Laser beam scanning method for forming via holes in polymer materials

The surface of a polymer dielectric layer is scanned repeatedly with a high energy continuous wave laser in a pattern to create via holes of desired size, shape and depth. This is followed by a short plasma etch. The via holes are produced at commercial production rates under direct computer control without use of masks and without damage to conductor material underlying the dielectric layer. A two-step technique usable to form a large hole to a partial depth in the dielectric layer and several smaller diameter holes within the large hole through the remainder of the dielectric layer depth allows formation of a large number of holes in a given area of a thick dielectric layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to a method and apparatus for producing 
via holes in polymer dielectrics without use of a mask. More particularly, 
the invention relates to a method for packaging electronic integrated 
circuit chips disposed on a substrate wherein via openings are produced in 
a polymer film overlaid on the chips to facilitate electrically 
interconnecting the chips through the vias thus formed. 
2. Description of the Prior Art 
Formation of via holes to allow for electrical connection between two or 
more layers of conductor separated by a layer of dielectric material has 
heretofore been accomplished by several methods, each of which has 
limitations. 
In one prior method, via holes are formed by depositing a metal masking 
layer over the dielectric layer, patterning the masking layer to expose 
areas of the dielectric layer where via holes are desired, and then 
selectively etching the exposed areas of the dielectric layer through the 
masking layer. Either chemical or plasma etching techniques are used to 
remove the dielectric from the exposed areas, although use of either 
technique has its own unique drawbacks. Certain dielectric materials 
cannot conveniently be chemically etched since they absorb chemical 
etchants, causing damage to the dielectric material underlying the metal 
masking layer. In addition, some chemical etchants are known to attack 
metallization, thus requiring special attention when the via being etched 
is designed to expose an aluminum interconnect pad on a very large scale 
integrated circuit (VLSI) chip. The anisotropic nature of plasma etching 
tends to produce barrel-shaped via holes. Depositing metallization in 
barrel-shaped via holes is very difficult. Moreover, thickness variations 
in the dielectric layer being etched may result in excessive plasma 
etching in certain areas with marginal etching in others. 
In another known method, photo-patternable polymer materials are applied to 
a substrate and then exposed to light in the areas where via holes are 
desired. After exposure, the polymer is developed, and baked at high 
temperatures. This method has three prominent limitations. First, it may 
only be used with photo-patternable materials. Second, the thickness of 
material which can be photopatterned is limited to approximately a maximum 
of 5 microns, due to the inherent properties of the material and 
sensitizers therefor. Third, the photo-patternable materials must be 
deposited in liquid form and then merely dried such that they may be 
easily developed. Prereacted film overlays, such as Kapton.RTM. polyimide 
film available from E.I. du Pont de Nemours and Company, Wilmington, Del., 
cannot be used. 
Another prior method involves forming pillars of conductor material on a 
substrate and filling in around the pillars with a dielectric. In this 
method, a layer of metallization on a substrate is first patterned and the 
metal is then built up by electroplating to form the pillars. Polymer 
material is then sprayed or spun onto the substrate in multiple coats, 
leaving sufficient curing time between coats to allow solvent and 
by-products of the curing process to escape. Enough coats are applied to 
completely cover the conductors on the substrate, but to barely cover the 
pillars. Following a short etch sufficient to uncover the top surfaces of 
the pillars, the pillars can function as metal-filled vias. This method 
has the disadvantages of requiring an excessive number of steps and the 
use of more difficult wet processing techniques. 
Eichelberger et al. U.S. Pat. No. 4,714,516, issued Dec. 22, 1987 and 
assigned to the instant assignee, discloses the use of an argon ion laser 
to form vias. Selected areas of the dielectric are damaged with laser 
energy and the damaged areas are then etched with oxygen plasma. The 
damaged portion of the dielectric etches at a much higher rate than the 
undamaged portion. One qualification on use of this approach arises 
because hole depth is limited by the amount of damage at the surface; that 
is, the via hole must be at least as wide at the top as the thickness of 
the dielectric material to be drilled. As a practical matter, via hole 
depths are limited to about 10 microns; therefore, thicker dielectric 
material requires extra steps in the formation of a via. For example, via 
formation would require at least the steps of damaging the dielectric with 
the laser, cleaning out the resulting hole with plasma, and thereafter 
damaging the dielectric at the bottom of the hole with the laser, followed 
by again cleaning out the hole with plasma. Another limitation to this 
approach is that the surface of the dielectric layer is exposed to 
excessive amounts of plasma. A substantial amount of plasma is required to 
clean out the holes and the undamaged dielectric surface tends to become 
rough from exposure to the plasma. The rough surface makes subsequent 
metallizations difficult to pattern. 
Pulsed lasers such as excimer and doubled YAG lasers have been used to 
directly ablate dielectric layers. By this technique, laser energy is 
absorbed in a thin layer of the dielectric in sufficient amounts to 
evaporate or ablate the thin layer of dielectric where the laser beam 
impinges. Multiple pulses are required to ablate holes of increased depth. 
Care must be taken to avoid ablating the underlying metal with laser 
pulses since the energy necessary to heat the polymer to ablation 
temperatures can also destroy the metallization underlying the dielectric. 
To effectively practice this method, only very small thicknesses of 
dielectric may be removed per pulse and, as a result, the method requires 
an excessive number of pulses. Lasers emitting visible light are not 
acceptable for this ablation technique because the penetration depth is 
too great. 
The direct laser ablation technique limits electronic package manufacturers 
to use of circular via holes. Round vias, however, are not as good as 
rectangular vias since they limit the area available for electrical 
contact; that is, rectangular vias are easily enlarged into larger 
rectangular vias without leaving residual materials in the bottom of the 
holes. Yet.the shape of the via holes formed using this technique is 
unavoidably circular because the laser energy is a focussed version of the 
laser cavity. Even if a non-circular mask opening is used as an aperture, 
the laser lens will nevertheless focus the energy as a circle. Changing 
via hole sizes requires a sophisticated zoom lens which can change focal 
length and f number to balance the conflicting optical requirements for 
hole size variation and constant depth of focus. 
Direct laser ablation techniques also do not allow for creation of 
sufficiently narrow via hole sizes in thick dielectrics, i.e., dielectrics 
too thick to be patterned by conventional photoresist techniques. The top 
of the hole is necessarily larger for larger thicknesses of dielectric, 
with the result that hole size at the top becomes too large when working 
with thick dielectrics. Yet thick dielectrics are desirable for many 
applications. When working with self-standing dielectric films, thicker 
films are easier to handle. Thick dielectrics are advantageous in systems 
where capacitance of conductor lines must be minimized. In addition, thick 
dielectrics are preferred for compliance with radiation hardness 
requirements - the dielectric must be sufficiently thick to absorb the 
secondary electrons generated by conductors overlying semiconductors. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a method for forming 
via holes of various sizes and shapes in dielectrics of various types and 
thicknesses, including fully-reacted films, at commercial rates and with a 
minimum number of processing steps. 
Another object is to provide a method for forming via holes of various 
sizes and shapes in dielectrics of various types and thicknesses, under 
direct computer control, without use of masks and without causing damage 
to the underlying conductor material. 
Another object is to provide a method for forming via holes in thick 
dielectrics in a stepped profile for minimum hole sizes and maximum hole 
densities. 
Briefly, in accordance with a preferred embodiment of the invention, a 
continuous wave laser beam is focussed to a very high energy density and 
dithered or scanned at a high scan rate, while also being advanced in a 
direction perpendicular to the dither direction so as to move over the 
entire area of a dielectric where a via hole is to be formed. The energy 
density in the focussed spot must be sufficient to heat a surface layer of 
the dielectric material within that spot to the point of vaporization or 
dissociation of the polymer. This constitutes what is known as ablation. 
The duty cycle must be such that the surrounding dielectric material is 
not heated to a degree which causes damage, and the wavelength of the 
laser must be such that a predominant portion of the energy is absorbed in 
a very thin uppermost section of the dielectric. The hole may be scanned 
in a raster format, which allows complete control over the size and shape 
of the hole. During this process, the laser removes a small amount of 
dielectric material on each scan and the scans are repeated until the 
underlying metallization is exposed. A computer controls the output of the 
laser, scanning or dithering (i.e., rapid to-and-fro movement) of the 
laser beam, and movement of an X-Y table carrying the structure undergoing 
ablation.

DETAILED DESCRIPTION OF A PREFERRED 
EMBODIMENT OF THE INVENTION 
FIG. 1A and 1B are top view illustrations of two alternative scan patterns 
which may be used to produce a typical via hole. FIG. 1A illustrates a 
raster scan pattern for application to a surface onto which a focussed 
laser beam spot 120 impinges. The parallel scans of the laser beam are all 
in the same direction, each being 25 microns in extent. The solid lines 
100, 102, 104, 106, 108, 10 and 122 indicate the paths along which the 
laser beam is scanned, in sequence, in the direction of the arrows. Each 
of these paths may be considered to be in an easterly direction (or +X 
direction in a Cartesian coordinate system). After each easterly scan, 
there is a relatively brief time delay before the next scan. During this 
time, the laser beam is blocked, or turned off, while it is repositioned 
relative to the area being scanned (as though the beam were moved 
generally westerly along the diagonal, dashed lines 101, 103, 105, 107, 
109 and 111, in sequence). Thus, path 101 is traversed after scan 100, and 
is followed by scan 102, then path 103, etc. The incrementation in the 
southerly (or --Y) direction is such that parallel scan lines 100, 102, 
104, 106, 108, 110 and 112 are spaced 1 micron apart. Scanning is done at 
the rate of three meters per second. The diameter of the maximum energy 
spot 120 is approximately 3 microns. After traversal of a raster has been 
completed, the process is repeated with the easterly scans progressing in 
the northerly (or+Y) direction relative to the area being scanned. 
FIG. 1B shows an alternative scan pattern which may be considered as a 
modified raster scan. This scan pattern differs from that shown in FIG. 1A 
in that the laser beam is scanned in both X directions. Thus, the laser 
beam scans along lines 130, 134, 138, and 142 by moving easterly (in the 
+X direction), while in alternate scans along lines 132, 136 and 140 the 
beam moves westerly (in the --X direction). The laser beam is turned off, 
or blocked, while it is incremented between successive east-west scans (as 
though it were moved southerly along each of the dashed lines 131, 133, 
135, 137, 139 and 141) by 1 micron. Spacing between adjacent scan lines is 
thus 1 micron. The maximum energy spot 150 is approximately 3 microns in 
diameter. The etching process for the scan pattern of FIG. 1A is thus 
similar to that for the scan pattern of FIG. 1B, while those skilled in 
the art will recognize that other scan patterns can be devised to effect 
the same etching process. 
As a specific example of the etching process according to the invention, a 
dielectric polymeric layer of Kapton polyimide or of silicone-polyimide, 
25 to 30 microns in thickness, can be etched in approximately 4 raster 
scans. Since the maximum energy spot size is approximately 3 microns in 
diameter, the spot overlaps the scan lines, which are spaced apart from 
each other by 1 micron, by a factor of 3. Thus in 4 raster scans, the same 
area is scanned by the laser beam a total of 12 times (i.e., 3 overlaps 
multiplied by 4 scans). A depth of approximately 2.5 microns of dielectric 
material is removed by each scan of the laser beam, so that approximately 
30 microns of material is removed during the 12 passes made by the laser 
beam. Since the beam is focussed to a spot of maximum energy in a diameter 
of 3 microns and the power within that spot is 0.25 watts, then the power 
density of the spot is 3.3.times.10.sup.6 watts per square centimeter 
(watt/cm.sup.2). At a laser wavelength of 351 nanometers, the surface 
layer, or depth of dielectric material to which the laser energy 
penetrates before being virtually completely absorbed, is 1-3 microns, 
depending on the material. For a laser beam dwell time of 1 microsecond, 
the approximate volume of material exposed to the laser beam is the 
product of the maximum energy spot size area and the absorption depth. 
This is a 3 micron depth multiplied by the roughly square (but with 
rounded corners) area exposed to a 3 micron diameter spot, or 
approximately 2.7.times.10.sup.-11 cm.sup.3. This extremely small volume 
can be heated to vaporization temperature in less than 1 microsecond by 
the 3.3.times.10.sup.6 watt/cm.sup.2 energy density spot. The system works 
effectively because very small volumes of material are vaporized at very 
fast rates, much like machining away a small amount of material in a 
conventional machining operation. 
FIGS. 2A through 2C illustrate progression of the via hole formation 
process of the invention from 1 raster scan to the next. A continuous wave 
laser beam 200 is focused to a very high energy density over a dielectric 
polymer layer 202, which is situated atop a metal contact pad 204 on a 
substrate 206. Substrate 206 may comprise an integrated circuit chip, in 
which case contact pad 204 comprises a chip pad thereon. Alternatively, 
substrate 206 may comprise a multilevel printed circuit board or ceramic 
multilayer substrate for hybrid assemblies in which case layer 204 is a 
metal run and layer 206 is insulation between successive, 
vertically-stacked metal runs. As another alternative, metal run 204 may 
be situated directly on a printed circuit board 206. FIGS. 2A and 2B show 
the effect of laser beam 200 being scanned across polymer layer 202. Beam 
scanning occurs at a rate of 3 meters per second. As shown in FIG. 2C, 
laser beam 200 is reflected when the beam reaches metal contact pad 204. 
Since well over half the energy of the beam is reflected by pad 204 and 
since pad 204 is highly thermally conductive and exhibits a substatially 
higer termal diffusivity than the material of polymer layer 202, the 
surface temperature of pad 204 remains well below the metal vaporization 
point. As a result, the drilling action ceases when the metal pad has been 
exposed. In the present embodiment, the laser beam impringes on polymer 
layer 202 only 1% of the time, such that only 1% of the laser power, or 
2.5 milliwatts, is applied to the via during its formation. This amount of 
power is well below the threshold for damage to adjacent areas of polymer 
202. The sides of the hole formed by this laser drilling action, such as 
sides 210 and 212, slope toward the center because they do not experience 
the overlapping scans that occur in the central portions of the hole and 
the etching process therefore proceeds more slowly at the sides of the 
hole. If the raster is scanned more times than required to reach 
underlying metal pad 204, sides 210 and 212 of the hole will be steepened 
somewhat from their illustrated slopes. 
The via holes produced by the laser beam are cleaned by a short plasma etch 
cycle which removes from underlying metal pad 240 any particles formed 
during the laser beam scanning, such as soot, any residual polymer, or any 
glass that may be residual from any glass removal operation performed when 
etching a pad mask, and cleans and roughens the upper surface of 
dielectric polymer layer 202 in order to enhance adhesion of subsequent 
metallization. The presently preferred plasma etch technique is to preheat 
both the part (i.e., subtrate) and chamber in which the part is placed, to 
a temperature of 110.degree. C., and then to etch the part for 2 minutes 
at a power of 150 watts in an atmosphere of 20% CF.sub.4 and 80% 0.sub.2. 
This assumes that four 2".times.2" substrates are etched in the chamber. 
The amount of polymer dielectric etched by this technique is between 2 and 
4 microns. 
FIG. 3 illustrates an application of the method of the present invention to 
increase the number of via holes possible in a given area in thick 
dielectric polymer layers. The dielectric polymer layer 300, as shown, has 
first been scanned by the laser beam across a relatively large area to 
produce the opening that extends from side 302 to side 304, but with only 
a sufficient number of scans to remove a portion of the depth of the 
polymer material. This is the result of large scans or dithers between 
side 302 and side 304, which cause ablation to a depth, in this case, of 
approximately one-half the thickness of polymer layer 300. Then the three 
smaller openings between side 306 and side 308, between side 310 and side 
312, and between side 314 and side 316, respectively, are formed using two 
additional raster scans of reduced dimensions. These three smaller 
openings extend all the way down to metal pads 318, 320 and 322 mounted on 
substrate 324, so as to constitute vias having sloping sidewalls to 
facilitate metallization. Despite the relative depth of these three via 
holes and the relative thickness of polymer layer 300, each of the three 
via holes is still relatively small in diameter at its respective top. 
FIG. 4 illustrates laser apparatus for forming via holes by the laser beam 
dither method of the present invention. Laser 410 may be a 3.5 watt 
Spectra Physics 2035-35 argon ion laser with ultraviolet optics. This is a 
continuous wave (CW) laser which emits ultraviolet radiation. The main 
ultraviolet line is 351 nanometers, and the laser power output is 
nominally 1.2 watts. The beam produced by the laser is modulated by a 
modulator, such as an acoustooptical modulator 412. One such 
acousto-optical modulator useable in the invention is a Newport Electro 
Optics Quartz 5080-3 modulator. The modulated laser beam 414 strikes a 
scanning mirror 416 and is reflected to a lens 418 which focuses it upon a 
substrate 420 situated on an XY positioning table 422 which allows 
movement of the chip in an X or Y direction relative to the laser beam. 
Substrate 420 may comprise a dielectric polymer layer overlying an 
integrated circuit chip. It will be appreciated by those skilled in the 
art that, in the alternative, other laser beam deflectors such as rotating 
mirrors, acousto optic deflection devices, or similar devices designed to 
deflect light in a precise manner, could be substituted for scanning 
mirror 416. The XY positioning table may be a Klinger Scientific model TCS 
250-200 table. Lens 418 focuses what is preferably a millimeter diameter 
laser beam 414 to a spot 424 having a maximum energy region diameter of 3 
microns. This lens is selected to keep the laser beam continually in focus 
on the area of dielectric polymer layer that is to be ablated. For 
example, lens 418 may comprise a field-flattening lens capable of focusing 
ultraviolet energy. 
The laser beam is scanned in the east/west (i.e.,.+-.X) direction 
preferably by a General Scanning G120D galvanometer 415 which sweeps 
scanning mirror 416 in response to a ramp voltage output from amplifier 
426. The ramp voltage appears as one or the other waveforms depicted in 
FIG. 5. For the raster scan of FIG. 1A, the laser beam is scanned in 
easterly directions only, and the waveform appears as designated for 
easterly scans; i.e., a relatively slowly rising ramp voltage followed by 
a relatively rapid flyback or return to the starting voltage level, 
designated 0, and a repetition of the rising ramp voltage, etc. During the 
rising voltage, the laser beam is on for either the full extent of the 
rise or some predetermined portion thereof, and during the falling voltage 
the beam is off. For the raster scan of FIG. 1B, the laser beam is 
dithered, or alternately scanned in an easterly direction and a westerly 
direction, and the waveform appears as designated for east/west scans; 
i.e., a rising ramp voltage (easterly scan) followed by a brief pause at 
the maximum voltage, and thereafter a falling ramp voltage (westerly scan) 
followed by a brief pause at the starting voltage level, designated 0, and 
a repetition of the rising ramp voltage, etc. 
As shown in FIG. 4, an AND gate 428 having one input energized by a 
controller 430 controls modulator 412 to either pass or block the laser 
beam. Controller 430, which may comprise a programmed IBM Personal 
Computer (PC), also controls positioning of XY table 422 through servo 
motor controls 432 for the north/south (i.e., .+-.Y) direction and 434 for 
the east/west direction. 
Controller 430 loads ON and OFF values in latches or registers 436 and 438 
which define the extent or limits of the east/west laser beam scanning 
introduced by galvanometer-controlled scanning mirror 416. Each respective 
one of latches 436 and 438 may comprise a Texas Instruments 74LS374 latch. 
Counter logic 440 operates under the control of controller 430 to provide 
an output count to ON and OFF comparators 442 and 444, respectively, each 
of which may comprise a Texas Instruments 74521 8-bit comparator, 
respectively. When the count from counter logic 440 equals the value 
stored in latch 436, comparator 442 produces an 0N output signal which 
sets a flip-flop 446. Likewise, when the count from counter logic 440 
equals the value stored in latch 438, comparator 444 produces an OFF 
output signal which resets flip-flop 446. Comparators 442 and 444, 
together with flip-flop 446, comprise switching circuitry 450. The Q or 
true output of flip-flop 446 supplies an input voltage to AND gate 428, 
while the Q or false output of flip-flop 446 supplies an input voltage to 
both counter logic 440 and controller 430. The count output from counter 
logic 440 is also supplied to a digital-to-analog converter 448, such as 
an Analog Devices AD656 converter, which generates the scanning ramp 
voltage that is supplied to amplifier 426. Thus when in the presence of an 
ON signal from comparator 442 AND gate 428 is enabled by controller 430, 
the output signal produced by AND gate 428 defines the time during which 
the laser beam is passed by modulator 412, and this in turn corresponds to 
the duration of the easterly or westerly scan of the laser beam on the 
substrate by galvanometer-controlled scanning mirror 416. 
In operation, the XY table 422 is commanded by controller 430 to the 
appropriate position for beginning the etching process. This is 
accomplished in a manner well known in the art by actuating motor controls 
432 and 434 to cause table 422 to rapidly arrive at the beginning scan 
position. At the same time, controller 430 loads latches 436 and 438 with 
the appropriate ON and OFF values for the area to be etched. As will be 
understood by those skilled in the art, the values loaded in latches 436 
and 438 may be selected to control both length and position of the laser 
beam scans, depending on the specific implementation of the invention. 
When the proper table position has been reached, controller 430 enables 
AND gate 428 in the manner previously mentioned, and the first easterly 
scan of the laser beam commences. At the completion of that scan, 
controller 430 moves the XY table in the southerly direction by 1 micron 
and initiates either another easterly scan of the beam, for the raster 
scan pattern of FIG. 1A, or a westerly scan of the beam, for the raster 
scan pattern of FIG. 1B, by driving galvanometer-controlled scanning 
mirror 416 in the appropriate direction. The process is repeated with the 
XY table being incremented in the southerly direction by step pulses from 
controller 430 for each scan of the laser beam resulting from motion of 
mirror 416. After completion of the first raster, the next raster is 
scanned with the XY table being incremented in the northerly direction for 
each scan of the laser beam resulting from motion of mirror 416. When the 
requisite number of raster scans have been made, controller 430 
deenergizes its input to AND gate 428 to shut off the laser beam, and 
appropriately energizes motor controls 432 and 434 to move the XY table to 
the next location where a via hole is desired. Hole size in the east/west 
direction is controlled by the numbers loaded into latches 436 and 438. 
Hole size in the north/south direction is controlled by the number of step 
pulses supplied from controller 430 to Y axis motor control 432. Y axis 
control 432 thus serves a dual purpose: first, acting as a slew servo to 
rapidly position XY table 422 in cooperation with X axis control 434, and 
second, acting as an incrementing servo to control the extent of the 
raster scan in the north/south or .+-.Y direction. Controller 430 controls 
both of these modes of operation of Y axis control 432. 
One suitable embodiment of counter logic 430 is shown in detail in FIG. 6. 
This implementation produces the raster scan shown in FIG. 1A. The logic 
comprises a counter 510, such as a Texas Instruments 74163 counter, which 
is initially reset by controller 430 via OR gate 514 and counts in 
response to clock pulses supplied via AND gate 512 when enabled by 
controller 430. The count output of counter 510 is supplied to ON and OFF 
comparators 442 and 444 and D/A converter 448, shown in FIG. 4. When the 
count output of counter 510 equals a predetermined value as detected by 
decoding logic 518, a reset signal is supplied to AND gate 516 which is 
enabled by the Q output voltage from flip-flop 446. 
For purposes of this description, it will be assumed that the values loaded 
in latches 436 and 438 of FIG. 4 do not represent the full extent of the 
east/west scan of the laser beam produced by mirror 416, but only that 
portion of the scan during which the laser beam is passed by modulator 
412. Thus, the scan (i.e., motion of mirror 416) is begun before the beam 
turns on and continues after the beam turns off. In this manner, the laser 
beam can be made to scan through only the most linear portion of each 
sweep of galvanometer 415 to achieve uniform ablation over the scanning 
area of the dielectric polymer layer. In addition, the beam may thus be 
controlled to pass through only the most central portion of lens 418 to 
ensure that beam distortion introduced by the lens is minimized. The XY 
table is incremented in the southerly direction during the flyback portion 
of the voltage waveform shown in FIG. 1A. The entire process is somewhat 
analogous to television raster scanning, a major exception being that 
incremental movement takes place in either direction on the Y axis, 
depending on the "frame" being scanned. 
The alternative scanning pattern shown in FIG. 1B is produced by a 
modification to the circuitry of FIG. 6. In this case, counter 510 is an 
up/down counter and the decoding logic 518 detects both a predetermined 
count and a zero count. The result of either detected count does not reset 
counter 510 but rather controls the direction of counting of the counter. 
Accordingly, in the presence of a signal from controller 430, clock pulses 
supplied through AND gate 512 drive counter 510 to continuously count up 
and down between a zero count and a predetermined count, and the count 
output of counter 510 is supplied to comparators 442 and 444 and to D/A 
converter 448. Because the laser beam now undergoes east/west scans, the 
voltage waveform output from D/A converter 448 is now generally a 
triangular wave, as indicated for the east/west scans in FIG. 5, rather 
than a sawtooth wave as indicated for the easterly scans in FIG. 5; that 
is, the signal controlling galvanometer mirror 416 ramps in two directions 
rather than just one. In addition, PC controller 430 must supply data to 
the on and off latches 436 and 438 to coincide with each east or west 
direction of scan. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modifications or changes. It is, therefore, to be 
understood that the appended claims are intended to cover all such 
modifications and changes as fall within the true spirit and scope of the 
invention.