|
Micropipes in SiC - Device Killers
Also, silicon carbide has found an application as a substrate material for III-nitrides device structure epitaxy, due to its good lattice match with III-nitrides and high thermal conductivity.
Most views on micropipes are based around Frank’s theory of a micropipe being the hollow core of a screw dislocation with a large Burgers vector. According to the theory, the diameter of the micropipe has a direct relationship with the magnitude of the Burgers vector. Generally speaking, a micropipe is a tube (with a diameter ranging from fractions of a micron to tens of microns) that propagates through SiC crystal in the direction parallel or close to the [0001] crystallographic axis. Today, the density of micropipe defects in standard SiC commercial wafers, which are being used as substrates for SiC device fabrication, exceeds 100 cm-2. These micropipes, originated from SiC substrates, penetrate in device structures during epitaxial growth and cause the device failure (Figure 1). Closing of micropipes during SiC LPE growth It has been shown that micropipes in SiC wafers may be closed during liquid phase epitaxial growth. A few independent researchers have proved this fact. The general observation was that the channel of the micropipe, penetrating from SiC substrate in to epitaxial layer, became smaller and smaller during Liquid Phase Epitaxy (LPE). Finally, the micropipe closes, forming a growth hillock on the epitaxial layer surface. In order to close micropipes during liquid phase epitaxial growth, a thick (up to 100 microns) SiC epitaxial layer should be grown on the initial SiC wafer. Due to the requirement of thick SiC layer formation, the growth temperature and supersaturation in a melt must be reasonably high and the solubility of SiC in a melt must also be high. During thick layer growth, the formation of other defects, such as foreign polytype inclusions in the grown layer, is possible. The height of growth steps forming due to step bunching on the SiC surface is larger for the thicker layers. If a thick layer is required to close micropipes, the amplitude of surface relief may reach a few microns, causing problems in growing a SiC device structure on such a surface. Experiments also show that it is difficult to close micropipes by this approach if the SiC wafer has a large misorientation angle. Filling the Micropipe Channels in Standard Commercial SiC
Currently TDI is manufacturing 50 mm diameter SiC epitaxial wafers with micropipe density less than 10 cm-2. Properties of SiC epitaxial layers with reduced micropipe density Step
bunching occurs during epitaxial growth and surface morphology depends strongly
on substrate orientation. The flattest surfaces are obtained on well-oriented
substrates. Step height increased with the increase of the tilt angle of the
substrate. The SEM image of 4H-SiC layer grown on Si face of 4H-SiC substrate
with tilt angle of 8 degrees is shown in the Figure 3. We etched SiC wafer with reduced micropipe density in molten KOH to count number of micropipes remained in the wafer after micropipe filling process. This method is time-consuming, but it provides reliable results on micropipe density measurements. If the micropipe is not closed during the epitaxial growth, it is clearly seen on the surface of the epitaxial layer. When the closing process is completed, a growth hillock can usually be seen on the layer surface at the point corresponding to the micropipe in the substrate. There is no difference in the micropipe closing process of 4H and 6H polytypes. However, the misorientation angle plays an important role in the micropipe closing process. It is more difficult to fill micropipe if the tilt angle is large. After epitaxial growth on on-axis wafers, some areas, about 1 cm2, having no micropipes were observed on the epitaxial layers. Micropipes were not opened after short etching (about 1 minute) at 500oC. Very important to note that micropipe filling process did not depend on size of SiC wafer. |
|
|
|
Copyright (c) 1997-2005 by TDI, Inc. |
