Page 74 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH06.qxd 04/04/2005 19:37 Page 6.13

PLASMA PROCESS CONTROL

PLASMA PROCESS CONTROL 6.13

Before After

Plasma Plasma
Plasma C Plasma C
sheath p sheath p
Resist Oxide C o Resist
Silicon Silicon
Probe Probe

FIGURE 6.12 Oxide etch.

the chamber and power level of each source was established such that arc transients were intentionally
created. The trace with more cycles for the given time period in Fig. 6.11 is the measured frequency
for the 2-MHz RF source and the trace with three complete cycles is the measured frequency of the
27-MHz RF source. The discontinuities in the traces with three complete periods are correlated to the
occurrence of arcs in the plasma. This demonstrates the rigid ability to maintain lock to the frequency
of multiple RF sources in a worse-case environment. Also apparent is the ability to detect plasma arc
transients. Extensions of arc detection include arc classification and suppression controls.
Figure 6.12 illustrates before and after effects of an oxide etch. The illustration on the left in this
figure shows the presence of oxide in a trench prior to initiating an etch. The circuit diagram of the
capacitor-divider circuit is a simplification of the resulting change in the impedance measured by the
voltage/current sensor and analysis module. As the ion bombardment penetrates the trench and
removes the oxide, the impedance of the plasma varies. Monitoring the harmonics generated from
the plasma has proven to be an effective method of monitoring the etch process. 49,53,54 Real-time
feedback control and monitoring of the impedance during the etch process has been demonstrated to
yield an etch rate and endpoint. Figure 6.13 yields an example of polysilicon etch when monitor-
55
ing the sixth harmonic of 13.56 MHz. The T1 and T2 labels on the plot indicate the etch start and

Etch Trend on 6th Harmonics
1.15 T2
BARC Etch Mask Etch Polyetch #1 Polyetch #2
1.13
1.11
1.09
Noimalized 1.05 T1 T2 T1 I V Phase
1.07
Z-mag
1.03
1.01
0.99
0.97
0.95
0.00 50.00 100.00 150.00 200.00 250.00 300.00
Time (s)
FIGURE 6.13 Endpoint detection.

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