U.S. patent application number 12/398275 was filed with the patent office on 2009-08-06 for method and apparatus for via drilling and selective material removal using an ultrafast pulse laser.
This patent application is currently assigned to LASERFACTURING INC.. Invention is credited to Tan DESHI.
Application Number | 20090194516 12/398275 |
Document ID | / |
Family ID | 36755398 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090194516 |
Kind Code |
A1 |
DESHI; Tan |
August 6, 2009 |
METHOD AND APPARATUS FOR VIA DRILLING AND SELECTIVE MATERIAL
REMOVAL USING AN ULTRAFAST PULSE LASER
Abstract
A method and apparatus for selective material removal and via
drilling for semiconductor applications using an ultrafast laser
pulse directly from an ultrafast pulse laser oscillator without
amplification are disclosed. The method and apparatus includes
techniques to avoid/reduce the cumulative heating effect and to
avoid machine quality degrading in multi shot ablation. Also the
disclosed method and apparatus provide a technique to change the
polarization state of the laser beam to reduce the focused spot
size, and to improve the machining efficiency and quality. The
disclosed method and apparatus provide a cost effective and stable
system for high volume manufacturing and inspection applications.
The disclosed method and apparatus have particular applications in,
but not limited to, drilling vias for interconnect formation,
selective material removal for application specific integrated
circuits, selective material removal for flash memory applications,
exposing layers for further semiconductor processing such as wire
bonding etc. The ultrafast laser oscillator can be a called a
femtosecond laser oscillator or a picosecond laser oscillator
depending on the pulse width of the laser beam generated.
Inventors: |
DESHI; Tan; (Wuhan,
CN) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1950 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
LASERFACTURING INC.
Burlington
CA
|
Family ID: |
36755398 |
Appl. No.: |
12/398275 |
Filed: |
March 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11048704 |
Feb 3, 2005 |
7528342 |
|
|
12398275 |
|
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|
|
Current U.S.
Class: |
219/121.71 |
Current CPC
Class: |
B23K 2103/56 20180801;
B23K 2101/42 20180801; H05K 3/0035 20130101; B23K 26/0624 20151001;
B23K 26/032 20130101; B23K 2103/50 20180801; B23K 26/389 20151001;
B23K 26/382 20151001; B23K 26/40 20130101; B23K 2103/08 20180801;
B23K 2103/172 20180801; B23K 2103/42 20180801; H05K 3/0038
20130101; B23K 2103/54 20180801 |
Class at
Publication: |
219/121.71 |
International
Class: |
B23K 26/38 20060101
B23K026/38 |
Claims
1. A laser machining method for via drilling and selective material
removal in a work piece or a semiconductor wafer using an ultrafast
laser pulse directly from an ultrafast laser oscillator without an
amplifier, comprising: emitting a pulsed laser beam from a diode
pumped or CW laser pumped solid state ultrafast laser oscillator
without an amplifier; controlling the laser pulse, to minimize the
cumulative heating effect and to improve the machining quality;
varying the diameter of the laser beam in at least one axis;
scanning the laser beam in two axes; and focusing the pulsed laser
beam on to a work piece; wherein a via is drilled or material is
selectively removed from the semiconductor wafer.
2. The method according to claim 1, further comprising changing the
polarization of the laser beam.
3. The method according to claim 1, further comprising moving the
wafer in three dimensions.
4. The method according to claim 1, further comprising injecting a
liquid or gas to assist in reducing the cumulative heating
effect.
5. The method according to claim 1, further comprising controlling
the scanning speed.
6. The method according to claim 1, further comprising imaging the
laser beam in order to align the laser beam with the wafer and to
monitor the machining process.
7. The method according to claim 1, further comprising using a
longer wavelength laser beam.
8. The method according to claim 1, further comprising controlling
the laser pulse energy and the pulse number.
9. The method according to claim 1, further comprising changing the
shape of the laser beam to improve the machining efficiency and
quality.
10. The method according to claim 1, further comprising reducing
the ablated feature size below the focused spot size by controlling
the laser threshold fluence.
11. The method according to claim 1, wherein the work piece is a
semiconductor wafer.
12. The method according to claim 1, for use in selective remove
material or a layer in semiconductor wafer by pulses from ultrafast
laser oscillator wherein: a layer of material can be selectively
removed without ablating the underlying material by precisely
controlling the pulsed laser fluence; the laser fluence of the
material depends on the material, the number of pulses at each scan
point, scanning speed, focused spot size, repletion rate of the
laser pulse, laser wavelength and the pulse width.
13. The method according to claim 12, wherein the selectively
ablated area can be round, square or of any desired shape depending
on the applications and further processing.
14. The method according to claim 12, wherein: overlying layers can
be removed layer by layer or a few layers together by controlling
the laser fluence; and each layer can vary in thickness from a few
micrometers to a few nanometers.
15. The method according to claim 1 for use in formation of
interconnect via in semiconductor wafer or multilayer printed
circuit board wherein: blind via holes are drilled through an
insulator layer and an conductive plate/layer causing minimal or no
damage to the underlying conductive layer; via interconnects are
then formed by filling the via holes formed between conductive
layers/plane with conductive material by metallization; and the
insulating layers are made of dielectric, glass or any other
insulating material and the conductive layer are made of
metals.
16. The method according to claim 15, wherein: the vias have a
smaller diameter at the lower portion of via compared to the upper
portion; via sidewall angles may range from 89 degrees to 1 degree
depending on the depth and diameter of the via; and the number of
layers though which via hole is drilled and the thickness of each
layer can vary depending on the application.
17. The method according to claim 15, wherein: the shape of via
hole can be round and slotted of single and multiple depths
depending on application; and the ablated feature size can be
reduced below the focused spot size by controlling the laser
threshold fluence.
18. The method of claim 1, wherein a thin film material to be
removed is a metal, dielectric, semiconductor, insulating material,
polymer, glass, silicon.
19. The method of claim 1, wherein debris is loosely bound to the
surface of the work piece and can be removed while machining using
pressurized gas assist and hence the process may not require post
processing.
20. The method of claim 1, where the via holes can be a through
hole or a blind hole in the work piece.
21. The method of claim 1; wherein micro cracks are minimized or
eliminated during the ablation process; wherein a recast layer
along the via side walls is minimized or eliminated to avoid
formation of voids during metallization of the via holes; and
wherein the via holes have a high aspect ratio and near vertical
sidewalls.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of pending U.S. patent
application Ser. No. 11/048,704, filed Feb. 3, 2005, which is
expressly incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method and apparatus for
via drilling and selective material removal using an ultrafast
pulse laser, and more specifically it relates to an apparatus and
method for via drilling and selective material removal using an
ultrafast pulse laser directly from an oscillator without an
amplifier, operating in picosecond and femtosecond pulse width
modes.
DESCRIPTION OF THE RELATED ART
[0003] Amplified short pulse lasers of pulse widths of 100
picosecond to 10 femtosecond are being used in general applications
to overcome the problem of long pulse lasers. There are several
advantages of short pulse lasers in comparison to long pulse
lasers. For example, since the duration of short pulse laser is
shorter than the heat dissipation time, the energy does not have
the time to diffuse away and hence there is minimal or no heat
affected zone and micro cracks. There is also negligible thermal
conduction beyond the ablated region resulting in negligible stress
or shock to the surrounding material.
[0004] Since there is minimal or no melt phase in short pulse laser
processing, there is no splattering of material onto the
surrounding surface. There is also no damage caused to the adjacent
structure since no heat is transferred to the surrounding material.
There are no undesirable changes in electrical or physical
characteristic of the material surrounding the target material.
There is no recast layer present along the laser cut side walls,
and this is vital for semiconductor applications. Amplified short
pulse lasers eliminate the need for any ancillary techniques to
remove the recast material within the kerf or on the surface. The
surface debris present does not bond with the substrate, and it is
easily removed by conventional washing techniques.
[0005] Machined feature size can be significantly smaller than the
focused laser spot size of the laser beam, and hence the feature
size is not limited by the laser wavelength.
[0006] Short pulse lases can be broadly divided in to two
categories. The first category is the femtosecond pulse with laser
(ranging from 10 fs-1 ps), and the second category is the pico
second pulse width laser (ranging from 1 ps-100 ps).
[0007] The femtosecond laser system (which is generally a
Ti-sapphire laser) generally consists of a mode locked femtosecond
oscillator module, which generates and delivers femtosecond laser
pulse of in the order of nanojoule pulse energy and 10-200 MHz
repletion rate. The low energy pulse is stretched in time prior to
amplification. Generally the pulse is stretched to Pico second
pulse width in a pulse stretcher module, using a dispersive optical
device such as a grating. The resultant stretched beam is then
amplified by several orders of magnitude in the amplifier module,
which is commonly called as regenerative amplifier or optical
parameter amplifier (OPA). The pump lasers generally used to pump
the gain medium in the amplifier are Q-switched
Neodymium-yttrium-lithium-floride (Nd-YLF) laser or Nd: YAG laser
with the help of diode pump laser or flash lamp type pumping. The
repletion rate of the system is determined by the repletion rate of
the pump laser. Alternatively if continuous pumping is used then
the repetition rate of the system is determined by the optical
switching within the regenerative amplifier. The resultant
amplified laser pulse is of Ps pulse width is compressed to
femtosecond pulse width in a compressor module. By this means
femtosecond pulse of mille joules to micro joules of pulse energy
of repletion rate 300 KHz to 500 Hz and average power less than 5 W
are produced.
[0008] The amplified femtosecond pulse has been used widely for
micro machining applications as described in U.S. Pat. No.
6,720,519, U.S. Pat. No. 6,621,040, U.S. Pat. No. 6,727,458 and
U.S. Pat. No. 6,677,552. The amplified femtosecond pulse, however,
suffers from limitations, which prevents it from being employed in
high volume manufacturing industrial applications. The system is
relatively unstable in terms of laser power and laser pointing
stability. Laser stability is very essential in obtaining uniform
machining quality (ablated feature size) over the entire scan
field. The average laser power is relatively low to meet the
industrial throughput requirements. The Amplified femtosecond laser
technology is relatively expensive, which increases manufacturing
costs considerably. The down time of the system is high due to the
complexity of the laser system. The laser system requires
relatively large floor space. There are relatively poor feature
size and depth controllability due to laser power fluctuation.
Experienced and trained professionals are required for the
maintenance of the system.
[0009] In contrast, an amplified pico second laser system has a
pico second oscillator, which delivers picosecond laser of
nanojoules pulse energy and is amplified by a amplifier. The pump
lasers generally used to pump the gain medium in the amplifier are
Q-switched Neodymium-yttrium-lithium-floride (Nd-YLF) laser or Nd:
YAG laser with the help of diode pump laser or flash lamp type
pumping. The repletion rate of the system is determined by the
repletion rate of the pump laser. Alternatively if continuous
pumping is used then the repetition rate of the system is
determined by the optical switching within the regenerative
amplifier. The resultant amplified pulse has repletion rate ranging
from 500 Hz to 300 KHz of average power 1 to 10 W. Although the
amplified picosecond laser is simple and compact in comparison to
the amplified femtosecond laser, it has, however, several
limitations, which prevents it from being used for high volume
manufacturing applications in industry.
[0010] The Amplified picosecond laser is more stable than an
amplified femtosecond laser system, but it is still unstable in
terms of laser power and laser pointing stability to meet the needs
for industrial high volume manufacturing applications. Laser
stability is very essential in obtaining uniform machining quality
(sblated feature size) over the entire scan field. The Amplified
picosecond femtosecond laser technology is also cheaper than
amplified femtosecond laser system, but it is still expensive,
which increases manufacturing costs considerably. It also has
relatively poor feature size and depth controllability due to laser
power fluctuation. The down time of the system is relatively high,
and the laser system requires relatively large floor space.
Experienced and trained professionals are required for the
maintenance of the system
[0011] Femtosecond laser with very low fluency is a promising
machining tool for direct ablating of sub-micron structures.
Fundamental pulses emitting from oscillator can be used to create
nano-features. But due to short time gap between the successive
pulses, there is considerable degrading of the machining quality,
which is explained below.
[0012] At the end of the irradiation of an individual laser pulse,
surface temperature rises to T.sub.max. Due to thermal diffusion,
the surface temperature decays slowly and eventually reduces to the
environment temperature T.sub.0. The time span of the thermal
diffusion .tau..sub.diffusion can be determined by the
one-dimensional homogeneous thermal diffusion equation. In the case
of multi-shot ablation, if the successive pulse arrives before
.tau..sub.diffusion (t<.tau..sub.diffusion), the uncompleted
heat dissipation will enhance the environment temperature. The
environment temperature after n laser shots for a pulse separation
of t at a time just before the next (or (n+1)th) shot can be
expressed by
T.sub.0(n)=T.sub.0+n.delta.T,
[0013] where, .delta.T is the temperature rise due to un-dissipated
heat at the end of a pulse temporal separation.
[0014] The actual surface temperature T.sub.max(n) after n
successive pulses can be written as:
T.sub.max(n)=T.sub.0(n)+T.sub.max
[0015] The enhanced surface temperature of the ablation front will
cause over heating and deteriorate the quality of ablation. In the
case of via drilling application, such over heating deteriorate the
geometry of via, causing barrel at the bottom of the hole.
[0016] The longer the time between successive pulses, the less is
the effect of the thermal coupling enhancing the surface
temperature. When pulse separation t is long enough that the heat
diffusion outranges the thermal coupling, the machining quality of
multi-shot ablation will be as good as that of single-shot
ablation.
[0017] In fact, thermal coupling effect of multi-shot ablation was
observed not only for nano-second pulses but also for ultrafast
laser pulses. Fuerbach [1], reported that to avoid degrading of
machine precision due to heat accumulating 1 .mu.s pulse separation
should be given for femtosecond pulses ablation of glass.
[0018] If pulse to pulse separation time is less than the
relaxation time/diffusion time of the ablated material, there is a
cumulative heating effect as described above. By this process the
subsequent pulses arrive before the sample surface dissipate the
heat generated by the previous pulse and relax to the state of the
underlying bulk material. These effect due to heat accumulation
increases with the increase in the pulse width, say from 1 fs to
100 ps. Also machining with ultrafast pulse laser directly from
oscillator, the feature quality is degraded. There are several
drawbacks related to the cumulative heating effects. It is
difficult use such a system for nanoscale maching applications due
to heat accumulation, and hence there is broadening of the feature
at the focused spot. The surrounding area will be damaged due to
heat accumulation, which is not accepted in many semiconductor
applications. There is more debris inside and around the ablated
feature, possibly resulting in considerable post processing. A
barrel shape may form at the bottom of the hole in via drilling
applications. There is relatively poor quality associate with the
ablated feature. Accordingly, there is a need for overcoming the
effect of cumulative heating, and such a technique is disclosed in
the present patent application.
[0019] Drilling Interconnect Via:
[0020] In recent years, demands for higher speed and smaller chips
have resulted in more complex chips having millions of
interconnections. Micro-vias are used to configure multilevel and
multilayer structures and integrate the components on
microprocessor, gate array, or high speed computer chip. On-chip
and chip-to-chip interconnections play the most significant role in
determining the size, power consumption, speed, reliability and
clock frequency and yield of circuit. The solution for future IC
packaging is 3D IC stacking using through chip interconnects. A 3D
IC is a stack of multiple dies with many direct connections
tunneling through them, dramatically reducing global interconnect
lengths and increasing the number of transistors that are within
one clock cycle of each other. Drilling interconnect via (in Si ICs
and Si interposer) are increasingly important in various
applications such as laying ground plane on the back side,
provision for an optical interconnect, chip scale packaging etc.
After drilling via, they are coated with a layer of insulating
material before the conductive material, typically copper, is
deposited to make the wire. One way of producing interconnect via
is by plasma etch equipment in conjunction with photolithography
process. But the technique is very expensive and very slow to meet
the industrial need. The fastest growing emerging tool for micro
via formation is laser drilling using solid state Nd: YAG UV laser.
UV wavelength in the range of 248 to 355 nm is absorbed by most
materials used in IC and semiconductor fabrication. Via of 25 .mu.m
diameter can be easily achieved with UV laser.
[0021] Interconnect vias, however, fabricated with a nanosecond
pulse laser as described in patents U.S. Pat. No. 6,631,558, U.S.
Pat. No. 6,706,997 etc. suffer from limitations. These limitations
include micro cracks, and a recast layer along the via sidewalls.
It also relatively difficult to selectively drill through a layer
without damaging the underlying layer, which is demanded in most
interconnect via applications. It is also relatively difficult to
remove surface debris due to molten material ejection from the via
hole by post process cleaning. This technique cannot generate via
holes in the submicron range, which is demanded by current and
future integrated circuits. It also causes damage to adjacent
structure due to heat dissipation. There is relatively poor via
depth control which is critical in interconnect via fabrication.
There is also relatively poor repeatability of via holes in terms
of diameter and depth. Lastly, there is relatively poor via shape
due to laser plasma shielding
SUMMARY OF THE INVENTION
[0022] One object of the present invention is to provide an
improved method and apparatus for micro/nano machining and to
ameliorate the aforesaid deficiencies of the prior art by using an
ultrafast pulse generated directly from the laser oscillator. The
laser oscillators preferably include a mode locked diode pumped
solid state laser system, which is stable and compact. The pulse
laser beam preferably has a pulse width of 1 fs to 100 ps and a
repletion rate from 1 MHz to 400 MHz, and it is controlled by an
electro optic modulator or an acousto optic modulator.
[0023] The modulated pulse is expanded to the required beam
diameter by using a combination of positive and negative lens to
act as a telescope. By varying the diameter of the laser beam, the
focused laser spot size can be varied. The pulsed laser beam is
preferably scanned by a two axis galvanometer scanner to scan the
pulse laser beam on the surface of the work piece in a
predetermined pattern. The scanning beam can be focused on a work
piece using a focusing unit or lens, which is preferably a scanning
lens, telecentic lens, F-.theta. lens, or the like, positioned a
distance from the scanning mirror approximately equal to the front
focal length (forward working distance) of the focusing lens. The
work piece is preferably positioned at approximately the back focal
length (back working distance) of the focusing lens.
[0024] In another aspect of the invention, the modulator controls
the laser pulse to minimize the cumulative heating effect and to
improve the machining quality. In addition to pulse control the
modulator controls the pulse energy and function as a shutter to on
and off the laser pulse when required.
[0025] In another aspect of the invention, the cumulative heating
effect can be minimized or eliminated by using a gas or liquid
assist. Due to the cooling effect of the assisted gas or liquid, it
is possible to minimize the cumulative heating effect even at a
relatively high repletion rate. Also the machining quality and
efficiency of processing are improved by using assisted gas or
liquid.
[0026] In another aspect of the invention, the cumulative heating
effect, quality of the machined feature and efficiency of the
process also depends on the scanning speed of the laser. The
scanning speed is controlled depending on the repletion rate of the
laser beam, the ablated feature size and the type of gas or liquid
assist used.
[0027] In another aspect of present invention, it is possible to
produce feature sizes of less than one twentieth of the focused
spot size of the ultrafast pulse laser beam. This can be achieved
by precisely controlling the laser threshold fluence slightly above
the ablation threshold of the material and by precisely controlling
the number of pulses and the duration between the pulses
(minimizing or eliminating the cumulative heating effect) using the
pulse modulation means disclosed in the present application. In
addition the stability of the laser pulse from the ultrafast laser
oscillator plays a vital role in machining feature of desired size
with repeatability and precision.
[0028] In another aspect of the present invention, a polarization
conversion module is used to vary the polarization state of the
laser beam along the axis. The modules uses a combination of a
telescopic arrangement with a retardation plate or birefringent
material in-between them. The resultant polarization state of the
beam can be a partially or fully radial polarization state. This
enables reduced focused spot size and improvement in the cutting
efficiency and quality compared to linear and circularly polarized
laser beams.
[0029] In another aspect of the present invention a piezo scanner
is used for scanning the laser beam in two axes rather than a
galvanometer scanner. This eliminates the distortion created at the
image field due to common pivot point of scanning on two axes. Also
the position accuracy and resolution is enhanced.
[0030] In another aspect of the present invention, a beam shaping
module is introduced to change the profile of the laser beam to the
desired profile using a combination of a MDT element and a quarter
wave plate. By carefully selecting the beam diameter and the length
of the MDT element the beam profile is varied for selective
material removal and via drilling application.
[0031] In another aspect of the present invention, the pulse energy
plays a vital role in micro and nano processing with high quality.
The pulse energy required to ablate a feature depends on the depth
of ablation, repeatability of feature size required and the feature
quality. The maximum depth that can be generated for a given
focused spot size of the laser beam depends on the pulse energy. As
the ablated feature becomes deeper it is difficult to remove the
ablated material from the hole and hence the ablated material
absorbs the energy of the subsequent pulse. Also the uncertainty in
the feature size obtained will depend upon the number of pulses
required to ablate the required feature. Due to the topography
generated and debris deposited in the crater by the ablation of the
first pulse the absorption of the successive pulses is different
due to the defects generated in the previous pulse, scattering of
the laser beam etc. Due to the above mechanism, the ablation
threshold of the successive pulses may vary. The uncertainty in the
diameter of ablated feature increases with an increase in the
number of pulses. Also, higher pulse energy generates sufficient
pressure for ejecting the debris out of the carter and hence the
successive pulse will interact with the fresh substrate. This
results in improved top surface and inner wall quality of the
ablated feature. Hence it is advantageous to use higher pulse
energy and a lower number of pulses to ablate a required
feature.
[0032] In another aspect of the invention, the effect of wavelength
on the cutting efficiency and stability of micron and nano
processing using laser pulses from an ultrafast laser oscillator is
disclosed. In ultrafast laser processing the wavelength of the
laser beam does not have a major impact on the threshold fluence of
the material as in the case of short pulse ablation in micron and
nanosecond pulse width techniques. Due to high peak power of the
laser caused by a short pulse width, the protons generated by the
laser beam start the ablation process rather than the protons
generated from the substrate. Hence absorption of the material at
different wavelength does not have a major influence in its
threshold fluence. Hence a laser beam having the fundamental
frequency will have a higher cutting efficiency than the second
harmonic frequency for a given focused spot size due to the higher
average power from the ultrafast laser oscillator at the
fundamental laser frequency. Similarly, the laser beam having the
second harmonic frequency will have a higher cutting efficiency
compared to a third harmonic frequency due to the greater average
power from the ultrafast laser oscillator at the second harmonic
frequency. Also the stability of the laser beam will deteriorate
with the reduction in wavelength by frequency doubling and
tripling, due to the increase in the optical components and the
sensitivity of the frequency doubling and tripling crystal and to
environmental factors such as temperature. Hence repeatability in
feature size and position accuracy may deteriorate compared to the
fundamental frequency from the ultrafast laser oscillator by
frequency doubling and tripling. Also the cost of the system may be
increased by frequency doubling and tripling due to the addition of
more optical components. In spite of the drawbacks of using
frequency doubled and tripled laser pulse, some applications may
demand the use of a shorter wavelength to achieve smaller feature
size and in sensitive material processing.
[0033] The method and apparatus of the present invention can be
utilized for selective removal of material using ultrafast laser
pulses directly from the oscillator. In ultrafast laser processing,
the threshold fluence of the material is clearly defined and hence
by controlling the pulsed laser fluence, material with a lower
threshold fluence can be selectively removed without ablating the
underlying material of higher threshold fluence.
[0034] In addition, the method and apparatus of the present
invention can be utilized for drilling interconnect vias on
multi-layer printed circuit boards or semiconductor wafers by using
ultrafast pulses generated directly from the laser oscillator.
[0035] The blind via holes are drilled through insulator layer and
conductive plate/layer causing minimal or no damage to the
underlying conductive layer. The insulating layer can be
dielectric, glass or any other insulating material. A via has a
smaller diameter at the lower portion of the via compared to the
upper portion. Via sidewall angles may range from 89 degrees to 1
degree depending on the depth and diameter of the via (top and
bottom via diameter required). Via interconnects are then formed by
filling via holes formed between conductive layers/plane with
conductive material. Sidewall angle is very critical for filling
via holes with conductive material without voids. In laser
processing as the depth of the via channel increases, barrel shape
channels are formed. These barrel shape holes results in voids when
filling the holes with conductive material, and it is not
acceptable for the formation of an interconnect. This barrel
formation can be avoided by the method and apparatus disclosed in
the present application. The number of layers though which via hole
is drilled can vary depending on the application. The method and
apparatus disclosed herein can be used to produce both round and
slotted blind vias of single and multiple depths.
BRIEF DESCRIPTION OF THE DRAWING
[0036] FIG. 1 is an illustration showing the laser apparatus for
micro and nano processing using ultrafast laser pulse from the
oscillator.
[0037] FIG. 2 is an illustration showing the apparatus to modulate
the ultrafast laser pulse from the oscillator using electro optic
modulator.
[0038] FIG. 3 is an illustration showing the mechanism of
eliminating the successive ultrafast laser pulse to reduce the
repetition rate by using electro optic modulator.
[0039] FIG. 4 is an illustration showing the introduction of a time
gap between groups of laser pulses using an electro optic
modulator.
[0040] FIG. 5 is an illustration showing the control mechanism
including a photo detector, electro optic modulator, XYZ
translation stage, galvanometer scanner and the imaging system
which are controlled by a processor control.
[0041] FIG. 6 is an illustration showing the apparatus to modulate
the ultrafast laser pulse from the oscillator using acousto optic
modulator.
[0042] FIG. 7 is an illustration showing the mechanism of
eliminating the successive ultrafast laser pulse to reduce the
repetition rate by using acousto optic modulator.
[0043] FIG. 8 is an illustration showing the introduction of a time
gap between groups of laser pulses using acousto optic
modulator.
[0044] FIG. 9 is an illustration showing the control mechanism
including a photo detector, acousto optic modulator, XYZ
translation stage, galvanometer scanner and the imaging system
which are controlled by a processor control.
[0045] FIG. 10 is an illustration showing a polarization conversion
module to change the polarization state of the ultrafast laser
beam.
[0046] FIG. 11 is an illustration showing a beam shaping module to
change the profile of ultrafast laser beam.
[0047] FIG. 12 is an illustration showing the 0% overlap between
consecutive ablated laser spots.
[0048] FIG. 12A is an illustration showing the edge quality of the
ablated feature with 0% overlap between consecutive ablated laser
spots.
[0049] FIG. 13 is an illustration showing the 50% overlap between
consecutive ablated laser spots.
[0050] FIG. 13A is an illustration showing the edge quality of the
ablated feature with 50% overlap between consecutive ablated laser
spots.
[0051] FIG. 14 is an illustration showing the edge quality of the
ablated feature with 90% overlap between consecutive ablated laser
spots.
[0052] FIG. 15 is an illustration showing the Gaussian energy
distribution of machining spots.
[0053] FIG. 16 is a graph showing the average laser power at
different laser wavelengths for a typical picosecond laser
oscillator.
[0054] FIG. 17 is an illustration of a multiple layer structure of
a semiconductor wafer.
[0055] FIG. 18 is an illustration of selective removal of a layer 1
without ablating the underlying layer2 using a laser beam from an
ultrafast laser oscillator.
[0056] FIG. 19 is an illustration of selective removal of multiple
layers 1, 2, 3 without ablating the underlying layer 4 using a
laser beam from an ultrafast laser oscillator.
[0057] FIG. 20A is an illustration of a multilayer semiconductor
wafer showing the protective, insulating and conductive layers.
[0058] FIG. 20B is an illustration showing a via channel drilled
through the insulating layer 1, 2 and conductive layer 1 without
ablating the underlying conductive layer 2 using a laser beam from
an ultrafast laser oscillator.
[0059] FIG. 20C is an illustration showing the via channel drilled
through multiple insulating and conductive layers without ablating
the underlying conductive layer using a laser beam from an
ultrafast laser oscillator.
[0060] FIG. 20D is an illustration showing a via channel drilled to
connect different conductive layers in the same conductive surface
using a laser beam from an ultrafast laser oscillator.
[0061] FIG. 20E is an illustration showing the interconnect
formation fabricated with a metallization process to connect the
conductive layers along the sidewall of a via channel.
[0062] FIG. 20F is an illustration showing the interconnect
formation fabricated by filling a via channel with conductive metal
to connect the conductive layers.
DETAIL DESCRIPTION OF THE DRAWING
[0063] One object of the present invention is to provide an
improved method and apparatus for micro/nano machining and to
ameliorate the aforesaid deficiencies of the prior art by using an
ultrafast pulse generated directly from a laser oscillator. The
laser oscillators preferably include a mode locked diode pumped
solid state laser system, which is stable and compact. The pulse
laser beam preferably has a pulse width of 1 fs to 100 ps and a
repletion rate from 1 MHz to 400 MHz, and it is preferably
controlled by an electro optic modulator or an acousto optic
modulator.
[0064] The modulated pulse is expanded to the required beam
diameter by using a combination of positive and negative lens to
act as a telescope. By varying the diameter of the laser beam the
focused laser spot size can be varied. The pulsed laser beam is
preferably scanned by a two axis galvanometer scanner in order to
scan the pulse laser beam on the surface of the work piece in a
predetermined pattern. The scanning beam can be focused on a work
piece using a focusing unit or lens, which is preferably a scanning
lens, telecentic lens, F-.theta. lens, or the like, positioned a
distance from the scanning mirror approximately equal to the front
focal length (forward working distance) of the focusing lens. The
work piece is preferably positioned at approximately the back focal
length (back working distance) of the focusing lens.
[0065] The modulator controls the laser pulse to minimize the
cumulative heating effect and to improve the machining quality. In
addition to pulse control, the modulator controls the pulse energy
and functions as a shutter to turn on and off the laser pulse when
required.
[0066] The cumulative heating effect can be minimized or eliminated
by using a gas or liquid assist. Due to the cooling effect of the
assisted gas or liquid, it is possible to minimize the cumulative
heating effect even at a relatively high repletion rate. Also the
machining quality and efficiency of processing are improved by
using assisted gas or liquid.
[0067] The cumulative heating effect, quality of the machined
feature and efficiency of the process also depends on the scanning
speed of the laser. The scanning speed is controlled depending on
the repletion rate of the laser beam, the ablated feature size and
the type of gas or liquid assist used.
[0068] In another aspect of the present invention, a polarization
conversion module is used to vary the polarization state of the
laser beam along the axis. The module uses a combination of a
telescopic arrangement with a retardation plate or birefringent
material in-between them. The resultant polarization state of the
beam can be a partially or fully radial polarization state. This
enables reduced focused spot size and improvement in the cutting
efficiency and quality compared to linear and circularly polarized
laser beams.
[0069] In another aspect of the present invention a piezo scanner
is used for scanning the laser beam in two axes rather than a
galvanometer scanner. This eliminates the distortion created at the
image field due to a common pivot point for scanning on two axes.
Also the position accuracy and resolution are enhanced.
[0070] In another aspect of the present invention, a beam shaping
module is introduced to change the profile of the laser beam to the
desired profile using a combination of a MDT element and a quarter
wave plate. By carefully selecting the beam diameter and the length
of the MDT element, the beam profile is varied for selective
material removal and via drilling applications.
[0071] In addition, the present invention is capable of producing a
feature size of less than one twentieth of the focused spot size of
the ultrafast pulse laser beam. This can be achieved by precisely
controlling the laser threshold fluence slightly above the ablation
threshold of the material and by precisely controlling the number
of pulses and the duration between the pulses (minimizing or
eliminating the cumulative heating effect) using the pulse
modulation means disclosed in this application. In addition, the
stability of the laser pulse from the ultrafast laser oscillator
plays a vital role in machining a feature of a desired size with
repeatability and precision.
[0072] In addition, the present application discloses pulse energy
that plays a vital role in micro and nano processing with high
quality. The pulse energy required to ablate a feature depends on
the depth of ablation, repeatability of the feature size required
and the feature quality. The maximum depth that can be generated
for a given focused spot size of the laser beam depends on the
pulse energy. As the ablated feature becomes deeper, it is
difficult to remove the ablated material from the hole and hence
the ablated material absorbs the energy of the subsequent pulse.
Also the uncertainty in the feature size obtained will depend on
the number of pulses required to ablate the required feature. Due
to the topography generated and debris deposited in the crater by
the ablation of the first pulse, the absorption of the successive
pulse is different due to the defects generated by the previous
pulse, scattering of the laser beam etc. Due to the above mechanism
the ablation threshold of the successive pulse may vary. The
uncertainty in the diameter of ablated feature increases with an
increase in the number of pulses. Also, higher pulse energy
generates sufficient pressure for ejecting the debris out of the
carter and hence the successive pulse will interact with the fresh
substrate. This results in an improved top surface and inner wall
quality of the ablated feature. Hence it is advantageous to use
higher pulse energy and a lower number of pulses to ablate a
required feature.
[0073] The present application discloses the effect of wavelength
on the cutting efficiency and the stability of micron and nano
processing using laser pulses from an ultrafast laser oscillator.
In ultrafast laser processing, the wavelength of the laser beam
does not have a major impact on the threshold fluence of the
material, as in the case of short pulse ablation in micron and
nanosecond pulse widths. Due to the high peak power of the laser
caused by short pulse widths, the protons are generated by the
laser beam to start the ablation process rather than being
generated from the substrate. Hence absorption of the material at
different wavelengths does not have a major influence in its
threshold fluence. Hence a laser beam having the fundamental
frequency will have a higher cutting efficiency than the second
harmonic frequency for a given focused spot size due to the higher
average power from the ultrafast laser oscillator at the
fundamental laser frequency.
[0074] Similarly, the laser beam having the second harmonic
frequency will have a higher cutting efficiency compared to a third
harmonic frequency due to the greater average power from the
ultrafast laser oscillator at the second harmonic frequency. Also
the stability of the laser beam will deteriorate with the reduction
in wavelength by frequency doubling and tripling, due to an
increase in the optical components and the sensitivity of the
frequency doubling and tripling crystal due to environmental
factors such as temperature. Hence repeatability in feature size
and position accuracy may deteriorate compared to the fundamental
frequency from the ultrafast laser oscillator by frequency doubling
and tripling. Also the cost of the system may increase by frequency
doubling and tripling due to the addition of more optical
components. In spite of the drawbacks of using frequency doubled
and tripled laser pulse, some applications may demand the use of
shorter wavelengths to achieve smaller feature size and in
sensitive material processing.
[0075] The method and apparatus of the present invention can be
utilized for selective remove material using ultrafast laser pulses
directly from the oscillator. In ultrafast laser processing the
threshold fluence of the material is clearly defined and hence by
controlling the pulsed laser fluence, material with a lower
threshold fluence can be selectively removed without ablating the
underlying material of the higher threshold fluence.
[0076] In addition, the method and apparatus of the present
invention can be utilized for drilling an interconnect via on
multi-layer printed circuit boards or semiconductor wafers by using
an ultrafast pulse generated directly from the laser
oscillator.
[0077] The blind via holes are drilled through an insulator layer
and a conductive plate/layer causing minimal or no damage to the
underlying conductive layer. The insulating layer can be
dielectric, glass or any other insulating material. A via has a
smaller diameter at the lower portion of the via compared to the
upper portion. Via sidewall angles may range from 89 degrees to 1
degree depending on the depth and diameter of via (top and bottom
via diameter required). Via interconnects are then formed by
filling the via holes formed between conductive layers/plane with
conductive material. Sidewall angle is very critical for filling
via holes with conductive material without voids. In laser
processing as the depth of the via channel increases, a barrel
shape channel is formed. These barrel shape holes results in voids
when filling the holes with conductive material, and it is not
acceptable for the formation of interconnect. This barrel formation
can be avoided by the method and apparatus disclosed in the present
application. The number of layers though which a via hole is
drilled can vary depending on the application. The method and
invention disclosed in the present application can be used to
produce both round and slotted blind vias of single and multiple
depths.
[0078] Exemplary embodiments of the present invention will now be
described in greater detail in reference to the figures.
[0079] One embodiment of the present invention is the method and
apparatus for micron and nano processing using ultrafast laser
pulse directly from the laser oscillator. The ultrafast laser
oscillator 1 generates laser pulse of a pulse width of 1 fs-100 ps.
The laser pulse is preferably of the wavelength 1200-233 nm, and
the repletion rate is preferably from 1 MHz to 400 MHz. Also the
laser beam is collimated and of a linear or circular polarization
state. The laser beam 20 incidents substantially normally on a wave
plate 2, which is preferably a half wave or a quarter wave plate to
change the polarization state of the incident laser beam 20. The
laser pulse 21 is modulated by beam modulating means 3. The
modulated laser pulse 22 is deflected by a mirror 4. The laser beam
23 is expanded or reduced in beam diameter by the optical lens 5
and 6, which are arranged and are of the keplerian telescope type
(where optical lens 5 and 6 are positive lens) or Galilean
telescope type (where optical lens 5 is a negative lens and optical
lens 6 is a positive lens for beam size expansion or vice versa for
beam size reduction).
[0080] The expanded laser beam 24 is passed through a diaphragm 7
to cut the edge of the Gaussian beam and to improve the quality of
the pulsed laser beam. The laser beam 25 is scanned in X and Y axes
by a two axis galvanometer scanner 10 after passing through a
mirror or polarizer 8. Camera 9 images the work piece through
polarizer 8, to align the work piece to the laser beam and to
monitor the machining process. The laser beam 26 from the
galvanometer scanner 10 is focused by an optical lens 11, which is
preferably a telecentric lens or f-theta lens or scan lens or
confocal microscopy lens. The lens 11 is positioned at the forward
working distance from the center of the two scanning mirrors in the
galvanometer scanner 10. The work piece/substrate 13 is placed at a
distance equal to the back working distance of the lens 11 from the
back face/out put of the lens 11. A gas assist system comprising of
one or more nozzles is positioned close to the work piece/substrate
13. Preferably the work piece/substrate 13 is placed on a three
axis mechanical translational stage 14. The translational stage 14
translates with respect to the laser beam 27 during and after laser
dicing of an area defined by a field of view of the scanning
lens.
[0081] During the micro and nano processing using ultrafast laser
pulse directly from oscillator, the laser beam 27 may be focused on
the top surface of the substrate/wafer 13 or located inside the
bulk of substrate material between the top and bottom surface of
the substrate 13. The location of the focus of the beam 27 depends
on the thickness of the substrate/wafer 13. When the material is
thicker, the focus of the laser beam 27 is further inside the bulk
of the substrate, away from the top surface of the substrate.
[0082] Depending on the pulse energy of the laser beam 27 from the
ultrafast laser oscillator 1 and the thickness of the
substrate/wafer 13, the laser beam 23 is expanded or reduced, thus
varying the energy density of the laser beam at the focused spot.
When the laser beam 23 is expanded in beam diameter, using
combination of optical lens 5 and 6, the focused spot size reduces
and hence increases the energy density at the focused laser spot.
Alternatively, when the laser beam 23 is reduced in beam diameter,
using the combination of optical lens 5 and 6, the focused spot
size increases and thereby reducing the energy density at the
focused laser spot.
[0083] The laser oscillator 1 generates a laser pulse of a pulse
width of 1 fs to 100 ps and a pulse repletion rate from 1 MHz to
400 MHz. The fundamental wavelength of the laser beam ranges from
1200 nm to 700 nm, second harmonic wave length 600 nm-350 nm and
third harmonics from 400 nm to 233 nm. The pulse energy generated
from this oscillator depends on the repetition rate of the system,
and a higher repletion rate will lower the pulse energy and vice
versa. Generally the average power of the laser from the oscillator
will be 0.2 W-30 W depending on the pulse width and wavelength of
the laser. A laser with a pulse width of 1 fs to 200 fs will have
an average power of 0.2 W to 10 W depending on the pump laser
power. Some of the commercially available femtosecond mode locked
diode pump solid state oscillators are manufactured by Coherent
Vitesse, Coherent Chameleon, Femtosource Scientific XL, Spectra
Physics Mai-Tai etc. Similarly, a laser with a pulse width of 1
ps-100 ps has an average power of 1 W-30 W at the fundamental
wavelength depending on the pump laser power. Some of the
commercially available picosecond mode locked diode pump solid
state oscillators include Coherent paladin, Time Bandwidth
Cheetah-X, Time Bandwidth Cougar, Lumera laser UPL-20.
[0084] Since the oscillator operates on diode pumped solid state
technology and involves minimal optical components, the system is
highly stable for industrial high volume manufacturing
applications. In ultrafast laser processing, the ablated feature
size/machined feature size depends on the energy stability/noise of
the laser. Based on Gaussian profile, for every 1% fluctuation in
the laser fluence/laser energy there will be 16% fluctuation in the
ablated/machined feature size in ultrafast laser processing. Most
industrial applications, however, demand strict feature size
control within 1-5%. Also pointing stability becomes a relatively
critical issue for machining a feature in micron and nano scale
industrial applications. This stringent industrial requirement can
be met by using a laser pulse directly from an ultrafast laser
oscillator.
[0085] Hence, using a laser pulse directly from an ultrafast laser
oscillator for micro/nano processing makes the ultrafast laser
technology viable for high volume manufacturing industrial
applications due to the following reasons. The system is stable in
terms of laser power and pulse to pulse energy due to Diode Pump
Solid State (DPSS) laser technology and minimal optical components.
The laser stability and the pulse to pulse energy stability are
critical in controlling and obtaining repeatability in the ablated
feature size. Good laser pointing stability is provide by DPSS
laser technology. Good beam quality is essential for micro/nano
processing. The laser power is high enough to meet the industrial
throughput in micro/nano processing application. The system is
simple and cost effective and reduces the manufacturing cost
considerably. There is a low cost of ownership due to efficient
DPSS technology. The down time of the system is relatively low. A
relatively small floor space is required for the laser system
[0086] In spite of the salient features mentioned above, micro/nano
processing by using laser pulse directly from ultrafast laser
oscillators limited due to several reasons. The cumulative heating
effect which results in poor machining quality. There is an absence
of a shutter mechanism to turn on and off the laser at high speed.
There is also an absence of means for controlling the pulse
energy.
[0087] To avoid surface modification around the structure which one
actually wants to generate, thermal diffusion of the heat flowing
out of the focal volume must overcome the deposited\laser energy.
In this case, there is no temperature raise around the focal area
and hence no cumulative heating effect is expected. Thus in order
to minimize the cumulative heating effect in multi shot ablation,
the pulse separation time t should be long enough that the heat
diffusion outranges the thermal coupling. Following are some of the
means for minimizing the cumulative heating effect and for
improving machining quality which are disclosed in the present
application. One technique is to control the laser pulse from the
ultrafast laser oscillator. Another is to use gas assisted
ablation. Still another is to scan the laser beam at a rate at
which each laser pulse irradiates at a different spot.
[0088] These techniques ensure that the machining precision after
many laser shots does not degrade in comparison to single pulse
damage spot.
[0089] Controlling the Laser Pulse from the Ultrafast Laser
Oscillator:
[0090] Alternatively, the repetition rate can be reduced by
increasing the resonator length, and hence a repletion rate as low
as 5 MHz-10 MHz can be realized by increasing the resonator length.
By reducing the pulse repetition rate the pulse energy can be
increased, which increases the range of material that can be
ablated and the feature size. The pulse energy, out of the mode
locked oscillator can be calculated by
[0091] Ep=PA/R, where Ep is the pulse energy, PA is the average
power and R, repetition rate of the system.
[0092] To completely eliminate the cumulative heating effect and to
improve the ablated feature quality, however, the repletion rate
should be reduced to less than 1 MHz, which means a resonator
cavity length of 150 m, which is difficult to realize. In order to
further reduce the repletion rate some external pulse control means
should be used. Also the pulse control means eliminates the need
for a shutter and pulse energy control mechanism. Two types of
pulse control means namely electro optic and acousto optic
modulation system are disclosed in the present application to
perform control of the repletion rate and control of the pulse
energy, and to operate as a laser shutter to turn on and off the
laser output when required.
[0093] Controlling the Laser Pulse by Electro Optic Modulator:
[0094] Depending on the application, the electro optic modulator is
known as pockels cells or a Q-switch or a pulse picker. The electro
optic modulator is used in combination with a polarizing beam
splitter or polarizer or prism for controlling the laser pulse. For
efficient pulse control, the electro optic modulator preferably has
a short rise time in the range of 20 ns to 10 ps, an energy/power
loss less than 10%, and a clear aperture diameter of 2-10 mm.
[0095] The antireflection coating and type of crystal in the
modulator depend on the laser wavelength, which may vary depending
on the application. The electro optic modulator is driven by a
driver which can be computer controlled. On sending the trigger
signal, which is preferably a voltage or power signal, to the
electro optic modulator from the driver the polarization state of
the laser beam is shifted from horizontal to vertical polarization
or vice versa. Vertical and horizontal polarizations are also
called as S and P polarizations. By changing the polarization, the
beam will be transmitted or deflected by the polarizing beam
splitter or a polarizer or prism, thus acting like a high speed
shutter and controlling the pulse. The deflected or transmitted
beam can be used for processing, but generally the transmitted beam
is used for laser processing, and the deflected beam is blocked by
the beam blocking means. FIG. 2 shows the working mechanism of the
electro optic modulator for pulse control. The pulsed laser from
the ultrafast laser oscillator 1 preferably has a repletion rate of
5 MHz to 200 MHz and passes through an electro optic modulator 3C
at S or P-polarization state. The electro optic modulator 3C is
driven by a driver 3D, which is controlled by a computer 3E. A
fraction of the laser beam 21 (less than 1% of energy) is deflected
by a partial coated mirror 3A on to a photo detector 3B which is
placed before the electro optic modulator as shown in the FIG. 2 to
obtain the signal from beam 21A and to synchronize the on/off of
the electro optic modulator 3C to avoid any clipping of the laser
pulse 21C. Due to the fast rise time of the electro optic modulator
30, the polarization state of any individual pulse or a group of
pulses can be shifted by 90 degrees to S or P polarization state
respectively. On passing through the polarizing beam splitter 3F
which is of the type plate polarizing beam splitter or cube
polarizing beam splitter or polarizer or prism, the S and P
polarized laser pulses are deflected at different angles. One of
the beams 21D can be blocked by a beam blocking means 3G and the
other beam 22 can be used for laser processing. FIG. 3 shows the
electro optic modulator for changing the polarization state of
alternative pulses, and FIG. 4 shows the electro optic modulator
changing the polarization state of the group of pulses. Thus by
using electro optic modulator 3C in combination with a polarizing
beam splitter 3F for controlling the laser pulse from ultrafast
laser oscillator, the repletion rate of the laser pulse can be
reduced to any required value as shown in FIG. 3 to
minimize/eliminate the cumulative heating effect and improve the
machining quality. Alternatively, a time gap is provided between
groups of laser pulses to minimize the cumulative heating effect
and to improve the machining quality as sown in FIG. 4. Further the
electro optic modulator serves as a shutter to enable or disable
the ultrafast laser pulse when required. Further the electro optic
modulator can be used to vary the pulse energy by varying the
voltage applied to the electro optic modulator from the driver.
Precise control of pulse energy/intensity control is essential for
varying the ablated feature size, selective material removal etc. A
central processor controller controls the photo detector, the
driver of the electro optic modulator, the imaging system, the XYZ
stages and the galvanometer scanner as shown in FIG. 5.
[0096] Controlling the laser pulse by Acousto Optic Modulator
[0097] The acousto optic modulator may have the following
specifications, and it may be used to control the laser pulse from
the ultrafast laser oscillator to minimize or eliminate the
cumulative heating effect and to improve the machining quality.
[0098] Rise time: 5-100 ns
[0099] Efficiency: 70-95%
[0100] Clear aperture: 0.5-5 mm
[0101] Centre frequency/carrier frequency: 25 MHz to 300 MHz
[0102] The laser pulse from the ultrafast laser oscillator passes
through the acousto optic Modulator (AOM) 3H, which is driven by a
driver 31, as shown in FIG. 6. The ultrafast laser is split into
first order beam 21E and zero order beams 22, where the first order
beam 21E is deflected at an angle called the Bragg angle to the
zero order beam 22 as shown in FIG. 6. The zero order beam 22 will
have the same polarization state of the input beam 21B and the
first order beam will have a polarization state ninety degrees to
the input beam 21B. Thus, if the input beam 21B is P polarized, the
zero order beam 22 will be P polarized, and first order beam 21E
will be S polarized and vice versa.
[0103] The Bragg angle is given by
[0104] .theta.=.lamda.f/v, where .lamda. is the wavelength of the
incident laser beam, f is the center frequency/carrier frequency of
the AOM and v is the velocity of the acoustic wave propagation in
the in the acoustic crystal.
[0105] The first order beam 21E or zero order beam 22 can be used
for laser processing, and the other beam is blocked by the beam
blocker 3G.
[0106] The ultrafast laser beam is a spectrum and the spectral
width increases with the reduction in pulse width. On passing
through the AOM 3H different wavelength in the laser spectrum will
have a different Bragg angle. Hence the first order beam 21E will
disperse resulting in an elliptical shape of the laser beam, which
will result in a poor beam quality and hence the machined feature
quality. The dispersion effect reduces with the increase in the
pulse width due to shorter spectral width and vice versa. Using the
first or zero order beams for material processing may not be a
problem above 1 ps pulse with but below 1 ps pulse width there will
be serious deterioration of the beam quality. The zero order beam
22 has no dispersive effect and can be used for processing, and the
first order beam 21E can be blocked by beam blocking means 3G as
sown in FIG. 6. By using an acousto optic modulator for controlling
the laser pulse from ultrafast laser oscillator the repletion rate
of the laser pulse can be reduced as shown in FIG. 7 to
minimize/eliminate the cumulative heating effect and improve the
machining quality. Alternatively, a time gap between groups of
laser pulses can be provided to minimize the cumulative heating
effect and to improve the machining quality as shown in FIG. 8.
Further the acousto optic modulator serves as a shutter to turn on
and off the ultrafast laser pulse when required. Also the electro
optic modulator can be used to vary the pulse energy by varying the
power applied to the acousto optic modulator from the driver.
Precise control of pulse energy/intensity control is essential for
varying the ablated feature size, selective material removal etc. A
central processor controller controls the photo detector, driver of
Acousto optic modulator, imaging system, XYZ stages and the
galvanometer scanner as shown in FIG. 9.
[0107] Polarization Conversion Module:
[0108] The laser beam 24 is passed through a polarization
conversion module 7A to change the polarization state of the laser
beam along the axis of the laser beam profile. In FIG. 10, a novel
yet simple technique is proposed for radial polarization
modulation. The first biconvex lens 200 focuses the collimated
laser beam into a tightly convergent beam 24A. As illustrated in
FIG. 10, light rays of a convergent beam travel different optical
path lengths when they transmit to a birefringent/retardation plate
plate 201. The retardation plate 201 can be a half-wave plate or a
quarter-wave plate. The light rays at the central part of the beam
travel a shorter distance than those at the edge. Consequently, the
polarization state is partially or completely modulated into
radial, depending on the beam convergence and properties of the
birefringent plate. The laser beam 24B is collimated by the lens
202. The lens 200 and 202 can be of the positive type or negative
type lens and may be combined like a telescope. It was found that
the polarization converted beam by the polarization conversion
module significantly improves the machining quality and throughput.
By converting the polarization state of the beam by the
polarization convertion module 7A there are significant advantages.
There is a significant reduction in debris generated due to
ablation. There is a reduction in the focused beam spot size by
10-30% compared to linear or circular polarization states. There is
an increase in the machining efficiency by 10-30% compared to
linaer or circular polarization states.
[0109] Scanning Module:
[0110] The scanning module 10 can be a galvo scanner or a piezo
scanner. The scanning module scans the laser beam in two axes. A
piezo scanner is preferred over a galvo scanner. There is a higher
scanning speed and hence improved machining quality and efficiency.
There is higher positioning accuracy and resolution. There is also
a minimization of the cumulative heating effect due to higher
scanning speed. Lastly, there is a common pivot point, and field
distortion at the image plane is avoided. Hence, it does not
require compensation software to eliminate the distortion.
[0111] Beam Shaping Module:
[0112] The beam shaping module is introduced to change the profile
of the laser beam to a hat top or any other profile required. The
beam shaping module is as shown in FIG. 11, and it preferably
includes a quarter wave plate 300 and a MDT crystal 301. The MDT
element is relatively cheap compared to beam shapers, consisting of
several micro lens or diffractive optics. The MDT element is based
on the phenomenon of internal conical reflection, and the resultant
beam profile depends on the diameter and wavelength of the incoming
beam and the length of the MDT element. By varying the diameter and
length of the MDT element, different beam profiles are possible.
The beam shaping module can be placed after the polarization
conversion module, or it can be absent depending on the
application.
[0113] Using Gas or Liquid Assist:
[0114] Use of assisted gas or liquid plays a vital role in
ultrafast laser machining. It provides a mechanical force to eject
the melt from the cut zone, and it cools the cut zone by forced
conversion.
[0115] By using assisted gas or liquid for ablating a feature using
a laser pulse from an ultrafast laser oscillator, the heat
diffusion time is reduced due to the cooling effect of gas or
liquid. Due to the reduction in the heat diffusion time it is
possible to minimize the cumulative heating effect and to improve
the ablated feature quality even at relatively high repletion
rates. Thus, by using assisted gas or liquid, the minimal/no
cumulative heating effect and quality machined feature can be
obtained at a repetition rate 2-10 times higher than at non gas
assisted process. Also the efficiency and overall quality of the
machining process can be improved by using assisted gas or liquid
due to the interaction of the gas or liquid jet with the work
piece. Also the gas or liquid assist the machining process by
efficiently carrying the debris from the cutting channel. These
assisted gases or liquid are delivered by single or multiple
nozzles 12 at a pressure, which is determined by the substrate
material, depth of cut, the type of nozzle used, distance of the
nozzle 12 from the work piece 12 etc. In case of assisted gas,
compressed gas from a gas tank is fed into the nozzle through a gas
inlet where a pressure gauge is set. The gas pressure can be
adjusted through a regulator installed upstream of the gas inlet.
In the case of liquid assisted cutting water or any other
appropriate liquid is mixed with compressed air and sprayed on the
substrate at the required pressure. The liquid pressure and ratio
of liquid to air is controlled by a regulator. Generally the gas or
liquid nozzles are positioned as close to the work piece as
possible for minimizing the gas or liquid usage and for improving
the machining quality and efficiency. Some examples of the gas used
to minimize the cumulative heating effect, improving the ablated
feature quality and improve the machining efficiency are air, HFC,
SF6, Nitrogen, Oxygen, argon, CF4, Helium, or a chlorofluorocarbon
or halocarbon gas. The commonly used liquid assists are water,
methanol, iso-propanol alcohol etc. A lower viscosity liquid will
improve the cutting quality and efficiency.
[0116] Scanning the Beam at High Speed:
[0117] By scanning the laser beam fast enough, each laser pulse
irradiates a different spot. The scanning speed required to
minimize the cumulative heating effect and increase the ablated
feature quality depends on the focused spot size d, pulse energy
Ep, scanning speed S, ablation threshold of material Eth and
repletion rate of the system R.
[0118] The distance between the two consecutive spot D is given
by
D=S/R
[0119] For example, if the repletion rate of the system is 1 MHz
and the scanning speed of 1000 mm/sec, the distance between the
consecutive pulses is 1 .mu.m. The overlap between the pulses Op
will determine the edge quality of the ablated feature. The ablated
feature Fd size can be as big as 2-3 times the focused spot size
and as small as 1/20th focused spot size depending on the laser
fluence/pulse energy and the material threshold. So if the ablated
feature size Fd is 1 .mu.m, the consecutive pulse will have 0%
overlap as shown in FIG. 12. Hence, there will be no cumulative
heating effect present. But the edge quality will be bad, if there
is 0% overlap between the pulses as shown in FIG. 12A. Generally to
obtain a uniform edge quality, 50% or more overlap between the
consecutive pulses is required. So in order to obtain 50% overlap,
as shown in FIG. 13, the scanning speed S should be reduced to 500
mm/sec. The resultant edge quality of the machined feature is as
shown in FIG. 13A. The overlap between the pulses Op can be
increased to 90% as sown in FIG. 14 by reducing the scanning speed
to 100 mm/sec. The cumulative heating effect increases with the
increase in the pulse to pulse overlap Op, but an overlap of 90% to
50% generally has minimal cumulative heating effect and better
machining quality for most applications. Generally the maximum
scanning speed of a commercially available galvanometer scanner is
3000-7000 mm/sec. Since it is very difficult to reduce the
repletion rate of the of the laser pulse from the ultrafast laser
oscillator below a certain limit due to the required resonator
length, the scanning speed of the laser beam plays a very important
role in improving the machining quality and reducing the cumulative
heating effect. The repetition rate of the system Ro for a given
pulse to pulse overlap Op is given by:
Ro=S/(1-Op).times.Fd
[0120] For example, if the maximum scanning speed of the
galvanometer scanner is 5000 mm/sec and the ablated feature size is
1 .mu.m, the repletion rate of the pulse from ultrafast laser
oscillator R can be as high as 50 Mhz for a pulse to pulse overlap
Op of 90%. But if the maximum scanning speed of the galvonometer
scanner is 1000 mm/sec, then for the same condition of 90% overlap
the repletion rate R can be only 10 MHz. Thus, the cumulative
heating effect and the ablated feature quality can be controlled by
varying the scanning speed for a given repletion rate of the
system, the pulse to pulse overlap and ablated feature size.
[0121] Depending on the depth of the feature required the laser
beam will be scanned along the same path few times at the optimal
scanning speed. This mechanism of scanning at high speed is
applicable for cutting a slot or via drilling by trepanning.
[0122] Machining Feature Size Below the Focused Spot Size
[0123] In addition, the present invention is capable of producing a
feature size of less than one twentieth of the focused spot size of
the ultrafast pulse laser beam. This can be achieved by precisely
controlling the laser threshold fluence slightly above the ablation
threshold of the material and by precisely controlling the number
of pulses and the duration between the pulses (minimizing or
eliminating the cumulative heating effect) using the pulse
modulation means disclosed in this invention.
[0124] The energy distribution of machining spot follows a Gaussian
profile, as sown in FIG. 15, thus, the fluence at any location of
the spot F (x,y) can be calculated from the maximum fluence Fmax
by
[0125] F(x,y)=Fmaexp(-2(x2+y2)/(D/2)2), where D denotes the
diameter of laser spot. Since the threshold Fth is precisely
defined at ultrafast pulse width, only the portion of laser spot
where f(x,y).gtoreq.Fth will induce material removal. The above
equation can be used to predict the size of the ablated feature. To
obtain a feature size 1/10th of the spot size, the maximum fluence
Fmax must be controlled just 2% higher than the ablation threshold
of the target material.
[0126] Also it is difficult to obtain a feature far below the
focused spot size of the laser beam due to the cumulative heating
effect, which causes the damaged site to enlarge and hence
difficult to machine sub micron and nano structures. As disclosed
in this application, the cumulative heating effect can be minimized
or eliminated by controlling the distance between the successive
pulses or by varying the scanning speed of the laser beam or by
using gas or liquid assist or any combination of the above. In
addition, the stability of the laser pulse from the ultrafast laser
oscillator plays a vital role in machining feature of a desired
size with repeatability and precision. For every 1% variation in
the laser fluence the feature size varies by 16% (which can be
derived from a Gaussian equation). The pulse to pulse energy from
the ultrafast laser oscillator is very stable due to fewer optical
components, diode pumping, sealed optical components and
environmentally (temperature, pressure) stabilization. Hence the
laser fluence variation is very minimal, which enables it to
generate micro and nano scale features with high repeatability and
precision.
[0127] Pulse Energy:
[0128] Pulse energy plays a vital role in micro and nano processing
with high quality.
[0129] Pulse energy is given by
[0130] Pe=Pavg/R, where Pavg is the average power of the laser and
R is the repletion rate.
[0131] The pulse energy required to ablate a feature depends mainly
on the threshold fluence of the material, feature size, maximum
depth of the feature required.
[0132] Maximum Depth:
[0133] The maximum depth that can be generated for a given focused
spot size of the laser beam depends on the pulse energy. As the
ablated feature becomes deeper, it is difficult to remove the
ablated material from the hole and hence the ablated material
absorbs the energy of the subsequent pulse. Thus, the depth limit
exhibits a logarithmic dependence on the pulse energy.
[0134] Feature Size Repeatability:
[0135] The uncertainty in the feature size obtained will depend on
the number of pulses required to ablate the required feature. Due
to the topography generated and debris deposited in the crater by
the ablation of the first pulse, the absorption of the successive
pulse is different due to the defects generated in the previous
pulse, scattering of the laser beam etc. Due to the above mechanism
the ablation threshold of the successive pulse may vary. The
uncertainty in the diameter of ablated feature increases with the
increase in the number of laser pulses. The greater the number of
pulses required for a given feature, the greater will be the
uncertainty of feature size and hence the repeatability. Hence it
is advantageous to use higher pulse energy and a lower number of
pulses to ablate a required feature. An optimal pulse energy and
number of pulse should be determined to ablate a feature to a
required specification.
[0136] Quality of the Ablated Feature:
[0137] Due to the change in the topography of the substrate and the
debris deposited in the crater by the initial pulse, the successive
pulse will scatter and hence there is a change in the threshold
fluence of the successive pulse. Higher pulse energy generates
sufficient pressure for ejecting the debris out of the carter and
hence the successive pulse can interact with the fresh substrate.
This results in an improved top surface and an inner wall quality
of the ablated feature.
[0138] Wavelength of the Laser Beam
[0139] In ultrafast laser processing, the wavelength of the laser
beam does not have a major impact on the threshold fluence of the
material as in case of short pulse ablation in micron and
nanosecond pulse width. Due to high peak power of the laser due to
short pulse width, the protons are generated by the laser beam to
start the ablation process rather than being generated from the
substrate. Hence absorption of the material at different
wavelengths does not have a major influence in its threshold
fluence. Hence, a laser beam having the fundamental frequency will
have a wavelength preferably in the range of 700 nm to 1200 nm. It
will have a higher cutting efficiency than the second harmonic
frequency (frequency doubled) of 350 nm-600 nm for a given focused
spot size due to the higher average power from the ultrafast laser
oscillator at the fundamental frequency. The fundamental laser
frequency power will be 50% to 300% higher than the second harmonic
frequency in the range of 233 nm to 400 nm, and hence it will have
50% to 300% higher material removal throughput.
[0140] Similarly, the laser beam having the second harmonic
frequency having the wavelength preferably in the range of 350 nm
to 600 nm, will have a higher cutting efficiency compared to third
harmonic frequency (Frequency tripled) due to the greater average
power from the ultrafast laser oscillator at second harmonic
frequency. The second harmonic laser frequency power will be 50% to
300% higher than the third harmonic frequency in the range of 233
nm to 400 nm, and hence it will have 50% to 300% higher material
removal throughput.
[0141] For example, the average power output at the fundamental
wavelength of 1064 nm is 16 W for a picosecond laser, such as the
Model UPL-20 Lumera laser, and the average power of second harmonic
frequency at 532 nm wavelength is 10 W (FCS-532-Lumeral laser) and
the third harmonic frequency at 355 nm wavelength is 3 W
(FCS-355-Lumera laser). Typical increases in laser power with the
laser wavelength for an ultrafast laser oscillator of a picosecond
pulse width is as shown in FIG. 16.
[0142] The stability of the laser beam will deteriorate with the
reduction in wavelength by frequency doubling and tripling, due to
the increase in the optical components and the sensitivity of the
frequency doubling and tripling crystal to environmental factors
such as temperature. This deterioration in the stability of the
laser beam will lead to relatively poor pulse to pulse energy
stability and beam pointing stability. Hence, repeatability in
feature size and position accuracy may deteriorate compared to the
fundamental frequency from the ultrafast laser oscillator by
frequency doubling and tripling.
[0143] Hence, the fundamental frequency will have better stability
in terms of pulse to pulse energy and pointing stability compared
to second harmonic frequency. Similarly, the second harmonic
frequency will have better stability in terms of pulse to pulse
energy and pointing stability compared to third harmonic frequency.
Also the cost of the system may increase by frequency doubling and
tripling due to the addition of more optical components.
[0144] In spite of the drawbacks of using frequency doubled and
tripled laser pulse, some applications may demand the use of
shorter wavelength to achieve smaller feature size and in sensitive
material processing.
[0145] Selective Material Removal
[0146] The method and apparatus of the present invention is capable
of selective remove material using ultrafast laser pulse from the
oscillator. In ultrafast laser processing the threshold fluence of
the material is clearly defined. Hence by controlling the pulsed
laser fluence, material with lower threshold fluence can be
selectively removed without ablating the underlying material of
higher threshold fluence. A structure, illustrated in FIG. 17,
includes multiple layers of different materials. Hence each layer
has different threshold fluence depending on the composition of the
material. If the layer 1 has the threshold fluence lower than the
underlying layer 2, then layer 1 can be ablated/removed without
ablating/machining the underlying layer 2 by controlling the laser
fluence as shown in FIG. 18. The selectively ablated area can be of
any desired shape depending on the application. The laser fluence
is controlled above the threshold fluence of layer 1 and lower than
the threshold fluence of the underlying layer 2. By this mechanism,
the laser pulse will not have sufficient energy to ablate/machine
the layer 2, but it will have sufficient energy to ablate the layer
1. For example, silicon has a lower threshold fluence than metal
such as copper, gold and aluminum. Hence, a silicon layer can be
removed without damaging the underlying metal layer such as copper,
gold or aluminum. Similarly a few layers of material can be removed
as shown in the FIG. 19 without affecting the underlying layer. The
overlying layer can be removed layer by layer or a few layers
together by controlling the laser fluence. Each layer can vary in
thickness from a few micrometers to a few nanometers. The laser
fluence of the material depends on the material, the number of
pulses at each scan point, scanning speed, focused spot size,
repletion rate of the laser pulse, laser wavelength and the pulse
width. Depending on the required feature shape (such as via, slot
etc) and the size of the feature, the threshold fluence of the
material at different layers should be determined.
[0147] It is not imperative that the entire overlying layer have a
lower threshold fluence then the underlying layer (which should not
be ablated). For precise machining, only the layer immediately
above the underlying layer where the ablation/machining should
stop, need to have the threshold fluence lowers than the underlying
layer. For example, if layer 3 has a higher threshold fluence than
layer 5, but layer 4 has lower threshold fluence than layer 5. By
controlling the laser fluence, layer 3 is first removed before
completely removing the layer 4 in the successive cycle.
[0148] Via-Interconnects
[0149] In addition, the method and apparatus of the present
invention can be utilized for drilling an interconnect via on
multi-layer printed circuit boards or semiconductor wafers and to
ameliorate the aforesaid deficiencies of the prior art by using an
ultrafast pulse generated directly from the laser oscillator.
[0150] The semiconductor wafer where the interconnect via is to be
formed includes a protective layer, an insulating layer and
conductive layers as shown in FIG. 20A. As shown in FIG. 20B blind
via holes are drilled through the protective or insulator layer 1
and through the conductive plate/layer 1 and insulating layer 2,
causing minimal or no damage to the underlying conductive layer 2.
The insulating layer can be dielectric, glass or any other
insulating material. The protective/insulating layer at the top
surface of the wafer may or may not be present depending on the
application. A via has a smaller diameter at the lower portion of
the via compared to the upper portion as shown in the FIG. 20B. A
via sidewall angle may range from 89 degrees to 1 degree depending
on the depth and diameter of via (top and bottom via diameter
required). Also via holes can be formed between more than 2
conductive layers as shown in FIG. 20C. Here via holes are drilled
between conductive layer 1 and conductive layer 3. Also different
layers can be connected on the same multi layer printed circuit
board or semiconductor wafer by drilling via holes as shown in FIG.
20D. The number of layers though which a via hole is drilled can
vary depending on the application. Via interconnects are then
formed by filling via holes formed between conductive layers/plane
with conductive material as shown in FIGS. 20E and 20F. This
process is also called metallization. Via holes are filled along
the sidewalls (type1) or completely filled (type-2) as shown in
FIGS. 20E and 20F. Sidewall angle is critical for filling via holes
with conductive material without voids. The slope enables smooth
flow of conductive material in via holes. In laser processing as
the depth of the via channel increases, barrel shape channels are
formed. These barrel shape holes result in voids when filling the
holes with conductive material, and it is not acceptable for the
formation of an interconnect. This barrel formation can be avoided
by the method and apparatus disclosed in the present application.
The method and apparatus disclosed in this application can be used
to produce both round and slotted blind vias of single and multiple
depths. Following are some of the advantages of forming
interconnect via holes using the method and apparatus disclosed in
this invention. Micro cracks are minimized or eliminated. A recast
layer along the via sidewalls is minimized or eliminated to avoid
formation of voids during metallization of the via holes. It is
possible to selectively drill a via hole through multiple layers
without damaging the underlying layer by controlling the threshold
fluence of the laser beam. FIG. 21 shows the difficulty in stopping
the ablation precisely at selected layer using nanosecond and
amplified ultrafast laser compared to the ultrafast laser system
disclosed in the present application. The present invention makes
it easy to remove surface debris by minimal post process cleaning,
since the debris does not adhere to the surface strongly. The
present invention can generate via holes in micron and nano scale,
which is demanded by current and future integrated circuits.
Minimal or no damage is caused to adjacent structure due to heat
dissipation. Via depth can be controlled very precisely. A very
high repeatability of via holes in terms of diameter and depth is
possible. FIG. 22 shows the poor repeatability of via holes formed
by nanosecond laser and amplified ultrafast laser compared to the
high repeatability of via holes formed by the ultrafast laser
system disclosed in this application. Moreover, barrel shape via
can be eliminated.
[0151] The invention has been described with reference to exemplary
embodiments. However, it will be readily apparent to those skilled
in the art that it is possible to embody the invention in specific
forms other than those of the embodiments described above. This may
be done without departing from the spirit of the invention. The
exemplary embodiments are merely illustrative and should not be
considered restrictive in any way. The scope of the invention is
given by the appended claims, rather than the preceding
description, and all variations and equivalents which fall within
the range of the claims are intended to be embraced therein.
* * * * *