Measurement of proximity effects in electron beam lithography

A test pattern which is applied to a wafer or mask by electron beam lithophy for measuring proximity effects. The pattern comprises two lines which intersect at a small angle, for example 1.degree. so that the proximity effects of the two lines combine within the angle to displace the angle vertex by an amount much larger than the proximity effect of an isolated line. A calibration scale is provided to measure this enhanced proximity effect by viewing the pattern with an optical microscope.

BACKGROUND OF THE INVENTION 
The invention is concerned with the measurement of proximity effects which 
occur in the fabrication of solid state microelectronic circuits by means 
of a technique known as electron beam lithography. In this technique a 
pattern of electronic circuits is formed on a mask or directly onto an 
oxide coated silicon wafer by means of an electron beam which is scanned 
over the surface of the mask or wafer. The pattern to be lithographed is 
stored in a computer memory and the computer controls the beam intensity 
or energy deposit in accordance with the stored pattern program. The wafer 
or mask is coated with an electron resist material which is exposed when 
it is impinged by the electron beam. Subsequent development of the mask or 
wafer will either remove the exposed or the unexposed portion of the 
resist layer depending on whether the resist is a positive or negative 
one. 
Very large scale integrated circuits of this type necessarily involve 
closely spaced patterns which require accurate location of electron resist 
pattern edges. The location of these pattern edges is a function not only 
of the beam location but also of the proximity effect of the particular 
wafer or mask being lithographed. The electron beam as it strikes and 
penetrates the electron resist material will be forward scattered by the 
molecules of the resist material and as it strikes the underlying silicon 
dioxide layer it will be backscattered through the resist layer again. 
This scattering of the electron beam causes the exposed area to overlap 
the beam edge. The amount of this overlap is a function of numerous 
factors such as the beam current density, the width of the line or area 
being lithographed, the resist material and the thickness thereof and the 
substrate material. Thus the magnitude of the proximity effect varies with 
the wafer or mask being lithographed and also with the characteristics of 
the lithography machine. If the magnitude of the proximity effect can be 
measured, it can be compensated for by modifying the computer software 
which controls the lithography process. The present invention provides a 
means and a method of rapidly and accurately measuring this proximity 
effect. 
SUMMARY OF THE INVENTION 
The invention involves the lithographing of a test pattern on the wafer or 
mask under test in such a way that the proximity effects to be measured 
are enhanced by the geometry of the test pattern. The enhanced proximity 
effect can then be measured by viewing the developed test pattern with a 
conventional optical microscope, utilizing a measuring scale which is part 
of the test pattern. The test pattern may comprise, for example, a pair of 
lithographed lines which cross at a small angle, for example, less than 
1.degree.. The measuring scale is lithographed along one of the lines and 
has its origin at the point where the centerlines of the two lines cross. 
Each of the lithographed lines will exhibit the aforementioned proximity 
effect so that each line as developed will be wider than the sweep of the 
electron beam which produced it. The combination of the two proximity 
effects of the lines crossing at the small angle will displace the vertex 
or corner between the two intersecting lines by a distance which is many 
times greater than the increase of width of each line due to the proximity 
effect. This displaced vertex can be optically measured using the 
aforementioned measuring scale. 
The invention thus provides a novel method of measuring proximity effects 
as well as a novel test pattern for carrying out the method. The invention 
can be used for in-process control and evaluation of the patterning of 
submicron dimensions in electron beam lithography and it can also be used 
to evaluate the scattering properties of new resist materials or of 
different thicknesses of known resist materials. The use of this technique 
can increase the accuracy of the patterning of microelectronic chips and 
thus also permit an increase in patterning density to yield further 
miniaturization of these devices. 
These and other objects and advantages of the invention will become 
apparent from the following detailed description and the drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Electron beam lithography (EBL) can be used to directly produce so called 
working copy masks which are used as photo masks for transferring the 
circuit patterns thereon to the silicon oxide coated silicon wafer which 
forms the substrate of the microelectronic circuits. This technique 
simplifies the production of these masks by eliminating several steps in 
the older photographic process which required the production of a large 
scale reticle followed by production of master masks which were reproduced 
from the reticle by photoreduction. The working copy masks were reproduced 
from the master masks. 
Direct writing electron beam lithography eliminates completely the use of 
any masks since the beam imprints or lithographs the desired circuit 
pattern directly onto the silicon wafer, as explained above, under the 
control of a computer in which the pattern is stored. The present 
invention is applicable to either of these two EBL techniques. 
The drawing shows the test pattern 3 of the present invention printed by 
direct writing electron beam lithography onto a wafer or mask indicated at 
8. A pair of benchmarks 4, each comprising a square group of four crosses, 
5 and 7, is also printed on the same wafer or mask, with the two 
benchmarks widely spaced apart so that each one is near an edge of the 
wafer or mask at nearly diametrically opposed points. The benchmarks are 
used to accurately measure the angle at which the two printed lines of the 
test pattern intersect. 
The test pattern 3 comprises a pair of intersecting lines VE.sub.o and 
VE.sub.1, which are printed so that they intersect at a very small angle, 
.theta., of the order of 1.degree. and preferably between 0.1.degree. and 
0.5.degree., inclusive. The size of the angle .theta. in the drawing has 
been greatly exaggerated to facilitate the explanation of the mode of 
operation of the pattern. The width of the printed lines VE.sub.o and 
VE.sub.1 may for example range from 0.1 microns to 4.0 microns to simulate 
typical pattern lines encountered in very large scale integrated 
microelectronic circuits. The centerlines of the two printed lines are the 
dashed lines 9 and 11, which intersect at the point 17. A measuring scale 
19 is printed parallel to one of the two lines, in this case parallel to 
and adjacent to the line VE.sub.1 and having its origin, O, lined up with 
the point of intersection 17 of the two center lines 9 and 11. The scale 
is calibrated from 0 to 0.50 in units of microns, as illustrated. The 
solid line edges of the lines VE.sub.o and VE.sub.1 represent the edges 
which would result in the absence of any proximity effect and thus 
represent the edges of the path of the electron beam as it sweeps over the 
wafer or mask to form the pattern. The edges of the two lines VE.sub.o and 
VE.sub.1 which form the angle .theta. are referenced as 13 and 15, 
respectively. The point of intersection of the lines referenced as 13' and 
15, respectively, is hereafter referred to as the vertex of the angle 
.theta.. The actual pattern edge as displaced by the proximity effect is 
represented by one or the other of the two dashed line curves labelled A 
and B, representing two different degrees or magnitudes of proximity 
effect. The smaller proximity effect represented by curve A might result 
from one or more of the following causes: a low dose, low backscatter by 
the substrate or mask, a narrow line width (VE.sub.o or VE.sub.1), an 
underdeveloped positive electron resist, the use of a low sensitivity 
resist, or a low beam current density. The opposite of one or more of 
these conditions will increase the width of the proximity effect to yield 
a pattern edge represented by curve B. The two curves A and B are known as 
critical dose curves since they represent the edge of the area in which 
the scattered electrons will be sufficiently intense to cause development 
of the electron resist. 
The distance of the point .psi..sub.A on curve A or .psi..sub.B on curve B 
from the vertex of the angle .theta. or point 17 is a relative measure of 
the magnitude of the proximity effect. These two points are the points on 
the curves A and B which are closest to the vertex of the angle .theta. or 
to the point 17. The dashed lines L.sub.A and L.sub.B are lines 
perpendicular to the line VE.sub.1 and to the measuring scale 19 and these 
lines pass through the points .psi..sub.A and .psi..sub.B, respectively, 
to provide scale readings related to the magnitude of the proximity 
effect. It can be seen that the spacing of the points .psi..sub.A and 
.psi..sub.B is much greater than the spacing of the two curves A and B at 
points more remote from the vertex or from the point 17. The reason for 
this is that the electron resist in the area between the points 
.psi..sub.A or .psi..sub.B and point 17 is exposed to the scattering 
effects from both of the lines VE.sub.o and VE.sub.1, whereas an isolated 
pattern edge would be subjected to the proximity effect from only one 
side. Also, simply increasing the width of two lines which cross at a 
small angle will displace the vertex thereof by many times the amount of 
the width increase. The 0.50 micron scale can be easiy read if the 
developed test pattern is viewed with a conventional optical microscope. 
With the actual test patterns having an angle of intersection of less than 
1.degree. as stated above, the enhancement of the proximity effect would 
be even greater than is illustrated by the drawing, in which the angle 
.theta. is approximately 20.degree.. Thus this test pattern and technique 
permits proximity effects in the submicron region to be enhanced to tens 
of microns which can be easily optically read and measured. 
The novel technique and method of measuring proximity effects of submicron 
dimensions comprises the steps of lithographing a pair of straight lines 
on a wafer or mask with an electron beam, with the lines crossing at an 
angle less than 1.degree. and further printing a measuring scale on said 
wafer or mask parallel to one of said lines and having its origin lined up 
with the intersection of said lines or with the intersection of the 
centerlines of said lines, and then optically measuring the distance from 
said vertex or intersection to the closest unexposed portion of said wafer 
or mask which lies between the said two lines. 
The steps in the printing and the use of novel test pattern will be 
described in connection with an electron beam lithography (EBL) machine 
made by ETEC Corporation and known as the LEBES, meaning Laboratory 
Electron Beam Exposure System. This machine can function as a scanning 
electron microscope (SEM) as well as an EBL machine. 
The wafer or mask 8 (or the sample) is first inserted into the stage of the 
LEBES and the two benchmarks 5 and 7 are exposed thereon by the electron 
beam, the sample is then removed and the benchmark patterns developed and 
the oxide underlying the benchmarks etched. The stage is a mechanically 
moveable table in which the sample is mounted for insertion into the 
evacuated base area of the column of the machine where it will be in the 
path of the focused and deflected electron beam. The sample is then 
re-inserted in the LEBES and aligned with the printed benchmarks. When so 
aligned the test pattern line VE.sub.o can be exposed parallel to the line 
R.sub.o which joins the centers of the two benchmarks 5 and 7. The sample 
is again removed and the line VE.sub.o developed. The sample is then 
re-inserted in the stage of the LEBES, but upon re-insertion it will be in 
a slightly different position due to mechanical backlash in the stage 
system. The LEBES includes a laser interferometer which can detect the 
stage position relative to its former position and this stage error can be 
used to realign the stage relative to the scanning or field axis by 
electrically varying the field control settings. The aforementioned stage 
error will normally be less than 1.degree. and thus can conveniently be 
used to determine the angle .theta. at which the two lines of the test 
pattern cross each other. Thus the angular change in the field axis 
required to achieve realignment can be noted and then the deflection field 
axis can be mis-aligned relative to the stage axis by the angle .theta. 
using the Alignment and .DELTA. Field controls of the Field Control Module 
of the LEBES. The line VE.sub.1 with the measuring scale 19 parallel 
thereto is then printed parallel to the line R.sub.1. The sample is then 
removed and developed to reveal the pattern edges such as the curves A or 
B. An optical microscope can then be used to locate the points .psi..sub.A 
or .psi..sub.B relative to scale 19 as explained above. A straight edge 
can be moved along the scale parallel to the lines L.sub.A and L.sub.B to 
read the numerical scale value corresponding to the points .psi..sub.A or 
.psi..sub.B. 
Another technique of printing the test pattern would be to first print the 
line VE.sub.o on the sample and then rotate the field axis of the LEBES 
through the desired angle .theta. by means of the alignment control 
poteniometers of the Field Control Module, after which the line VE.sub.1 
and the scale 19 would be printed. This technique would require that these 
potentiometers be calibrated to relate the settings thereof to the field 
axis rotation. 
The scale 19 can be calibrated in full scale units of distance or at a 
reduced scale representing the proximity effect of an isolated pattern 
edge. This reduced scale would be related to the full scale by a factor 
equal to the enhancement of the proximity effect due to the use of the 
test pattern, which factor is approximately equal to the reciprocal of tan 
.theta. when the origin of scale 19 is lined up with the vertex of angle. 
Such a scale may also be calibrated to provide direct readings of 
proximity effects when it has its origin lined up with the point 17, as 
illustrated in scale 19 in the drawing. 
While the invention has been described in connection with illustrative 
embodiments, obvious variations therein will occur to those skilled in the 
art, accordingly the invention should be limited only by the scope of the 
appended claims.