Electron beam pattern generator control

A method is provided for controlling an electron beam pattern generator which writes a pattern on a substrate with an electron beam. A pattern store holds the defining coordinates of a set of trapezia 9, 13, 14, 15, 16 which together constitute the pattern. The set of trapezia in the store are divided into groups written in turn on the substrate, successive groups containing successively coarser 13, 14, 15, 16 to finer 9 pattern details. The groups are preceded in the pattern store by respective data words which instruct a trapezium generator to produce scanning steps of successive groups at successively higher to lower frequencies respectively. Coarse detail is thereby written quickly and fine detail receives the correct electron exposure.

This invention relates to a method of controlling an electron beam pattern 
generator comprising an electron beam generation and deflection system for 
writing a pattern on a substrate with a spot formed by an electron beam, a 
pattern store for holding the defining coordinates of a set of trapezia 
which together constitute the pattern, and a trapezium generator for 
taking the defining coordinates of each trapezium in turn and for 
generating scanning steps for the electron beam deflection system to write 
each trapezium in turn on the substrate. 
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 a 
masking layer on the surface of the semiconductor substrate which can be 
used subsequently in the processing of the semiconductor wafer, for 
example to mask a metalisation layer. 
Such an electron beam pattern generator and its control arrangements are 
described in the article "An Electron Beam Maskmaker" by J. P. Beasley and 
D. G. Squire in IEEE Transactions on Electron Devices, Vol ED-22, No. 7, 
July 1975. Therein it is described how the pattern can be written as the 
summation of a large number of small rectangles, each of which is written 
by moving the electron beam in a succession of small steps in a spiral 
pattern to fill in the rectangle. Triangles, or trapezia in general, may 
be written by this method. 
It is desired to use such an electron beam pattern generator as a 
production machine. There is therefore a strong incentive to increase the 
writing speed of the beam to improve production throughput. The adoption 
of the rectangle writing method, now referred to in the art as 
vectorscanning, provides an improvement in writing speed over simple 
raster scanning with beam modulation since time is not wasted traversing 
the beam across areas of substrate which are devoid of pattern. 
It is a further requirement of electron beam pattern generators that they 
shall be capable of writing patterns which contain submicron detail as 
well as large area detail, for example, connection pads. Such submicron 
detail may comprise fine lines which may be only a few spot diameters in 
width. In this event the problem of adequate exposure for these fine lines 
arises. Usually, the beam current will have been chosen to provide 
adequate exposure for the large area detail and it is not desirable to 
increase the current for the fine detail since this will involve an 
enlargement of spot size and other undesirable changes in operating 
conditions of the electron beam column. The electron dose, given by the 
dwell time of the spot at each step, which is required to define a pattern 
is dependent upon the geometry and area of the pattern details. For 
example a 0.25 .mu.m wide pattern requires some four times the dose used 
to define large area detail. 
It is an object of the invention to maximise writing speed and at the same 
time to provide correctly exposed fine detail. 
The invention provides a method of controlling an electron beam pattern 
generator comprising an electron beam generation and deflection system for 
writing a pattern on a substrate with a spot formed by an electron beam, a 
pattern store for holding the defining coordinatees of a set of trapezia 
which together constitute the pattern, and a trapezium generator for 
taking the defining coordinates of each trapezium in turn and for 
generating scanning steps for the electron beam deflection system to write 
each trapezium in turn on the substrate, the method being characterised in 
that the set of trapezia in the pattern store are divided into groups 
which are written in turn on the substrate, successive groups comprising 
those trapezia which define successively coarser to finer pattern details, 
and in that the groups are each preceded in the pattern store by 
respective data words which instruct the trapezium generator to generate 
the scanning steps of the successive groups at successively higher to 
lower frequencies respectively. In many patterns used in semiconductor 
integrated circuits only a small fraction of the pattern is of very fine 
detail. Consequently in many cases only two groups of trapezia are needed, 
the trapezia in one group being written before the trapezia of the other 
group. 
As has been mentioned previously, a common subsequent stage in processing 
the wafer is to apply a metallization layer by evaporation to the whole 
substrate after dissolving away the exposed resist. After this 
metallization, a second solvent is used to dissolve away the resist under 
the metallization, taking that part of metallization away and leaving only 
that part of the metal layer which was deposited directly on the substrate 
and is required to form an interconnection pattern. If this removal of 
metal on top of resist is to occur, the second solvent must be able to 
attack the resist at its edges under the metal. If, as a result of over 
exposure, the resist edge is poorly defined as a sloping surface the 
metallization can effectively seal in the resist and render subsequent 
removal of metal difficult. It is a benefit of the invention that the 
method of writing trapezia achieves a more nearly constant and correct 
resist exposure per unit area in all areas, either of coarse or fine 
detail. This results in more sharply defined resist edges after the action 
of the first solvent and so greatly reduces the risk of resist "seal-in" 
at the metallization stage.

Referring to FIG. 1, a semiconductor substrate 1 is shown in the target 
area of an evacuated electron beam column 2. An electron gun 3 provides an 
electron beam 4 which is focused as a spot on the substrate by focusing 
electron optics not shown. Typically the spot will have a gaussian 
distribution of intensity and will have a diameter of 0.12 to 0.1 .mu.m 
measured at the half intensity points. Deflection coils X and Y are 
provided for deflecting the electron in two dimensions across the 
substrate. A trapezium generator 5 is provided to receive the defining 
coordinates of a trapezium and to output deflection currents for the X and 
Y coils having values which change in steps at a defined stepping rate by 
amounts corresponding to typically 0.10 .mu.m on the substrate in a 
sequence which exposes the whole area of the trapezium. A trapezium 
coordinate store 6 is provided to hold the coordinates of a sub-set of 
trapezia for immediate access by the trapezium generator. The full set of 
trapezia are held in a pattern store 7, typically a magnetic disc store, 
and loaded in sub-sets into the trapezium coordinate store as required. 
The trapezium generator 5 is provided with a connection 8 to switch the 
electron gun 3 off when moving the beam from the end of one trapezium to 
the start of the next. The compilation and coding of the data held in the 
pattern store will be dealt with later. 
FIG. 2 shows a portion of a typical pattern required on a semiconductor 
substrate, being a gate electrode structure for a field effect transistor. 
The gate electrode is the structure of two fine lines 9 shown between the 
dotted lines 10. Typically these lines will need to be 0.2 .mu.m wide. 
Outside the dotted lines is coarse detail pattern comprising connection 
pads for the gate electrodes. FIG. 3 shows the circled portion of FIG. 2 
enlarged to show details of constituent trapezia and scanning patterns. 
The trapezia comprise rectangles 11, 12, triangles 13, 4, and squares 15, 
16. In square 15 a scanning pattern for the electron spot is shown as a 
sequence of spot positions shown as circles drawn at the half-intensity 
radius. The defining coordinate for the square could be the position of a 
corner and the side length, assuming the square to be orthogonal with the 
X and Y deflection directions. Arbitrary trapezia could be defined as the 
coordinates of the corners. The trapezium generator is programmed to take 
these defining coordinates and calculate the sequence of spot positions 
at, for example 0.1 .mu.m intervals which will completely expose the whole 
trapezium. The pattern shown is a boustrophedon which zig zags across the 
area without wasted motion, as in a ploughing pattern. Other economical 
movement patterns such as spirals could be used. The fine detail 9 is 
drawn as two lines of spots. 
The electron dose at each spot position is controlled by the spot dwell 
time at that position and hence by the spot stepping frequency. For 
example at a stepping frequency of 10 MHz, the dwell time at each position 
is about 100 ns. By reducing the stepping frequency the dwell time, and 
hence dose, can be increased, assuming constant electron beam current as 
is desirable for constant spot size. In electron lithography the electron 
dose required at each point in a pattern to achieve uniform resist 
exposure all over the pattern is dependent upon the geometry and area of 
the pattern details. This is due to the spot shape and to electron scatter 
effects which produce some exposure at points a few spot diameters from 
the point of spot incidence, referred to in the art as the proximity 
effect. For example, a 0.25 .mu.m wide line requires some four times the 
dose used to define large area, or coarse, detail. 
The information regarding coarse and fine pattern details comes into 
existence at a computer aided design (C.A.D.) stage when the designer 
specifies the nature of the circuit function required to a computer 
programmed to carry out detailed layout and electrode design. In 
accordance with the invention it is at this stage that the list of 
defining coordinates for all trapezia are divided into groups, usually two 
groups, one listing all coarse trapezia and the other listing all fine 
detail trapezia. The computer output, listing all members of both groups, 
will usually be on a magnetic tape. 
To run the electron beam pattern generator (EBPG) the data of the groups 
will normally be required at a higher rate and with more immediate access 
than is possible with a magnetic tape. The data will usually be loaded 
onto a magnetic disc, reference 7 in FIG. 1, driven in response to the 
requirements of the EBPG. It is at this stage that a pattern data word in 
the disc information preceding each group is changed to indicate a 
percentage of a predetermined base clock or writing frequency in 
dependence on the group structure detail. A high percentage is selected 
for the coarse detail group and a low percentage for the fine detail 
group. Typically 10 MHz might be chosen for the coarse group and 2.5 MHz 
for the fine group. The base frequency might be 10 MHz, the two 
percentages being 100% and 25% respectively. In operation the EBPG will 
read the data word preceding the fine detail group and set the writing 
frequency accordingly. The fine detail trapezia will then be written. Then 
the data word preceding the coarse detail group would be read, the writing 
frequency set accordingly and the coarse detail trapezia written. 
Without the improvement in accordance with the invention the whole pattern 
would have to be written at 2.5 MHz stepping frequency. Since the bulk of 
most patterns is coarse detail, with the invention the coarse detail is 
written at 10 MHz, i.e. four times faster. In general the total writing 
time is reduced by about 75%. 
Also, without the invention, the coarse detail would be overexposed, being 
written at 2.5 MHz. FIG. 4 shows the effect of overexposure on the edge 
definition of the resist after the exposed resist has been dissolved away. 
The edge 17 of the resist 18 has a sloping profile. When the metallization 
19 is applied, the sloping edge is covered by metal, effectively sealing 
the resist against the action of the next solvent chosen to dissolve the 
resist and hence remove the metal on top of the resist. FIG. 5 shows the 
resist edge 20 achieved with correct exposure. The resist edge is undercut 
to an extent and is exposed to the action of the next solvent, giving 
excellent metal lift-off. 
There follows an example of a Pattern File Header which would precede a 
group of information on the disc store. 
The RTL/2 specification of the header is 
MODE FILEHEDMODE ( 
INT DATADSA; The address of the next sector in the file. 
ARRAY (32)BYTE FILNAM The file name. 
INT FDSIZE The number of sectors occupied by the file data. 
ARRAY (100) INT User file header information. 
HEDDAT); 
HEDDAT FREQUENCY CONTROL DATA HELD IN THE FOLLOWING HEDDAT WORDS. 
HEDDAT(30) The beam stepping frequency, expressed as a percentage (in units 
of 0.1%) of the base frequency specified to the EXPOSE process. 
HEDDAT(31)-(37) The remaining 7 frequencies as a percentage in units of 
0.1%. 
HEDDAT(30) gives the stepping frequency for the group and HEDDAT(31) to 
(37) gives seven other lesser frequencies which might be required. Each 
HEDDAT entry is specified a four digit hexadecimal word. Thus if the 
stepping frequency is to be 100% of the base frequency, the hexadecimal 
word will be 03E8, i.e. 1000 units of 0.1%. For 40% of the base frequency 
the word would be 0190 i.e. 400 units of 0.1%.