A Wafer-Scale Etching Technique for High Aspect Ratio Implantable MEMS Structures

Abstract

Microsystem technology is well suited to batch fabricate microelectrode arrays, such as the Utah electrode array (UEA), intended for recording and stimulating neural tissue. Fabrication of the UEA is primarily based on the use of dicing and wet etching to achieve high aspect ratio (15:1) penetrating electrodes. An important step in the array fabrication is the etching of electrodes to produce needle-shape electrodes with sharp tips. Traditional etching processes are performed on a single array, and the etching conditions are not optimized. As a result, the process leads to variable geometries of electrodes within an array. Furthermore, the process is not only time consuming but also labor-intensive. This report presents a wafer-scale etching method for the UEA. The method offers several advantages, such as substantial reduction in the processing time, higher throughput and lower cost. More importantly, the method increases the geometrical uniformity from electrode to electrode within an array (1.5 ± 0.5 % non-uniformity), and from array to array within a wafer (2 ± 0.3 % non-uniformity). Also, the etching rate of silicon columns, produced by dicing, are studied as a function of temperature, etching time and stirring rate in a nitric acid rich HF-HNO(3) solution. These parameters were found to be related to the etching rates over the ranges studied and more-importantly affect the uniformity of the etched silicon columns. An optimum etching condition was established to achieve uniform shape electrode arrays on wafer-scale.

Fig. 1

Cartoon image of the Utah electrode array.

Fig. 2

Schematic illustration of a wafer containing arrays after using a dicing saw to make 1.5 mm cuts into the silicon wafer. For scale, the length of the columns is 1.5 mm.

Fig. 3

Cartoon image of the etching concept. Dicing saw is used to make orthogonal cuts in 2 mm thick silicon wafer to yield 1.5 mm long columns (a). A two step wet etching technique is used, where in the first step (b), called dynamic etching, the width of the columns is isotropically thinned, and in the second step (c), called static etching, the tips of the electrodes are preferentially sharpened. The arrows indicate pronounced etching.

Fig. 4

Photograph of the custom Teflon wafer holder for wafer- scale etching of electrode arrays.

Fig. 5

Photographs of the wafer scale etching system. (a) Shows dynamic etching setup, the width of the columns is isotropically thinned by the aggressive flush of acid. (b) Shows static etching setup, the tips of the electrodes are sharpened by the preferential etching.

Fig. 6

Plot of the column etch rate and non-uniformity in column width as a function of stir-bar speed. Each data point represents 3 etch experiments.

Fig. 7

Model for reaction mechanism. Here x = 0 represents silicon surface. The plot has been adapted from Robbins and Schwartz's work on planar silicon surface [10-13] to explain the etching mechanism in the UEA.

Fig. 8

Arhenius plot of etch rate versus temperature and the non-uniformity (%) in tip geometry also as a function of temperature.

Fig. 9

Plot of etching time versus temperature.

Fig. 10

SEM images showing progression of electrode formation from dicing through etching (af). The rectangular columns (a) are transformed into sharp electrodes (d) during static etching (8 min). The high etch rate (20 um/min), causes significant change in the geometry of the electrodes. The etching time can be varied to achieve desired final electrode shape. The geometry can range from thick, rigid, “missile” shaped electrodes with a rounded or pointed tip to extremely thin, flexible electrodes.

Fig. 11

A scaled drawing of an electrode side view showing the distance from the electrode base at which all, measurement were made. Listed is the non-uniformity percentage ± the standard deviation for 350 electrodes measured from 5 wafers.

Fig. 12

SEM micrograph of electrode arrays after static etching, performed on a 75 mm diameter wafer. Note the sacrificial features (extra row of electrodes, fins, corner posts) surrounding the UEA. These additional features help achieve uniformity in electrode geometry and later in tip exposure during photoresist coating [16].