of oxide in BHF : 0.1 µm /min Time of etch : 1 min
75 11. Second phase KOH etching from both sides KOH concentration : 40% Temperature : 60 o C Etch depth from one side : 122.5 µm Total silicon thickness removed from through etching area : 300 µm Total silicon thickness removed from beam area : 245 µm Etch rate : 44 µm / hr Time of etch : 167 mins `
76 Masking material (SiO2) Etch rate : 0.08 µm / hr Thickness of oxide removed from proof-mass area : 0.25 µm Thickness of oxide remaining in proof-mass area : 0.5 µm 12. Oxide etching using BHF in the proof-mass area Etch rate of Oxide in BHF : 0.1 µm /min Time of etch : 5.0 min 13. Third and final phase KOH etching from both sides KOH concentration : 40% Temperature : 60 o C Etch depth from one side : 22 µm Total silicon thickness removed from through etching area : 344 µm
77 Total silicon thickness removed from beam area : 289 µm Beam width achieved : 344 289 = 55 µm Proof-mass thickness achieved : 344 44 = 300 µm Etch rate : 44 µm / hr Time of etch : 30 mins Masking material (SiO2) etch rate : 0.08 µm / hr Thickness of oxide removed from contact area : 0.04 µm Thickness of oxide remaining in contact area : 0.86 µm 14. Oxide etching using BHF in the contact area Etch rate of Oxide in BHF : 0.1 µm /min Time of etch : 8.6 min
78 Maximum Oxide thickness left in the frame area: 0.2µm (Refer Table 3.1) 15. Evaporate 1500 A o Al for contact pad (Mask 5) Al MASK-5
79 Maximum Thickness of Silicon dioxide (in microns) remaining after Area Oxidation 1 st Phase KOH Etch 1 st Phase BHF dip 2 nd Phase KOH Etch 2 nd Phase BHF dip 3 rd Phase KOH Etch 3 rd Phase BHF dip Beam 0.15 0.1 0 - - - - Proofmass 0.9 0.85 0.75 0.5 0 - - Contact 1.8 1.75 1.65 1.4 0.9 0.86 0 Frame 2.0 1.95 1.85 1.6 1.1 1.06 0.2 Table 3. 1 Thickness of Silicon dioxide in different regions at different stages of etching Fig 3.7 Top view of the processed silicon die
80 3.4.2 Top glass wafer processing 1. 100 mm Pyrex 7740 Glass wafer with a thickness of 500µm Glass 2. Evaporate 1500 A 0 Al, on one side of the wafer 3. Pattern Al using Mask 6 Al
81 4. Pre dicing trenches are formed in top glass wafer with depth and width of 100 µm. MASK 6 3.4.3 Bottom glass wafer processing 1. 100 mm Pyrex 7740 Glass wafer with a thickness of 500µ Glass
82 2. Evaporate 1500 A o Al, on one side of the wafer Al 3. Pattern Al using Mask 7 MASK 7 4. Wafer edge is also diced for placing electrode on Silicon during top glass anodic bonding process 5. The three separately processed wafers are anodically bonded together and diced using mechanical dicing equipment. The exploded view of
83 the diced chip is shown in fig 3.8. The total size of the chip is 9 mm x 8 mm x 1.344 mm Fig 3.8 Exploded view & assembled view of the accelerometer chip 3.5 Results and discussion Fig 3.9 shows the Scanning Electron Microscope (SEM) picture of the proof-mass cross-section. The designed dimensions of the proofmass with rectangular cross-section were 2500 µm x 2500 µm x 300 µm. Due to anisotropic etching, the fabricated proof-mass has hexagonal cross - section and the dimensions achieved are 2458 µm x 2458 µm x 300 µm. This is fairly good result taking into account the complexity of fabrication. Fig 3.10 shows the SEM picture of the beam cross-section. The designed dimensions of the rectangular cross section of beam were 150 µm x 55 µm and the achieved dimensions are 176 µm x 49 µm. Here, the thickness of beam is reduced by 6 µm, and width increased by 26 µm, so the moment of inertia of beam reduces by 0.83
84 times compared to the designed rectangular cross-section beam. This is expected to cause 20% increased deflection & sensitivity. Fig 3.9 SEM picture of the proof-mass and beam Fig 3.10 SEM picture of the beam cross section
85 Fig 3.11 Sharp corners realized on proof-mass and L-beams Fig 3.12 Photograph of the diced sensor chip in its final form Fig 3.11 shows the clean and sharp corners realized on the corners of the proof-mass and on L-beams. This validates the corner compensation design technique used in the thesis. Fig 3.12 shows the diced chip in its final form. As shown here the top glass is diced in such a way that all the electrical pads are accessible for further wire bonding.