Trikon Has Last Word With Omega Etch
For several types of power semiconductor, significant advantages in device performance can be achieved by the use of a silicon trench technology. Many of the details of the trench etch, such as bottom rounding and sidewall smoothness have an effect on the device performance. One of the most important parameters is the trench depth. For example, in low voltage power MOSFETs the trench depth has a big impact on the gate drain capacitance. A SEM of a typical power device silicon trench is shown in figure 1.
Controlling the trench etch depth is more difficult than for most dry etch applications, because for most power devices there is no etch stop layer at which the etch can be terminated using conventional optical emission spectroscopy (OES). Controlling the depth simply by time is subject to many possible inconsistencies, such as variations due to chamber condition or loading effects due to mask open area changes from device to device. Typically only ±4% depth accuracy wafer to wafer is achieved with timed etches, and in many cases this level of control is insufficient for high device yield.
In order to improve the trench depth repeatability to the levels required for robust manufacturing, Trikon has integrated in-situ interferometric endpoint detection (IEPD) capability to its well established Omega ICP etch chamber. The design of the ICP chamber is particularly suited to the hardware integration. IEPD requires a direct line of sight normal to the wafer surface, and as the ICP single turn antenna is around the outside of a ceramic wall, this means that the viewport is easily placed in the centre of the chamber lid without interfering with the RF source (figure 2). The ICP chamber with IEPD is available on both Trikons Omega fxP cluster tool and the new Omega i2L platforms (see side panel).
The IEPD system works by monitoring light reflected from a section of the wafer surface, at a chosen wavelength. The intensity of reflected light seen is a result of the interference between reflections from different parts of the wafer surface, such as the top of the mask, the bottom of the mask, and the silicon etch surface exposed for etch.
As the etch proceeds, the silicon etches faster than the mask and so the distance between the two surfaces increases, changing the optical path length and so the interference conditions. The variation in interference produces an oscillation in intensity, the frequency of which is related to the etch rate.
A typical IEPD trace is shown in figure 3. Here the faster oscillation is caused by the silicon trench etch, whereas the slow overall variation in signal is due to etching of the mask. By monitoring this signal, and compensating for the mask etch rate if necessary, it is possible to continually measure the etch depth on every wafer with a very high degree of accuracy. This ability means that each etch can be terminated at a pre-programmed depth even though no etch stop is present, automatically compensating for any etch rate variations from wafer to wafer and cassette to cassette.
The benefits of using IEPD to the customer are many. The most immediate is a significant improvement in trench depth repeatability. Tests show that wafer to wafer uniformity is typically 0.8% for a 6 ìm deep trench. This improved consistency in trench depth can lead directly to higher device yield compared to a timed etch, as