Hints, Tips and Solutions

Q. How can I properly setup Athena Monte Carlo simulation of ion implantation in Silicon Carbide?

A. Monte Carlo module of Athena allows accurate simulation of ion implantation in 4H-SiC and 6H-SiC [1]. It is known that channeling effects are very pronounced in SiC since its lattice has several wide open channels ( e.g. 〈0001〉, 〈1120〉, and 〈1123〉 for 4H-SiC) and therefore implant profiles strongly depend on wafer orientation and ion beam direction. Another important aspect of ion implantation simulation in SiC is the fact that most of commercially available standard 〈0001〉 wafers are offered with miscut of 3.5° - 8.5° usually toward 〈1120〉 direction. The miscut is introduced primarily to facilitate epitaxial growth. In the same time the miscut alters ion implantation, because implanting normal to the wafer surface is equivalent to a tilt of the beam in the (1120) plane.

In order to perform Monte Carlo implant simulation in Athena you should first specify simulation plane. The Monte Carlo module of Athena simulates ion trajectories in 3D and then registers the stopping points and consequently doping concentration by projecting 3D result onto simulation plane. The module also assumes that configuration of material interfaces don’t change in direction normal to this simulation plane. The simulation plane is always perpendicular to the wafer surface and its orientation is specified by parameter ROT.SUB in the the INITIALIZE statement relative to the wafer major flat (see Figure 1). To our knowledge most commercial 4H-SiC and 6H-SiC wafers have major flat parallel to (1100) plane as in Figure 1.

Figure 1. SiC wafer orientation relative to simulation plane in Athena BCA Module. Also shown are major plane projections on the wafer surface.

 

So, if you want your Athena simulation plane to be parallel to the major flat, 〈1100〉, you have to specify ROT.SUB=0. If you want your simulation plane to be parallel to 〈1120〉 crystallographic direction you should specify ROT.SUB=90.

The ion beam direction is specified by parameters TILT and ROTATION in the IMPLANT statement. The TILT is the angle (in degrees) between the ion beam vector and the normal to the wafer surface. The ROTATION is the angle between the ion beam vector plane perpendicular to the wafer surface and the simulation plane. For example, if the ion beam vector plane is parallel to 〈2110〉 crystallographic direction in Figure 1 you have to specify ROTATION=30 (see Figure 2). Alternatively, if the ion beam vector plane is perpendicular to simulation plane you have to specify ROTATION=90.

Figure 2. 3D SiC wafer and simulation plane orientation. ROT.SUB=0 for simulation plane parallel to (1100).

 

The wafer miscut is specified by parameters MISCUT.TH and MISCUT.PH. Parameter MISCUT.TH specifies the angle between actual wafer surface and the XZ plane (see Figure 1). Parameter MISCUT.PH specified the angle (measured counter-clockwise from the major flat direction) toward which the wafer was cut. So, if the wafer is cut toward <1100> (i.e. X direction in Figure 1) you need to specify MISCUT.PH=0. This type of miscut is not used very often. Majority of commercial wafers (see, for example, [2] and [3}) have miscut toward <1120>. So, practically in all cases you need to set MISCUT.PH=90 and MISCUT.TH=4 or MISCUT.TH=8 depending of “off axis” angle in commercial wafer specifications (see, [2] and [3]).

So, a typical initialize of Monte Carlo implant statements will be as follows:

init sic_4h rot.sub=0
implant aluminum energy=50 dose=1e13 bca n.ion=10000 tilt=4 rotation=90
miscut.th=4 miscut.ph=90

This will correspond to implant to a wafer with standard miscut usually specified as “Off axis 4° toward 〈1120〉 ± 0.5°” in commercial wafer specifications. The simulation plane will be parallel to the major flat, and ion beam will be tilted by 4 degrees in the plane perpendicular to the simulation plane.

The following examples closely demonstrate equivalent implant directions based on miscut MISCUT.TH/PH or TILT/ROTATION specifications.

 

EXAMPLE 1:

This example demonstrates equivalent implant directions with wafer’s major flat, 〈1100〉.

init sic_4h rot.sub=0

implant aluminum n.ion=40000 dose=0.63e13 tilt=0 rot=0 energy=60 bca

The equivalent implant direction with miscut wafer 4° 〈1120〉 toward will be

init sic_4h rot.sub=0

implant aluminum n.ion=40000 dose=0.63e13 tilt=4 rot=90 miscut.th=4 miscut.ph=90 energy=60 bca

The two profiles will be statistically equivalent with slight shift due to cos(4°) projection of the second profile.

 

EXAMPLE 2:

This example demonstrates equivalent implant directions with wafer’s major flat, 〈1120〉. NOTE, MISCUT.TH/PH are measured from the internal crystallographic coordinate system, not the laboratory one defined by ROT.SUB.

init sic_4h rot.sub=90

implant aluminum n.ion=40000 dose=0.63e13 tilt=0 rot=0 energy=60 bca

The equivalent implant direction with miscut wafer 4° toward 〈1120〉 will be

init sic_4h rot.sub=90

implant aluminum n.ion=40000 dose=0.63e13 tilt=4 rot=0 miscut.th=4 miscut.ph=90 energy=60 bca

The two profiles will be statistically equivalent with slight shift due to cos(4°) projection of the second profile.

 

EXAMPLE 3:

The following example demonstrates equivalent implant conditions for 3° tilted off [0001] direction toward 〈1120〉 implant for ideal, and for miscut 4° toward 〈1120〉 4H-SiC wafer.

init sic_4h rot.sub=0

implant aluminum n.ion=40000 dose=0.63e13 tilt=3 rot=0 energy=60 bca

The equivalent implant direction with miscut wafer 4° toward 〈1120〉 will be

init sic_4h rot.sub=90

implant aluminum n.ion=40000 dose=0.63e13 tilt=1 rot=0 miscut.th=4 miscut.ph=90 energy=60 bca

The two profiles will be statistically equivalent with slight shift due to both profiles projection on the vertical, Y, coordinate.

 

References

  1. I. Chakarov and M. Temkin, “Modeling of Ion Implantation in SiC Crystals”, Nucl. Instr. And Meth. In Phys. Res. v. B-242, p.690, 2005.
  2. http://www.qualitymaterial.net/products.html
  3. http://www.semiconductorwafers.net/4H-Semi-insulating-SiC.html

Download PDF version of this article