DRY ETCHING

Diffusion of phosphorus

Oxidation and patterning

A

m jjjjjj^ m

nisotropic etchin through n-layer (a) (b) (c)

Selective p/n etching

(d) V777A n-layer

^H Oxide Figure 5.16 Process flow of etched-pattern technique

process and are not exposed to the etchant, which helps in stopping microdamage on the surfaces.

5.4 DRY ETCHING

As discussed in Sections 5.2 and 5.3, bulk-micromachining processes can yield SCS microstructures using crystal-orientation-dependent and dopant-concentration-dependent wet chemical etchants, such as EDP, KOH, and hydrazine to undercut the SCS structures from a silicon wafer. However, the type, shape, and size of the SCS structures that can be fabricated with the wet chemical etch techniques are severely limited. A dry-etch-based process sequence to produce suspended SCS mechanical structures and actuators has been developed (Zhang and McDonald 1992). The process is called single-crystal reactive etching and metallisation (SCREAM). SCREAM uses RIE processes to fabricate released SCS structures with lateral feature sizes down to 250 nm and with arbitrary structure orientations on a silicon wafer. SCREAM includes process options to make integrated, side-drive capacitor actuators. A compatible high step-coverage metallisation process using metal sputter deposition and isotropic metal dry etch is used to form side-drive electrodes. The metallisation process complements the silicon RIE processes that are used to form the movable SCS structures.

For the SCREAM process, mechanical structures are defined with one mask and are produced from a silicon wafer. The process steps used in SCREAM are illustrated in

Si substrate

~U1_T

Si substrate (d)

Si substrate

Aluminum

Thermal SiO2 Si substrate

Photoresist

Figure 5.17 SCREAM process flow for a straight cantilever beam with integrated electrodes (Zhang and McDonald 1992)

Figure 5.17. This figure shows the SCREAM process sequence for the fabrication of a straight cantilever beam including the integrated metal electrodes adjacent to each side of the beam. A layer of SiO2, used as an etch mask, is thermally grown on the silicon substrate. The pattern to produce free-standing SCS structures is created using photolithography. The photoresist pattern on the SiO² is transferred to the silicon dioxide using fluorocarbon-based oxide etching plasma. The photoresist is then stripped by an O2 plasma etch and the SiO² pattern is subsequently transferred to the silicon substrate using a Cl^– -based or a HBr-based RIE. Following the silicon etch, a sidewall silicon dioxide layer is thermally grown in wet O² at high temperatures. The thermal oxidation process reduces possible damage on the sidewalls of the silicon steps created during the RIE process. The lateral dimensions of the SCS structures are reduced during the thermal oxidation. A metal layer is conformably deposited on top of the thermal oxide using a sputter deposition system. Before the metal deposition, contact windows are opened to allow electrical contact to both the silicon substrate and the movable silicon structures.

After the metal sputter deposition, photoresist is spun on the metal layer to refill the few-microns-deep trenches. A metal side-electrode pattern is created in the photoresist using

136 SILICON MICROMACHINING: BULK

a wafer stepper to expose the photoresist. The metal side-electrode is then transferred to the metal by a metal RIE process. After the metal electrodes are patterned, the SiO² is etched back with fluorocarbon-based RIE. Finally, the silicon mechanical structures are released from the silicon substrate using an RIE process.

The SCREAM process described in the preceding text can be used to fabricate complex, circular and triangular structures in SCS. These structures can include integrated, high-aspect-ratio, and conformable capacitor actuators. The capacitor actuators are used to generate electrostatic forces and so produce micromechanical motion.

Worked Example E5.5: Fabrication of Straight Cantilever Beam with Integ-rated Aluminum Electrodes Using SCREAM

Objective:

To fabricate a free-standing cantilever beam 200 nm long, 0.8 um wide, and 3.5 urn thick and coated with a 150 nm-thick SiO2 layer (Zhang and McDonald 1992).

Process Flow:

The steps are based on the process shown in Figure 5.17, with the metal being aluminum.

1. The starting substrate is an arsenic-doped, 0.005 fi-cm, n-type (100) silicon wafer. A layer of SiO² that is 400 nm thick is thermally grown on the substrate in a conven-tional furnace in a steam O2 ambient at 1100°C. The oxide is used as an etch mask.

The pattern to produce free-standing SCS structures is created using photolithography in positive photoresist spun on the oxide layer on silicon. A wafer stepper is used to expose the photoresist. The minimum feature size of the SCS beam structures is 400 nm (see Figure 5.17(a)).

2. The photoresist pattern on the oxide is transferred to the oxide in a CHF3/O2 plasma etch at flow rates of 30 seem5 and 1 seem, a chamber pressure of 30 mTorr and a DC self-bias of 470 V in a conventional parallel-plate RIE tool. The etch rate of the oxide is 23 nm/min. The photoresist on top of the oxide is then stripped off by an O2 plasma etch (Figure 5.17(b)).

3. The oxide is then transferred to the silicon substrate using a Cl2/BCl3/H2 RIE in a commercial RIE tool. Three etch steps are required to accomplish a deep vertical silicon etch (Figure 5.17(c)). The parameters for these etch steps are given in Table 5.3.

Table 5.3 Silicon etch parameters (Zhang and McDonald 1992) Step

1 sccm is standard cubic centimeter per minute.

BURIED OXIDE PROCESS 137 4. Following the silicon etch, a sidewall SiO2 layer of 150 nm is thermally grown

in wet O² at 1000°C. The thermal oxidation process reduces possible damage on the sidewalls of the silicon steps created during the Cl²/BCl³/H² RIE. The lateral dimensions of the SCS structures are reduced during the thermal oxidation (Figure 5.17(d)).

5. A 400 nm layer of Al with a step coverage of 60 percent on the sidewalls is conformably deposited using DC magnetron sputter deposition in a commercial depo-sition tool. The sputtering is performed at a pressure of 9 mTorr and at a temperature of 20 °C in argon gas and a beam current of 5 A. Before the Al deposition, contact windows are opened to allow electrical contact to both the silicon substrate and the movable silicon structure (see Figure 5.17(e)).

6. After the Al deposition, positive photoresist is spun on the Al layer at 2.5 krpm for 45 seconds (3.6 urn thick) to fill the 3.8 urn-deep trenches. An Al side-electrode pattern is created in the resist using a wafer stepper to expose the resist (Figure 5.17(f)).

7. The Al side-electrode pattern is then transferred to the Al by means of an aluminum dry-etch process; this step is required to clear the conformal Al layer deposited on the high steps where the resist has been removed. The etch process is an anisotropic RIE process that utilises flow rates of 20 sccm of Cl² and 40 sccm of barium trichloride (BCl³) at a chamber pressure of 50 mTorr and a DC self-bias voltage of 250 V. The photolithography and the Al RIE steps produce smooth edges on the Al pattern over a topography with 3.8 nm steps (Figure 5.17(g)).

8. After the Al electrodes are patterned, the SiO2 is etched back with a CF4 plasma in a standard parallel-plate RIE tool. Anisotropic etch profiles are preferred to the oxide etch-back to remove the oxide from the bottom of the trenches but to retain the SiO2

on the top and sidewalls of the SCS structures. Therefore, a low chamber pressure of 10 mTorr and a high DC self-bias of 600 V are selected in the CF4 RIE process (Figure 5.17(h)).