Method of using an electron beam

When an electron beam is used to effect a process at two adjacent surface areas of a target, such as a semiconductor wafer coated with an electron sensitive resist, various alignment errors can occur including wafer distortion. The provision of a reference marker, for example a square-etched depression, at the surface of the target between the adjacent surface areas enables detection of any misalignment. Thus, after effecting the process at one of the areas, an electron beam having substantially the same size and shape as the reference marker is directed toward the predetermined position of the reference marker. Back-scattered electrons are then detected to give a signal representative of any deviation between the actual position and the predetermined position of the reference marker so that the electron beam can be correctly aligned before carrying out the process at the second of the two surface areas. For increased accuracy of alignment, an array of reference markers may be provided between the two adjacent areas.

This invention relates to a method of using an electron beam to effect 
sequentially a process at two adjacent surface areas of a target. 
An important application of electron beam technology is in the manufacture 
of semiconductor devices. In particular, a so-called electron beam pattern 
generator can be used to direct an electron beam towards a target in the 
form of a semiconductor substrate coated with an electron sensitive 
resist. By computer control of the beam a predetermined pattern can be 
drawn in the resist. The exposed parts (or, in the case of a negative 
resist, the unexposed parts) of the resist are then removed selectively 
using an appropriate chemical. The remaining parts of the resist form on 
the surface of the semiconductor substrate a masking layer which can be 
used subsequently in the processing of the semiconductor wafer. 
Unfortunately, the area (sometimes called the deflection field) which can 
be scanned by the electron beam is somewhat restricted because of the 
occurrence of electron optical aberrations which increase markedly as the 
electron beam deviates more and more from the optical axis. This presents 
a problem when large areas of resist are to be exposed. However, in the 
manufacture of semiconductor devices such as integrated circuits, it is 
usually the case that the same, relatively small pattern has to be drawn 
repeatedly across a large semiconductor substrate. Because of this, the 
beam can be used to draw one pattern at a first area of the substrate 
before moving the substrate to introduce a new area of the substrate to 
the electron beam. Thus the same (or a different) pattern can be drawn at 
this new area without the need for the beam to have an unduly large 
deflection field. In fact, as long as the substrate can be moved in two 
transverse directions the whole of a resist layer present on a 
semiconductor substrate having, for example, a diameter of 4 inches can be 
exposed by sequentially drawing the same pattern at relatively small 
areas, typically 3 mm square, across the surface of the substrate. Not 
surprisingly, this technique has come to be known as the step-and-repeat 
method. 
A conventional pattern generator produces an electron beam which, at the 
target, is circular and has a Gaussian intensity distribution. As the beam 
has a typical diameter of 0.2 micrometer it can fairly be described as a 
point beam. 
If the pattern to be drawn at a particular area of a substrate can be 
divided into basic rectangular elements, as is usually the case, then the 
point beam is made to draw the outline of each rectangle and then to fill 
it in by scanning the rectangle before proceeding to the next rectangle. 
When the complete pattern has been drawn at that area the substrate is 
moved so that the same pattern can be drawn at the next area as described 
above. 
Before the semiconductor substrate is exposed to the electron beam it is 
usual to perform an initial, relatively coarse alignment of the substrate. 
Nevertheless, it is still possible for alignment errors to be introduced 
during device manufacture as a result of instabilities in either the 
electron beam pattern generator or in the semiconductor substrate. For 
example, the semiconductor substrate can become distorted as a result of 
the various treatments to which it is subjected. To compensate for any 
such errors it is usual to employ a reference marker system on the 
substrate surface. Thus, the electron beam can be directed towards the 
predetermined position of a marker to derive a signal representative of 
the deviation between the actual position and the predetermined position 
of that marker. This signal can then be used either to correct the 
substrate movement or to add a correction factor to the electron optical 
system so that the next time the electron beam draws the pattern it does 
so at the correct location. 
The reference marker may be a depression in the semiconductor substrate, 
for example a square of 20 micrometers.times.20 micrometers. When the 
areas at which the pattern is to be drawn are square such a marker may be 
located at the four corners of each such area. Before drawing the pattern 
at any given area the beam is directed, in turn, towards the four 
reference markers at the corners of that area. The beam is scanned, for 
example, 8 times across each edge of the marker with the scan direction 
being transverse to the edge in question. By observing the back-scattered 
electrons the deviation between the actual position and the predetermined 
position of the marker can be determined. To minimize errors, similar 
information is gathered from each of the four markers at the corners of a 
particular area before the pattern is drawn at that area. 
More recently there have been developments in the technology of pattern 
generators which allow the spot size of the electron beam to be varied. 
For example the paper entitled "Variable spot-shaped electron beam 
lithographic tool" by E. V. Weber and R. D. Moore in the Journal of Vacuum 
Science Technology, 16(6), November/December 1979 describes a spot-shaped 
beam which can be varied in size up to 4 micrometers.times.4 micrometers. 
Other systems which have been described have an even greater range over 
which the spot size can be varied. Clearly, the variable spot-shaped 
electron beam system is ideally suited to drawing patterns in a resist 
coating on a semiconductor substrate, particularly when the pattern can be 
decomposed into basic rectangular elements. In this case the time taken to 
a draw a pattern at a given area can be decreased considerably. This has 
the important consequence that pattern generators producing variable 
spot-shaped electron beams can have an increased throughput as compared 
with their point beam counterparts. 
Of course, the problem of alignment errors is still present so that the 
need for a compensating system has not been removed. In their paper 
(mentioned above) Weber and Moore describe an alignment system which 
involves the scanning of reference markers present at the four corners of 
each area of the substrate where a pattern is to be drawn. The resulting 
back-scattered electron signals can then be processed to determine 
position errors at the four corners of each area. 
Although the scanning of reference markers, as previously described in 
relation to the point beam pattern generator, is effective in determining 
position errors of markers it is a relatively slow technique which, when 
used in conjunction with the variable spot-shaped electron beam pattern 
generator, inevitably erodes the advantage resulting from the reduced 
exposure time. 
According to the present invention there is provided a method of using an 
electron beam to effect sequentially a process at two adjacent surface 
areas of a target, a reference marker being provided between the areas, 
which method, after effecting the process at one of the areas, includes 
the steps of directing an electron beam having substantially the same size 
and shape as the marker towards the predetermined position of the marker, 
detecting the resulting backscattered electrons using a detector to 
provide a signal representative of any deviation between the actual 
position and the predetermined position of the marker, and responding to 
the signal by compensating for any such deviation when effecting the 
process at the second of the areas. 
By using an electron beam whose size and shape is substantially the same as 
the reference marker, the beam current can be considerably higher than 
that of a point beam. This has the advantage that the number of 
back-scattered electrons can be significantly higher so that the detection 
of position errors can be carried out more quickly. 
The reference marker may have a different topology to the surrounding area 
of the target and/or it may be constituted by an area of material having a 
different back-scattering coefficient to the material to the target. For 
example, when the target is a silicon substrate coated with an 
electron-sensitive resist the reference marker may be an island of silicon 
oxide or of a metal such as tantalum formed directly on the substrate 
surface. Alternatively, the reference marker may be a depression at the 
surface of the target formed, for example, by etching with an appropriate 
chemical. Of course, the reference marker may have any geometric shape, 
but matching the shape of the electron beam to the reference marker is 
particularly straightforward when the reference marker is square. 
The provision of a plurality of similar reference markers between the two 
areas of the target is particularly advantageous not only because the 
effects due to the presence of a poorly-defined reference marker can then 
be minimized, but also because the signal-to-noise ratio of the detected 
signal can be increased for improved accuracy and greater speed. 
Thus in one form of a method in accordance with the invention an array of 
similar reference markers is provided between the two surface areas of the 
target. After effecting the process at one of the areas and before 
effecting the process at the other area the method includes the step of 
directing the electron beam toward the predetermined position of each of 
the reference markers in turn. 
The signals obtained by directing the electron beam toward each of the 
reference markers can be integrated to give an average value for the 
deviation between the predetermined position and the actual position of a 
reference marker. 
In order to determine the extent and the direction of any such deviation it 
is preferable that the back-scattered electrons are detected using two 
pairs of detectors arranged such that the detectors of at least one pair 
provide different signals when the predetermined position of the marker 
deviates from the actual position. Thus the differential signal from a 
detector pair represents the extent of the deviation in the direction 
parallel to the line joining the two detectors of that pair. 
In a modified form of a method in accordance with the invention an array of 
similar reference markers is again provided between the two surface areas 
of the target. In this method, however, after effecting the process at the 
one area and before effecting the process at the second area, the electron 
beam is directed towards the area of each reference marker in turn such 
that the beam is directed towards the predetermined position of one 
reference marker only. As explained in more detail hereinafter, this 
method allows accurate determination of position errors using only a 
single detector. 
The array of reference markers mentioned above may be an irregular array, 
but the detection of alignment errors is particularly straightforward when 
the markers are arranged in a regular manner, for example in a single row 
or in a plurality of rows and columns.

It should be noted that, for the sake of clarity, the Figures are not drawn 
to scale. 
In FIG. 1 a semiconductor wafer 1 is located on a movable table 2 of a 
variable spot-shaped electron beam pattern generator. The wafer 1 is 
coated on the major surface directed awy from table 2 with a layer 3 of 
electron sensitive resist. The wafer 1, which is generally circular, may 
have a diameter of approximately 4 inches and it is divided into areas 3 
millimeters square where a process is to be effected sequentially using 
the electron beam 4. For example, at each such area an integrated circuit 
may be formed in conventional manner. For the sake of clarity these areas 
are not shown in FIG. 1. However, FIG. 2 shows several such areas 5 on a 
greatly distorted scale. The spacing between adjacent areas 5 may be for 
example 100 micrometers. None of the circuit elements of the integrated 
circuits is formed in the space between adjacent areas, because this 
represents the so-called scribe lane where the semiconductor wafer 1 will 
later be severed to divide it into individual integrated circuits. 
In the vertical direction, between adjacent areas 5, there is provided a 
two-dimensional regular array 6 of similar reference markers 7 in the form 
of depressions at the surface of the wafer 1. For the sake of clarity FIG. 
1 only shows one such array, whereas FIG. 2 (again for the sake of 
clarity) shows an array of 35 markers only. 
The markers may, in fact, be squares of 3 micrometers spaced apart by 6 
micrometers. Therefore, for a scribe lane 100 micrometers wide and 3 
millimeters long the array will comprise 10 columns of 300 markers. 
These reference markers 7 may be formed by chemically etching the silicon 
wafer in known manner. Depending on the way in which the etching is 
performed the depressions may have vertical sides as shown in FIG. 3a, 
sloping sides and a horizontal bottom as shown in FIG. 3b, or four sloping 
sides which meet at a point, as shown in FIG. 3c. Typically these 
depressions may be 1 micrometer deep. 
In the electron beam pattern generator the variable spot-shaped beam is 
used to selectively expose one of the areas 5 of the wafer 1 which is 
coated with resist. This process is carried out to define in the resist a 
pattern so that selective removal of the unexposed (or, as the case may 
be, the exposed) parts of the resist 3 leave a masking layer for use in 
subsequent processing of the semiconductor wafer in the manufacture of 
integrated circuits. After having exposed one such area 5a the table 2 is 
moved to introduce an adjacent area 5b into the deflection field of the 
electron beam. The table is adapted to move in directions parallel and 
orthogonal to the scribe lanes in which the reference markers 7 are 
provided. 
In between effecting the process at the areas 5a and 5b, that is to say 
after exposing area 5a and before exposing area 5b it is arranged that the 
electron beam has a square shape with sides of 3 micrometers. This beam, 
having the same size and shape as the markers 7 is directed towards the 
predetermined position of each of the markers 7 in turn. For this purpose 
the beam is stepped, relative to the wafer 1, from marker to marker by an 
amount equivalent to the spacing of the markers. Each time the beam is 
directed towards a marker the back-scattered electrons are detected using 
four detectors 9, two in the X-directon and two in the Y-direction. Each 
detector 9 of a pair is equally spaced from the marker so that a 
differential signal is set up across the detectors of a pair if the 
predetermined position of a marker deviates from its actual position. As 
shown schematically in FIG. 1 the detectors 9 are connected to a 
differential amplifier D. FIG. 1 shows two detectors only, the other two 
detectors being provided in the direction orthogonal to the plane of the 
paper. 
The differential signal obtained from the differential amplifier D is 
representative of both the extent and the direction of any deviation 
between the predetermined position and the actual position of the 
reference marker 7 in question. 
By stepping the beam from marker to marker similar information is gained at 
each site. Thus this information can be fed into an integrator I to give 
an average value, represented by the output signal 0, for the deviation 
between the predetermined position and the actual position of a reference 
marker. 
The output signal 0 can be used in compensating for any such deviation when 
effecting the electron beam exposure process at the next area 5b. 
Consequently, the next time the electron beam draws a pattern it does so 
at the correct location. This may be achieved by adding a correction 
factor to either the table movement or to the electron optical system. 
By using this method account can be taken of any alignment errors such as 
distortions which occur in the wafer 1 during processing, and the 
appropriate correction can be made when exposing the electron-sensitive 
resist to the electron beam. 
By using a variable spot-shaped beam position errors can be detected very 
quickly, so much that the table can be moving between subsequent 
processing treatments while the alignment error detection is taking place 
simultaneously. Thus, the throughput of semiconductor wafers in the 
pattern generator can be significantly increased. 
In a modified form of a method in accordance with the invention the same 
two dimensional array 6 of similar reference markers 7 can be provided 
between adjacent areas 6 of a semiconductor wafer 1 as described above 
with reference to FIG. 1. After effecting the electron beam process at one 
such area and before moving the semiconductor wafer in order to effect the 
same process at the adjacent area the electron beam is directed towards 
the area of each reference marker in turn, but the beam is stepped, 
relative to the wafer 1, from the area of one marker to the area of an 
adjacent marker by an amount which is either more or less but not the same 
as the spacing of adjacent reference markers. Thus in this case, for 
example the beam may be stepped by 6.05 micrometers. Again the beam has 
the same size and shape as the individual reference markers 7, that is to 
say it is 3 micrometers square, but in contrast with the previous method 
the beam is directed towards the predetermined position of the reference 
marker at the center of the array only. As such, assuming that there are 
no alignment errors, the beam will be coincident with one reference marker 
only, viz. the marker at the center of the array, and it will be 
misaligned with all the other markers. This enables a single detector to 
be used in determining position errors of the reference markers, the 
signals from al the markers being utilized in determining any such errors. 
FIG. 4 shows three plots of the signals obtained under different 
circumstances. Plot A represents no deviation between the predetermined 
position and the actual position of the central reference marker. Plot B 
represents the situation where the beam has been aligned with a reference 
marker in the negative X-direction and Plot C represents the situation 
where the beam has been aligned with a reference marker in the positive 
X-direction. Similar information for the Y-direction can of course be 
obtained by plotting the signal intensity for the reference markers in the 
Y-direction. This information indicates the extent of any correction which 
may be needed to ensure that the electron beam is correctly aligned when 
effecting the electron beam process at subsequent areas. As before the 
correction factor may be applied to the movement of the table on which the 
semiconductor wafer is mounted or to the electron optical system. 
The embodiments described here are given merely by way of example. In the 
light of the above description it will be clear to the person skilled in 
the art that many modifications are possible within the scope of the 
invention.