Non ideal effects of the ICP process

Non ideal effects exist in the ICP process due to experiment inaccuracy. The effects include notching, loading effect, and grass formation etc. The detail process will be shown in the following.

Notching effect

Deep reactive ion etching through the silicon device layer is an essential step in microstructure fabrication. However, plasma etching the silicon over an insulator layer has a silicon notching problem at the silicon/insulator interface. The

Tungsten ball Switch Fingers

(l) Wire bonding and tungsten ball attachment (Step 13)

phenomenon was shown in Fig. 3.2 [37]. The positive charge accumulation in the isolation layer causes further deflection of reactant ion and forwarding scattering. The poor profile caused by the notching may result in degraded performances. As this undercutting is aspect ratio dependent, the profiles and the characteristics of the final devices may further vary across the wafer, affecting the repeatability and reliability, especially for the thick device wafer.

Fig. 3.2 Notching effect [37]

The problem was encountered in our device structure, as shown in Fig. 3.3 (a).

The bottom of the comb finger was over etched due to the plasma bombarding on the interface between silicon and oxide layer. Therefore, the deep ion etching was divided into two steps to prevent the silicon over etching. First, the etch rate of the deep silicon etching should be controlled accurately. Thus, the comb fingers should be etched a little thickness to estimate the etch rate and subsequently performed remainder thickness. The better profile of comb fingers was shown in Fig. 3.3 (b)

Insulation layer

Fig. 3.3 (a) Notching occurred on the fingers bottom side, (b) improvement result

Loading effect

The loading effect is an important issue for the ICP etching process. This effect indicates that the etch rate will be severely reduced when the number of chip and percentage of the open area is too large [38]. It also happens in our device during the back side ICP etching. As mentioned above, the etch rate in the ICP process is 0.6 μm per cycle with the six chips but is about 0.56 μm with ten chips bonded on the carrier wafer. The open area is proportional to the number of chips bonded on the wafer.

More chips on the handle wafer also cause poor heat dissipation and reduce the etch rate of silicon.

Grass formation

The other issue is the grass formation in the ICP process. The basic process of ICP etching can be divided into two major steps. The first step is a sidewall passivation step to achieve a straight sidewall feature with the polymer gas (C4F8) and the second step is silicon etching by etching gas (SF6). The periodic change of gases for etching (SF6) and passivation (C4F8) can lead to very high aspect ratios and very high etch rates.

(a) (b) Silicon over etched

However, ICP etching has its own specific problems, which include “grass”

formation, due to non-uniform etching [39]. To solve this problem, we decrease the C4F8 flow time (passivation time) from 10 s to 7 s and SF6 etching time is not changed.

The experiment results are shown in Fig. 3.4. The sidewall of the finger with decreased C4F8 gas flow time is changing dramatically. A major problem during etching silicon is the forming of “grass” on the surface. As a result of particulate material inadvertently deposited on the silicon surface, the material can locally mask the silicon during etching. This material can be formed during etching due to redeposition and growing of polymer material from the sidewall passivation step. By decreasing the cycle time of C4F8, the grass formation was remarkably decreased

Fig. 3.4 SEM photo with different C4F8 flow time (a) SF6: C4F8 = 11s:10s (b) SF6: C4F8

= 11s:7s

Sidewall flatness

The results of ICP process will influence the silicon nitride deposition and electrical measurement. The experiment results in the ICP etching is shown in Fig. 3.5.

The polymer adhered on the sidewall of the finger is produced by the passivation gas in the ICP process.

(a) (b)

Therefore, two solutions are proposed in the following. One is to adjust the fabrication parameters, such as the ratio of etching and passivation time or the RF power, to prevent formation of more polymers. But the etch rate of silicon are dependent on these fabrication parameters. The other is cleaning by chemical solution, such as NH3 solution (NH4OH: H2O2: H2O = 1: 4: 20). As mentioned in section 3.1, NH3 solution can remove unwanted particles and ensure the silicon surface not to be etched. But the finger structure fragility was taken into account when the device was immersed in the solution which heated to the temperature of 85^οC. Fortunately, the device cleaned by NH3 solution was all right and the polymer was removed, as shown in Fig. 3.5 (b).

Fig. 3.5 (a) Polymer produced in the ICP etching (b) polymer removal by NH3 solution

Plasma permeate problem

This problem occurred on the back side ICP etching process. As mentioned above, the diced chip had to be bonded on the carrier wafer with the heat dissipation glue. The plasma may permeate through the interface between the front side and the surface of the carrier wafer if the heat dissipation glue did not cover completely the edge of the diced chip. The results are shown in Fig. 3.6.

Polymer

(a) (b) Polymer

Fig. 3.6 Experiment results in the plasma permeate on the front side