Fully Recessed Oxide Growth

DESCRIPTION

This example shows the evolution of an oxide profile during semi-recessed oxidation. It uses the Deal-Grove model to calculate the oxide growth rate at the silicon/oxide interface, and treats the deformation of the oxide and nitride as viscous incompressible flow. The input file for the simulation is in the "examples/exam7" directory, in the file "fullrox.s4".

#Recessed LOCOS cross section: recess 0.3um, grow 
0.54um
#option quiet

#------------Substrate mesh definition
line y loc=0  spac=0.05 tag=t
line y loc=0.6 spac=0.2
line y loc=1            tag=b

line x loc=-1   spac=0.2 tag=l
line x loc=-0.2 spac=0.05
line x loc=0  spac=0.05
line x loc=1  spac=0.2 tag=r

region silicon xlo=l xhi=r ylo=t yhi=b
bound  expo    xlo=l xhi=r ylo=t yhi=t

init or=100

#-----------Anisotropic silicon etch
etch    silicon left p1.x=-0.218 p1.y=0.3 p2.x=0 p2.y=0

#----------Pad oxide and nitride mask
deposit oxide thick=0.02
deposit nitride thick=0.1
etch    nitride left p1.x=0
etch    oxide   left p1.x=0
plot.2d grid bound

#----------Field oxidation
meth compr

plot.2 bound y.mi=-0.5 line.b=2
diffuse tim=90 tem=1000 weto2 movie="plot.2 b cle=f 
axi=f"
stru    outf=fc.mesh

plot.2d bound flow vleng=0.1

The first line

option quiet

asks for as little output as possible.

The grid definition comes next. First the vertical lines are defined:

line y loc=0  spac=0.05 tag=t
line y loc=0.6 spac=0.2
line y loc=1            tag=b

Ninety minutes in 1000C steam grows about 0.54 microns of oxide on <100> silicon. In the process, 0.24 microns of silicon is consumed. The second line is around the expected final depth of the oxide, and there is one more line at 1 micron to round out the grid.

line x loc=-1   spac=0.2 tag=l
line x loc=-0.2 spac=0.05
line x loc=0  spac=0.05
line x loc=1  spac=0.2 tag=r

The x lines run from -1 to 1 for symmetry, with the mask edge at 0. The extra refinement around 0.25 microns is to prepare for the silicon etch, which will be at an angle of 54 degrees and therefore has a lateral extent of 0.3 microns / tan 54 = 0.218.

region silicon xlo=l xhi=r ylo=t yhi=b
bound  expo    xlo=l xhi=r ylo=t yhi=t
init or=100

The region statement identifies the entire area as silicon substrate. It refers to the tags defined on the x and y lines. These tags are used to label lines uniquely so that new lines can be added or subtracted easily without renumbering. The boundary statement identifies the top surface as being exposed. (SUPREM-IV.GS does not assume the top is exposed.) Layer depositions, oxidations and impurity predepositions only happen on "exposed" surfaces, so this statement must not be omitted. The initialize command causes the initial rectangular mesh to be generated. The substrate orientation is <100>.

#-----------Anisotropic silicon etch
etch    silicon left p1.x=-0.218 p1.y=0.3 p2.x=0 p2.y=0

The silicon substrate is etched. The etch statement removes all silicon found lying to the left of the line joining the points (-0.218, 0.3) and (0,0).

#----------Pad oxide and nitride mask
deposit oxide thick=0.02
deposit nitride thick=0.1
etch    nitride left p1.x=0
etch    oxide   left p1.x=0
plot.2d grid bound

The pad oxide is put down, then nitride is deposited on top and patterned. The patterning is presumed to remove the underlying pad oxide also. The plot command shows the structure before oxidation and is shown in Figure 1.

#----------Field oxidation
meth compr

The compressible flow model is chosen for oxidation. The incompressible model could also have been used, but for this example where the stress is not desired, the compressible model is faster and nearly as accurate.

plot.2d bound y.mi=-0.5 line.b=2
diffuse tim=90 tem=1000 weto2 movie="plot.2 b cle=f 
axi=f"

The diffuse statement is the point of the exercise. For 90 minutes, oxide grows at the silicon interface. The new oxide being formed pushes up the old oxide and nitride layers which cover it, causing the characteristic bird's head profile. The movie parameter lists any extra actions to take at each time step. In this case, the boundary is plotted without erasing or re-scaling the picture. A series of outlines is generated, one from each time step, illustrating the evolution of the oxide profile (Figure 2). The first plot.2d bound defined a plotting window large enough so that the subsequent plots without axes would fit inside it.

plot.2d bound flow vleng=0.1

The flow pattern at the last time step can be plotted with the statement above. The parameter vleng is the length to draw the longest velocity vector. The other vectors are scaled proportionally. This results in Figure 3. The flow is normal at the interface but becomes more vertically oriented away from it.