U.S. patent application number 13/735394 was filed with the patent office on 2013-08-15 for apparatus for minimizing sample damage during the ablation of biological tissue using a focused ultrashort pulsed laser beam wherein the slope of fluence breakdown is a function of the pulse width.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Richard Alan Haight, Peter P. Longo, Daniel P. Morris, Alfred Wagner.
Application Number | 20130211388 13/735394 |
Document ID | / |
Family ID | 46278033 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130211388 |
Kind Code |
A1 |
Haight; Richard Alan ; et
al. |
August 15, 2013 |
Apparatus for Minimizing Sample Damage During the Ablation of
Biological Tissue Using a Focused Ultrashort Pulsed Laser Beam
Wherein the Slope of Fluence Breakdown is a Function of the Pulse
Width
Abstract
An apparatus for laser induced breakdown of a tissue. The
apparatus has a source of a beam of one or more laser pulses in
which each pulse has a pulse width equal to or less than a pulse
width value corresponding to a change in slope of a curve of
fluence breakdown threshold (Fth) as a function of laser pulse
width (T), the change occurring at a point between first and second
portions of the curve, the first portion spanning a range of
relatively long pulse width where Fth varies with the square root
of pulse width (T1/2) and the second portion spanning a range of
short pulse width relative to the first portion with Fth versus T
slope which differs from that of the first portion; and an optical
system focusing the one or more pulses of the beam to a point above
the surface of the tissue to cause laser induced break down of the
tissue.
Inventors: |
Haight; Richard Alan;
(Mahopac, NY) ; Longo; Peter P.; (Hopewell
Junction, NY) ; Morris; Daniel P.; (Purchase, NY)
; Wagner; Alfred; (Brewster, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation; |
|
|
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
46278033 |
Appl. No.: |
13/735394 |
Filed: |
January 7, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12545216 |
Aug 21, 2009 |
8389890 |
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13735394 |
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09933461 |
Aug 20, 2001 |
7649153 |
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12545216 |
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09210226 |
Dec 11, 1998 |
6333485 |
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09933461 |
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Current U.S.
Class: |
606/4 ;
606/13 |
Current CPC
Class: |
B23K 26/066 20151001;
B23K 26/402 20130101; B23K 2103/30 20180801; B23K 2103/50 20180801;
A61B 18/26 20130101; A61B 18/20 20130101; A61B 2018/00577 20130101;
A61F 2009/00863 20130101; G03F 1/72 20130101; B23K 26/0624
20151001; A61B 2018/00625 20130101; A61F 9/0084 20130101; B23K
26/362 20130101; B23K 26/361 20151001; B23K 26/40 20130101; A61F
9/00814 20130101; A61B 18/203 20130101 |
Class at
Publication: |
606/4 ;
606/13 |
International
Class: |
A61B 18/20 20060101
A61B018/20; A61F 9/008 20060101 A61F009/008 |
Claims
1. A method of removal of a first biological tissue from the
surface of a second biological tissue using laser induced breakdown
of the first biological tissue which comprises: a. generating a
beam of one or more laser pulses in which each pulse has a pulse
width equal to or less than a pulse width value corresponding to a
change in slope of, a curve of fluence breakdown threshold (Fth) as
a function of laser pulse width (T), said change occurring at a
point between first and second portions of said curve, said first
portion spanning a range of relatively long pulse width where Fth
varies with the square root of pulse width (T1/2) and said second
portion spanning a range of short pulse width relative to said
first portion with a Fth versus T slope which differs from that of
said first portion; and b. focusing said one or more pulses of said
beam to a point above the surface of the first biological
tissue.
2. The method according to claim 1 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point above the surface of the tissue; and c.
scanning said beam along a predetermined path in a transverse
direction.
3. The method according to claim 1 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point above the surface of the tissue; and c.
scanning said beam along a predetermined path in a longitudinal
direction in the tissue to a depth smaller than the Rayleigh
range.
4. The method according to claim 1 wherein the breakdown includes
changes caused by one or more of ionization, free electron
multiplication dielectric breakdown, plasma formation and
vaporization.
5. The method according to claim 1 wherein the breakdown includes
plasma formation.
6. The method acct ding to claim 1 wherein the breakdown includes
disintegration.
7. The method according to claim 1 wherein the breakdown includes
ablation.
8. The method according to claim 1 wherein the breakdown includes
vaporization.
9. The method according to claim 1 wherein said beam is obtained by
chirped-pulse amplification (CPA) means comprising means for
generating a short optical pulse having a predetermined duration;
means for stretching such optical pulse in time; means for
amplifying such time-stretched optical pulse including solid state
amplifying media; and means for recompressing such amplified pulse
to its original duration.
10. A method of removal of a first biological tissue from the
surface of a second biological tissue using laser induced breakdown
(LIB) of a tissue with a pulsed laser beam, the tissue being
characterized by a relationship of fluence breakdown threshold
versus laser pulse width that exhibits a rapid and distinct change
in slope at a predetermined laser pulse width where the onset of
plasma induced breakdown occurs, said method comprising the steps
of: a. generating a beam of one or more laser pulses in which each
pulse has a pulse width equal to or less than said predetermined
laser pulse width; and b. focusing said beam to a point above the
surface of the first biological tissue material so that the laser
beam defines a spot and has a lateral gaussian profile
characterized in that fluence at or near the center of the beam
spot is greater than the threshold fluence whereby the laser
induced breakdown is ablation of an area within the spot.
11. The method according to claim 10 wherein the spot size is a
diffraction limited spot size providing an ablation cavity having a
diameter less than the fundamental wavelength size.
12. A method for laser induced breakdown (LIB) of a tissue with a
pulsed laser beam, the tissue being characterized by a relationship
of fluence breakdown threshold versus laser pulse width that
exhibits a rapid and distinct change in slope at a predetermined
laser pulse width where the onset of plasma induced breakdown
occurs, said method comprising the steps of: a. generating a beam
of one or more laser pulses in which each pulse has a pulse width
equal to or less than said predetermined laser pulse width; and b.
focusing said beam to a point above the surface of the tissue which
is biological tissue, the pulse width is 10 to 10,000 femtoseconds
and the beam has an energy of 10 nanojoules to 1 millijoule.
13. A method according to claim 12 wherein said point is moved as
said tissue is removed from said surface so that said point is
substantially maintained at a fixed distance above said ablated
surface.
14. A method for laser induced breakdown of a first biological
tissue on a second biological tissue which comprises: a.
determining, for a selected tissue, a characteristic curve of
fluence breakdown threshold (Fth) as a function of laser pulse
width; b. identifying a pulse width value on said curve
corresponding to a rapid and distinct change in slope of said Fth
versus pulse width curve characteristic of said tissue; c.
generating a beam of one or more laser pulses, said having a pulse
width at or below said pulse width value corresponding to said
distinct change in slope; and d. focusing said one or more pulses
of said beam to a point above the surface of the first biological
tissue.
15. The method according to claim 14 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point at or beneath the surface of the first
biological tissue; and c. mining said beam along a predetermined
path in a transverse direction.
16. The method according to claim 14 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point above the surface of the tissue; and c.
scanning said beam along a predetermined path in a longitudinal
direction in the tissue to a depth smaller than the Rayleigh
range.
17. The method according to claim 14 wherein the breakdown includes
changes caused by one or more of ionization, free electron
multiplication, dielectric breakdown, plasma formation, and
vaporization.
18. The method according to claim 14 wherein the breakdown includes
plasma formation.
19. The method according to claim 14 wherein the breakdown includes
disintegration.
20. The method according to claim 14 wherein the breakdown include
ablation.
21. The method according to claim 14 wherein breakdown includes
vaporization.
22. The method according to any one of claim 12, or 14 wherein said
beam is obtained by chirped-pulse amplification (CPA) means
comprising means for generating a short optical pulse having a
predetermined duration; means for stretching such optical pulse in
time; means for amplifying such time-stretched optical pulse
including solid state amplifying media; and means for recompressing
such amplified pulse to its original duration.
23. A method according to claim 10, 12, 14 or 15 wherein said
laser-induced breakdown removes said first biological tissue from
said surface resulting in an ablated surface.
24. A method far laser induced breakdown (LIB) of a first
biological tissue on a second biological tissue with a pulsed laser
beam, the tissue being characterized by a relationship of fluence
breakdown threshold versus laser pulse width that exhibits a rapid
and distinct change in slope at a characteristic laser pulse width,
said method comprising the steps of: a. generating a beam of one or
more laser pulses in which each pulse has a pulse width equal to or
less than said characteristic laser pulse width; and b. focusing
said beam to a point above the surface of the first biological
tissue.
25. The method according to claim 24 wherein the pulse width is 10
to 10,000 femtoseconds and the beam has an energy of 1 nanojoule to
1 microjoule.
26. The method according to claim 24 wherein the spot size is
varied within a range of 1 to 100 microns by changing the f number
of the laser beam.
27. The method according to claim 24 wherein the spot size is
varied within a range of 1 to 100 microns by varying the target
position.
28. The method according to claim 24 wherein the tissue is
transparent to radiation emitted by the laser and the pulse width
is 10 to 10,000 femtoseconds, the beam has an energy of 10
nanojoules to 1 millijoule.
29. The method according to claim 24 wherein the tissue is
biological tissue, the pulse width is 10 to 10,000 femtoseconds and
the beam has an energy of 10 nanojoules to 1 millijoule.
30. The method according to claim 24 wherein the laser beam has an
energy in a range of 10 nanojoules to 1 millijoule.
31. The method according to claim 24 wherein the laser beam has a
fluence in a range of 100 millijoules per square centimeter to 100
joules per square centimeter.
32. The method according to claim 24 wherein the laser beam defines
a spot in or on the tissue and the LIB causes ablation of an area
having a size smaller than the area of the spot.
33. The method according to claim 24 wherein the laser beam has a
wavelength in a range of 200 nanometers to 2 microns.
34. The method according to claim 24 wherein the pulse width is in
a range of a few picoseconds to femtoseconds.
35. The method according to claim 24 wherein the breakdown includes
changes caused by one or more of ionization, free electron
multiplication, dielectric breakdown, plasma formation, and
vaporization.
36. The method according to claim 24 wherein the breakdown includes
plasma formation.
37. The method according to claim 24 wherein the breakdown includes
disintegration.
38. The method according to claim 24 wherein the breakdown includes
ablation.
39. The method according to claim 24 wherein the breakdown includes
vaporization.
40. The method according to claim 24 wherein the spot size is
varied by flexible diaphragm to a range of 1 to 100 microns.
41. The method according to claim 24 wherein a mask is placed in
the path of the beam to block a portion of the beam to cause the
beam to assume a desired geometric configuration.
42. The method according to claim 24 wherein the laser operating
mode is non-TEMoo.
43. The method according to claim 24 wherein the laser beam defines
a spot and has a lateral gaussian profile characterized in that
fluence at or near the center of the beam spot it greater than the
threshold fluence whereby the laser induced breakdown is ablation
of an area within the spot.
44. The method according to any one of claim 24, 25, 28 or 38
wherein said beam is obtained by chirped-pulse amplification (CPA)
means comprising means for generating a short optical pulse having
a predetermined duration; means for stretching such optical pulse
in time, means for amplifying such time-stretched optical pulse
including solid state amplifying media; and means for recompressing
such amplified pulse to its original duration.
45. A method for laser induced breakdown of a first biological
tissue on a second biological tissue which comprises: a. generating
a beam of one or more laser pulses in which each pulse has a pulse
width equal to or less than a pulse width value corresponding to a
change in slope of a curve of fluence breakdown threshold (Fth) as
a function of laser pulse width (T), said change occurring at a
point between first and second portions of said curve, said first
portion spanning a range of relatively long pulse width where Fth
varies with the square root of pulse width (T1/2) and said second
portion spanning a range of short pulse width relative to said
first portion with a Fth versus T slope which differs from that of
said first portion; and b. focusing said one or more pulses of said
beam to a region of least confusion above the surface of the first
biological tissue.
46. The method according to claim 45 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point at or beneath the surface of the first
biological tissue; and c. scanning said beam along a predetermined
path in a transverse direction.
47. The method according to claim 45 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial tar point above the surface of the first biological tissue;
and c. scanning said beam along a predetermined path in a
longitudinal direction in the tissue to a depth smaller than the
Rayleigh ange.
48. The method according to claim 45 wherein the breakdown includes
changes caused by one or more of ionization, free electron
multiplication, dielectric breakdown, plasma formation, and
vaporization.
49. The method according to claim 45 wherein the breakdown includes
plasma formation.
50. The method according to claim 45 wherein the breakdown includes
disintegration.
51. The method according to claim 45 wherein the breakdown includes
ablation.
52. The method according to claim 45 wherein the breakdown includes
vaporization.
53. A method for laser induced breakdown (LIB) of a first
biological tissue on a second biological tissue with a pulsed laser
bears the tissue being characterized by a relationship of fluence
breakdown threshold versus laser pulse width that exhibits a rapid
and distinct change in slope at a predetermined laser pulse width
where the onset of plasma induced breakdown occurs, said method
comprising the steps of: a. generating a beam of one or more laser
pulses in which each pulse has a pulse width equal to or less than
said predetermined laser pulse width; and b. focusing said beam to,
a region of least confusion above the surface of the first
biological tissue so that the laser beam defines a spot and has a
lateral gaussian profile characterized in that fluence at or near
the center of the beam spot is greater than the threshold fluence
whereby the laser induced breakdown is ablation of an area within
the spot.
54. The method according to claim 53 wherein the spot size is a
diffraction limited spot size providing an ablation cavity having a
diameter less than the fundamental wavelength size.
55. A method for laser induced breakdown (LIB) of a first
biological tissue with a pulsed laser beam, the first biological
tissue being characterized by a relationship of fluence breakdown
threshold versus laser pulse width that exhibits a rapid and
distinct change in slope at a predetermined laser pulse width where
the onset of plasma induced breakdown occurs, said method
comprising the steps of: a. generating a beam of one or more laser
pulses in which each pulse has a pulse width equal to or less than
said predetermined laser pulse width; and b. focusing said beam to
a region of east confusion above the surface of the first
biological tissue which is biological tissue, the pulse width is 10
to 10,000 femtoseconds and the beam has an energy of 10 nanojoules
to 1 millijoule.
56. A method for laser induced breakdown of a first biological
tissue on a second biological tissue which comprises: a.
determining, for a selected first biological tissue, a
characteristic curve of fluence breakdown threshold (Fth) as a
function of laser pulse width; b. identifying a pulse width value
on said curve corresponding to a rapid and distinct change in slope
of said Fth versus pulse width curve characteristic of said first
biological tissue; c. generating a beam of one or more laser
pulses, said having a pulse width at or below said pulse width
value corresponding to said distinct change in slope; and d.
focusing said one or more pulses of said beam to a region of least
confusion above the surface of the first biological tissue.
57. The method according to claim 56 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start point at or beneath the surface of the first
biological tissue, and c. scanning said beam along a predetermined
path in a transverse direction.
58. The method according to claim 56 and further including: a.
identifying a pulse width start point; b. focusing the laser beam
initial start region of least confusion above the surface of the
tissue; and c. scanning said beam along a predetermined path in a
longitudinal direction in the tissue to a depth smaller than the
Rayleigh range.
59. The method according to claim 56 wherein the breakdown includes
changes caused by one or more of ionization, free electron
multiplication, dielectric breakdown, plasma formation, and
vaporization.
60. The method according to claim 56 wherein the breakdown includes
plasma formation.
61. The method according to claim 56 wherein the breakdown includes
disintegration.
62. The method according to claim 56 wherein the breakdown include
ablation.
63. The method according to claim 56 wherein breakdown includes
vaporization.
64. The method according to any one of claim 55 or 56 wherein said
beam is obtained by chirped-pulse amplification (CPA) means
comprising means for generating a short optical pulse having a
predetermined duration; means for stretching such optical pulse in
time; means for amplifying such time-stretched optical pulse
including solid state amplifying media; and means for recompressing
such amplified pulse to its original duration.
65. A method for laser induced breakdown (LIB) of a first
biological tissue on a second biological tissue by plasma formation
with a pulsed laser beam, the first biological tissue being
characterized by a relationship of fluence breakdown threshold
versus laser pulse width that exhibits a distinct change in slope
at a characteristic laser pulse width, said method comprising the
steps of: a. generating a beam of one or more laser pulses in which
each pulse has a pulse width equal to or less than said
characteristic laser pulse width, said characteristic pulse width
being defined by the ablation (LIB) threshold of the tissue as a
function of pulse width where the ablation (LIB) threshold function
is no longer proportional to the square root of pulse width; and b.
focusing said beam to a region of least contusion above the surface
of the first biological tissue and inducing breakdown by plasma
formation in the first biological tissue.
66. A method according to claim 1, 10, 12, 14, 15, 24, 45, 53, 55,
65, 56 or 57 wherein said first biological tissue is disposed on
the second biological tissue.
67. A method according to claim 66 wherein said second biological
tissue is the same as said first biological tissue.
68. A method according to claim 66 wherein said second biological
tissue is different than said second biological tissue.
69. A method according to claim 66 wherein said second biological
tissue is a first tissue and said tissue is a second biological
tissue.
70. A method according to claim 69 wherein said first tissue is a
tumor and wherein said second tissue is normal tissue.
71. A method according to claim 70 wherein said second tissue is a
retina of an eye and said tumor is disposed on said retina.
72. A method according to claim 24, 45, 53, 55, 65, 56 or 57
wherein said laser-induced breakdown removes said first biological
tissue from said surface resulting in an ablated surface.
73. A method according to claim 72 wherein said region of least,
confusion is moved as said first biological tissue is ablated so
that said region of least confusion is substantially maintained at
a fixed distance above said ablated surface.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to methods utilizing lasers
for modifying internal and external surfaces of material such as by
ablation or changing properties in structure of materials. This
invention may be used for a variety of materials.
BACKGROUND OF THE INVENTION
[0002] Laser induced breakdown of a material causes chemical and
physical changes, chemical and physical breakdown, disintegration,
ablation, and vaporization. Lasers provide good control for
procedures which require precision such as inscribing a micro
pattern. Pulsed rather than continuous beams are more effective for
many procedures, including medical procedures. A pulsed laser beam
comprises bursts or pulses of light which are of very short
duration, for example, on the order of 10 nanoseconds in duration
or less. Typically, these pulses are separated by periods of
quiescence. The peak power of each pulse is relatively high often
on the order of gigawatts and capable of intensity on the order of
1013 w/cm2. Although the laser beam is focused onto an area having
a selected diameter, the effect of the beam extends beyond the
focused area or spot to adversely affect peripheral areas adjacent
to the spot. Sometimes the peripheral area affected is several
times greater than the spot itself. This presents a problem,
particularly where tissue is affected in a medical procedure. In
the field of laser machining, current lasers using nanosecond
pulses cannot produce features with a high degree of precision and
control, particularly when nonabsorptive wavelengths are used.
[0003] It is a general object to provide a method to localize laser
induced breakdown. Another object is to provide a method to induce
breakdown in a preselected pattern in a material or on a
material.
[0004] U.S. Pat. No. 5,656,186 to Mourou et al. is directed to a
method for laser-induced breakdown. The teaching of Mourou et al.
requires that the laser beam be focused to a point at or beneath
the surface from which material is to be removed. Applicants have
discovered that it is undesirable to focus at or beneath the
surface since this results in undesired damage beneath the surface,
in particular to the substrate (or underlayer) on which the
material that is to be removed is disposed. This is particularly a
problem where the underlayer is very sensitive to the laser light
and/or can be easily damaged by the laser light. Applicants have
unexpectedly discovered that the light source should be focused
above the surface to be removed toward this undesired change.
SUMMARY OF THE INVENTION
[0005] In one aspect the invention provides a method for laser
induced breakdown of a material with a pulsed laser beam where the
material is characterized by a relationship of fluence breakdown
threshold (Fth) versus laser beam pulse width (T) that exhibits an
abrupt, rapid, and distinct change or at least a clearly detectable
and distinct change in slope at a predetermined laser pulse width
value. The method generating a beam of laser pulses in which each
pulse has a pulse width equal to or less than the predetermined
laser pulse width value. The beam is focused to a point above the
surface of a material where laser induced breakdown is desired. The
beam is focused to have the region of least confusion above the
surface of a material where laser-induced breakdown is desired.
[0006] In one aspect, the invention may be understood by further
defining the predetermined laser pulse width as follows: the
relationship between fluence breakdown threshold and laser pulse
defines a curve having a first portion spanning a range of
relatively long (high) pulse width where fluence breakdown
threshold (Fth) varies with the square root of pulse width (T1/2).
The curve has a second portion spanning a range of short (low)
pulse width relative to the first portion. The proportionality
between fluence breakdown threshold and pulse width differ in the
first and second portions of the curve and the predetermined pulse
width is that point along the curve between its first and second
portions. In other words, the predetermined pulse width is the
point where the Fth versus .tau.p relationship no longer applies,
and, of course, it does not apply for pulse widths shorter than the
predetermined pulse width.
[0007] The scaling of fluence breakdown threshold (Fth) as a
function of pulse width (T) is expressed as Fth proportional to the
square root of T1/2) is demonstrated in the pulse width regime to
the nanosecond range. The invention provides methods for operating
in pulse widths to the picosecond and femtosecond regime where we
have found that the breakdown threshold (Fth) does not vary with
the square root of pulse width (T1/2).
[0008] Pulse width duration from nanosecond down to the femtosecond
range is accomplished by generating a short optical pulse having a
predetermined duration from an optical oscillator. Next the short
optical pulse is stretched in time by a factor of between about 500
and 10,000 to produce a timed stretched optical pulse to be
amplified. Then, the time stretched optical pulse is amplified in a
solid state amplifying media. This includes combining the time
stretched optical pulse with an optical pulse generated by a second
laser used to pump the solid state amplifying media. The amplified
pulse is then recompressed back to its original pulse duration.
[0009] In one embodiment, a laser oscillator generates a very short
pulse on the order of 10 to 100 femtoseconds at a relatively low
energy, on the order of 0.001 to 10 nanojoules. Then, it is
stretched to approximately 100 picoseconds to 1 nanosecond and
0.001 to 10 nanojoules. Then, it is amplified to typically on the
order of 0.001 to 1,000 millijoules and 100 picoseconds to 1
nanosecond and then recompressed. In its final state it is 10 to
200 femtoseconds and 0.001 to 1,000 millijoules. Although the
system for generating the pulse may vary, it is preferred that the
laser medium be sapphire which includes a titanium impurity
responsible for the lasing action.
[0010] In one aspect, the method of the invention provides a laser
beam which defines a spot that has a lateral gaussian profile
characterized in that fluence at or near the center of the beam
spot is greater than the threshold fluence whereby the laser
induced breakdown is ablation of an area within the spot. The
maximum intensity is at the very center of the beam waist. The beam
waist is the point in the beam where wave-front becomes a perfect
plane; that is, its radius of curvature is infinite. This center is
at radius R=0 in the x-y axis and along the Z axis, Z=0. This makes
it possible to damage material in a very small volume Z=0, R=0.
Thus it is possible to make features smaller than spot size in the
x-y focal plane and smaller than the Rayleigh range (depth of
focus) in the Z axis. It is preferred that the pulse width duration
be in the femtosecond range although pulse duration of higher value
may be used so long as the value is less than the pulse width
defined by an abrupt or discernable change in slope of fluence
breakdown threshold versus laser beam pulse width.
[0011] In another aspect, a diaphragm, disk, or mask is placed in
the path of the beam to block at least a portion of the beam to
cause the beam to assume a desired geometric configuration. In
still further aspects, desired beam configurations are achieved by
varying beam spot size or through Fourier Transform (FT) pulse
shaping to cause a special frequency distribution to provide a
geometric shape.
[0012] It is preferred that the beam have an energy in the range of
10 nJ (nanojoules) to 1 millijoule and that the beam have a fluence
in the range of 0.1 J/cm2 to 100 J/cm2 (joules per centimeter
square). It is preferred that the wavelength be in a range of 200
nm (nanometers) to 1 urn (micron).
[0013] Advantageously, the invention provides a new method for
determining the optimum pulse width duration regime for a specific
material and a procedure for using such regime to produce a
precisely configured cut or void in or on a material. For a given
material the regime is reproducible by the method of the invention.
Advantageously, very high intensity results from the method with a
modest amount of energy and the spot size can be very small. Damage
to adjoining area is minimized which is particularly important to
human and animal tissue.
[0014] These and other object features and advantages of the
invention will be become apparent from the following description of
the preferred embodiments, claims, and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic representation of a laser induced
breakdown experimental system which includes a chirped pulse
amplification laser system and means for detecting scattered and
transmitted energy. If the sample is transparent, then transmitted
energy can also be measured.
[0016] FIG. 2 is a plot of scattered energy versus incident fluence
obtained for an opaque (gold) sample using the system in FIG. 1
operated at 150 femtoseconds (fs) pulse duration.
[0017] FIG. 3 is a plot of calculated and experimental values of
threshold fluence versus pulse width for gold, with experimental
values obtained for the gold sample using the system of FIG. 1
operated at 800 nm wavelength. The arrow shows the point on the
plot where the Fth proportional to T1/2 no longer applies, as this
relationship only holds for pulse widths down to a certain level as
shown by the solid line.
[0018] FIG. 4 is a graphical representation of sub-spot size
ablation/machining in gold based on arbitrary units and showing Fth
the threshold fluence needed to initiate material removal; Rs the
spot size of the incident beam and Ra the radius of the ablated
hole in the x-y plane.
[0019] FIG. 5 is a schematic illustration of a beam intensity
profile showing that for laser micro-machining with ultrafast pulse
according to the invention, only the peak of the beam intensity
profile exceeds the threshold intensity for ablation/machining
[0020] FIGS. 6A and B are schematic illustrations of a beam showing
the placement of a disk-shaped mask in the beam path.
[0021] FIG. 7 is a plot of scattered plasma emission and
transmitted laser pulse as a function of incident laser pulse
energy for a transparent glass sample, SiO2.
[0022] FIG. 8 is a plot of fluence threshold (Fth) versus pulse
width (T) for the transparent glass sample of FIG. 7 showing that
Fth varying with T1/2 only holds for pulse widths down to a certain
level as shown by the solid line. Previous work of others is shown
in the long pulse width regime (Squares, Smith Optical Eng 17, 1978
and Triangles, Stokowski, N B S Spec Bui 541, 1978).
[0023] FIG. 9 is a plot of fluence threshold versus pulse width for
corneal tissue, again showing that the proportionality between Fth
and pulse width follows the T1/2 relationship only for pulse widths
which are relatively long.
[0024] FIGS. 10 and 11 are plots of plasma emission versus laser
fluence showing that at 170 (FIG. 10) pulse width the Fth is very
clearly defined compared to 7 nm (FIG. 11) pulse width where it is
very unclear.
[0025] FIG. 12 is a plot of impact ionization rate per unit
distance determined by experiment and theoretical calculation.
[0026] FIGS. 13A and B are schematic illustrations of beam profile
along the longitudinal Z axis and sharing precise control of
damage-dimension along the Z axis.
[0027] FIG. 14a shows light source focused above the surface to be
ablated.
[0028] FIG. 14b shows a light source focused beneath the surface of
the layer to be ablated.
[0029] FIG. 15 is an optical system for repairing a photomask. The
laser beam illuminates an aperture which is then imaged below the
surface of the mask.
[0030] FIG. 16 is a general optical system. A slightly converging
laser beam is focused to a gaussian spot slightly above the plane
of the mask.
DETAILED DESCRIPTION
[0031] Pulses of light emitted by a laser are capable of removing
material from a sample. Typically, the light is focused onto the
surface of the sample to both 1) increase the intensity of the
light and 2) localize the region of material removal. We have found
that it is particularly advantageous to focus the laser beam above
the surface of the material to be ablated, rather than focusing at
or below the surface. In any situation where material must be
removed from the surface without risking damage or ablation of the
underlying substrate, our focusing technique is crucial. By
focusing above the surface, the maximum intensity of laser light
occurs away from the sample. The light intensity then decreases
monotonically as the laser beam moves toward and into the sample
(see FIG. 14a). This procedure ensures that the laser intensity
inside the sample is always less than the intensity at the surface
of the material to be ablated. Since it is often necessary to
confine ablation and potential damage to the surface of the sample,
focusing above the plane of the sample and then adjusting the
intensity of the light to the minimum necessary for ablation
ensures that no region beneath the surface of the sample will be
ablated or damaged.
[0032] Our focusing technique is particularly critical when
ultrashort (<10 psec) pulsed lasers are utilized for ablation.
Since ultrashort pulsed laser ablation is non-thermal, the ablated
region is limited to the spatial extent of the focused laser light.
Therefore, to eliminate ablation and damage to the underlying
substrate, it is essential to ensure that the peak light intensity
occurs outside the sample. Our method ensures this. Our technique
contrasts significantly with laser ablation in which the objective
is to remove large quantities of material without regard to
inflicting damage to the underlying substrate. In this case, by
focusing the laser beam at or below the surface the laser intensity
peaks inside the material (see FIG. 14b). This results in the
maximum ablation efficiency, a desirable characteristic when
drilling a simple hole without regard to substrate damage.
[0033] The following are examples which illustrate the benefit of
focusing the laser beam above the surface of the sample to avoid
sample damage. In the course of developing a tool to ablate
chromium defects on a quartz photomask, we focused a femtosecond
pulsed laser beam onto the surface of the chromium. We found that
it was difficult to avoid damaging the underlying quartz substrate
using this approach, since the normal variation in focus which
typically occurred would often cause the laser beam to be focused
inside the quartz. When the beam focus occurred in the quartz, the
laser intensity was sufficient to damage the quartz rendering the
photomask unusable. However, by focusing the laser beam above the
chromium surface, we could adjust the laser intensity so that the
Cr was ablated while the quartz was unaffected. Similarly, in
removing unwanted biological tissue such as a tumor from an organ,
focusing the laser beam above the surface of the tumor results in
its removal without damaging the underlying tissue. Consider a
tumor attached to the retina of the eye. Focusing a pulsed laser at
or below the surface of the tumor can result in a maximum laser
intensity in the retina rather than the tumor. As the tumor is
slowly ablated by the laser beam, the laser intensity at or below
the retina will be greater than the laser intensity at the tumor if
the laser is focused at or below the surface of the tumor. Focusing
above the surface of the tumor ensures that the laser intensity is
as small as possible at the retina and decreases with increasing
depth into the retina. This minimizes inadvertent damage to the
retina.
[0034] There are numerous methods of controlling the focal position
of the laser beam which will ensure that the maximum intensity
occurs above the surface of the material to be ablated. FIG. 2
shows a schematic diagram of the optical system for repairing
chromium defects on a photomask. An objective lens forms an image
of the mask on a video camera such as a CCD array. By adjusting the
distance (F) between the mask and the objective, the image can be
brought into a sharp focus. The optimal distance provides a high
quality image of the mask with excellent spatial resolution. This
optimal distance can be determined by visually inspecting the
sharpness of the image on the CCD array as the distance is varied,
using either a computer to perform an analysis of the mask image or
manually determining the best focused image. Alternatively, a
height sensor can be used to maintain the optimum distance between
the mask and the objective lens. The laser beam shown in FIG. 15
illuminates an aperture, which in turn is imaged onto the mask
using the relay lens and the mirror. We intentionally adjust the
position of the relay lens to ensure that the image of the aperture
is focused somewhat above the plane of the mask when the objective
lens is adjusted to the optimal imaging distance. Since the
objective lens has a large numerical aperture, small variations in
the distance between the mask and objective can result in large
changes in the effective optical intensity at or below the mask
surface. Typically, the uncertainty in the distance F shown in FIG.
15 is comparable to the depth of focus of the optical system. The
depth of focus (D) is related to the wavelength of the light (W)
and the numerical aperture of the objective lens (NA), and is given
approximately by: D.about.W/(2*(NA).sup.2)
[0035] We adjust the position of relay lens to form an image of the
aperture a distance slightly greater than D above the plane of the
mask, with the mask image at best focus. For the mask repair tool
we constructed, the numerical aperture is approximately 0.95 and
the wavelength of light is 400 nm. Using the method described
above, we would typically form an image of the aperture
approximately 300 nm above the plane of the mask. This ensures that
the peak optical intensity from the laser beam occurs above the
mask rather than at or below the surface of the mask, even in cases
where the mask image is slightly out of focus (due to the typical
uncertainty in determining the optimum focus). The slight defocus
in the aperture image at the mask plane has a negligible effect on
the spatial resolution of the ablated region.
[0036] A more general optical approach is shown in FIG. 16. As in
the case described above, the distance (F) between the objective
lens and the sample is adjusted to bring the image of the sample to
the best focus (highest spatial resolution image). However, in this
case the laser beam does not illuminate an aperture. Rather, the
entire beam enters the objective lens and is focused to a gaussian
spot. By adjusting the convergence angle of the laser beam, either
through internal adjustments in the laser or through the use of a
weak external lens system, the laser beam is brought to a focus
slightly above the surface of the sample. The laser intensity is
adjusted independently (using filters or a combination of a
waveplate and frequency doubling crystal) to a value such that
ablation just occurs at the sample surface. Since the peak laser
intensity occurs above the surface of the sample rather than inside
the sample, the possibility of ablation or damage to the underlying
material in the substrate is minimized.
[0037] In both cases described above, the focus of the laser beam
above the surface of the sample can be maintained by 1)
establishing a fixed offset in the relative focus of the sample
image and the laser beam, and 2) maintaining the sample at the
optimal distance from the objective by monitoring the sample image
and/or a height sensor. If a significant depth of material must be
removed (e.g. greater than the depth of focus of the optical
system), then the distance between the sample and the objective
lens can be continuously varied by monitoring the depth of the
ablated material and moving either the objective or the sample in
the Z direction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] FIG. 14a shows light source focused above the surface of the
layer to be ablated.
[0039] FIG. 14b shows a light source focused beneath the surface of
the layer to be ablated.
[0040] FIG. 15 is an optical system for repairing a photomask. The
laser beam illuminates an aperture which is then imaged above the
surface of the mask.
[0041] FIG. 16 is a general optical system. A slightly converging
laser beam is focused to a gaussian spot slightly above the plane
of the mask.
[0042] Referring to FIG. 1 there is shown an apparatus for
performing tests to determine the laser induced breakdown threshold
as a function of laser pulse width in the nanosecond to femtosecond
range using a chirped-pulse amplification (CP A) laser system. The
basic configuration of such a CPA system is described in U.S. Pat.
No. 5,235,606 which is assigned to the assignee of the present
invention and which has inventors in common with this present
application. U.S. Pat. No. 5,235,606 is incorporated herein by
reference in its entirety.
[0043] Chirped-pulse amplification systems have been described by
Jeffrey Squier and Gerard Mourou, two of the joint inventors in the
present application, in a publication entitled Laser Focus World
published by Pennwell in June of 1992. It is described that CPA
systems can be roughly divided into four categories. The first
includes the high energy low repetition systems such as ND glass
lasers with outputs of several joules but they may fire less than 1
shot per minute. A second category are lasers that have an output
of approximately 1 joule and repetition rates from 1 to 20 hertz.
The third group consists of millijoule level lasers that operate at
rates ranging from 1 to 10 kilohertz. A fourth group of lasers
operates at 250 to 350 kilohertz and produces a 1 to 2 microjoules
per pulse. In U.S. Pat. No. 5,235,606 several solid state
amplifying materials are identified and the invention of U.S. Pat.
No. 5,235,606 is illustrated using the Alexandrite. The examples
below use Ti:Sapphire and generally follow the basic process of
U.S. Pat. No. 5,235,606 with some variations as described
below.
[0044] The illustrative examples described below generally pertain
to pulse energies less than a microjoule and often in the nanojoule
range with pulse duration in the range of hundreds of picoseconds
or less and the frequency on the order of 1 kilohertz. But these
examples are merely illustrative and the invention is not limited
thereby.
[0045] In a basic scheme for CPA, first a short pulse is generated.
Ideally the pulse from the oscillator is sufficiently short so that
further pulse compression is not necessary. After the pulse is
produced it is stretched by a grating pair arranged to provide
positive group velocity dispersion. The amount the pulse is
stretched depends on the amount of amplification. Below a
millijoule, tens of picoseconds are usually sufficient. A first
stage of amplification typically takes place in either a
regenerative or a multipass amplifier. In one configuration this
consists of an optical resonator that contains the gain media, a
Pockels cell, and a thin film polarizer. After the regenerative
amplification stage the pulse can either be recompressed or further
amplified. The compressor consists of a grating or grating pair
arranged to provide negative group velocity dispersion. Gratings
are used in the compressor to correspond to those in the stretching
stage. More particulars of a typical system are described in U.S.
Pat. No. 5,235,606, previously incorporated herein by
reference.
[0046] An important aspect of the invention is the development of a
characteristic curve of fluence breakdown threshold Fth as a
function of laser pulse width specific to a material. Then identify
on such curve, the point at which there is an abrupt, or distinct
and rapid change or at least a discernable change in slope
characteristic of the material. In general it is more desirable to
operate past this point because of the more precise control of the
laser induced breakdown (LIB) or ablation threshold.
Example 1
Opaque Material
[0047] FIG. 1 shows an experimental setup for determining threshold
fluence by determining scattered energy versus incident fluence and
by determining threshold fluence versus pulse width. The system
includes means for generating a pulsed laser beam as described
earlier, and means, typically a lens, for collecting emission from
the target to a photomultiplier tube. Change of transmission
through a transparent sample is measured with an energy meter.
[0048] FIG. 2 shows a plot of data obtained from an absorbing
medium which is gold using 150 fs pulse and FIG. 3 shows threshold
fluence pulse width. The arrow in FIG. 3 identifies the point at
which the relationship between the threshold fluence and pulse
width varies dramatically.
[0049] In experimental conditions with wavelength of 800 nm and 200
fs pulses on gold (FIG. 3), the absorption depth is 275 A with a
diffusion of 50 A. In the case of nanosecond pulses the diffusion
length, which is on the order of 10 urn (micron) in diameter, is
much longer than the absorption depth, resulting in thermal
diffusion being the limiting factor in feature size resolution.
Empirical evidence for the existence of these two regimes is as
exhibited in FIG. 3. Here both experimental and theoretical
ablation thresholds are plotted as a function of pulse width. An
arrow at approximately 7 picoseconds pulse width (designated herein
as T or .tau.p) delineates the point (or region closely bounding
that point) at which the thermal diffusion length (lth) is equal to
the absorption depth (I/a). It is clear that for a smaller size
spot a shorter (smaller) pulse is necessary. For spot size on the
order of 1000.ANG. or less, pulse width on the order of 100
femtoseconds or less will be needed. It is clear from the figure
that this is the point at which the ablation threshold transitions
from a slowly varying or nearly constant value as a function of
pulse width to one that is dramatically dependent on pulse time.
This result is surprising. It has been that the electron
thermalization time for laser deposited energy in gold is on the
order o.English Pound. or less than, 500 fs and the
electron-lattice interaction time is 1 ps. The consequences of this
for ultrafast laser pulses is that the energy is contained within
the beam spot. In fact for energies at or near the threshold for
ablation, the spatial profile of the laser beam will determine the
size and shape of the region being ablated (FIGS. 4 and 5).
[0050] Additional experiments were performed to measure the amount
of recombination light produced as a function of the fluence
impinging on a gold film. The technique involved is based upon the
experimental setup previously described. A basic assumption is that
the intensity of the light is proportional to the amount of
material ablated. In FIG. 4, the material removed is plotted as a
function of fluence. A well defined threshold fluence is observed
at which material removal is initiated. By having only a small
fraction of the gaussian beam where the fluence is greater than the
threshold, the ablated region can be restricted to this small area.
In FIG. 4, Ra is the radial position on the beam where the fluence
is at threshold. Ablation, then, occurs only within a radius Ra. It
is evident that by properly choosing the incident fluence, the
ablated spot or hole can in principle be smaller than the spot
size, Rs. This concept is shown schematically in FIG. 5. Although
the data for a 150 fs pulse is shown in FIG. 4, this threshold
behavior is exhibited in a wide range of pulse widths. However, sub
spot size ablation is not possible in the longer pulse regimes, due
to the dominance of thermal diffusion as will be described below.
Additional experiments on opaque materials used a 800 nm
Ti:Sapphire oscillator whose pulses were stretched by a grating
pair, amplified in a regenerative amplifier operating at 1 kHz, and
finally recompressed by another grating pair. Pulse widths from 7
ns to 100 fs were obtained. The beam was focused with a 10*
objective, implying a theoretical spot size of 3.0 urn in diameter.
A SEM photo-micrograph of ablated holes obtained in a silver film
on glass, using a pulse width of 200 fs and a pulse energy of 30 nJ
(fluence of 0.4 J/cm2) produced two holes of diameter approximately
0.3 urn in diameter. Similar results have been obtained in
aluminum.
[0051] These results suggest that by, producing a smaller spot size
which is a function of numerical aperture and wavelength, even
smaller holes can be machined. We have demonstrated the ability to
generate the fourth harmonic (200 nm) using a nonlinear crystal.
Thus by using a stronger objective lens along with the 200 nm
light, holes with diameters of 200 angstroms could in principle be
formed.
[0052] These examples show that by using femtosecond pulses the
spatial resolution of the ablation/machining process can be
considerably less than the wavelength of the laser radiation used
to produce it. The ablated holes have an area or diameter less than
the area or diameter of the spot size. In the special case of
diffraction limited spot size, the ablated hole has a size
(diameter) less than the fundamental wavelength size. We have
produced laser ablated holes with diameters less than the spot
diameter and with diameters 10% or less of the laser beam spot
size. For ultrafast pulses in metals the thermal diffusion length,
lth=(Dt)1/2 (where D is the thermal diffusivity and t the pulse
time), is significantly smaller than the absorption depth (I/a),
where a is the absorption coefficient for the radiation.
[0053] Those skilled in the art will understand that the basic
method of the invention may be utilized in alternative embodiments
depending on desired configurations of the induced breakdown.
Examples include, but are not limited to using a mask in the beam
path, varying spot size, adjusting focus position by moving the
lens, adjusting laser cavity design, Fourier Transform (FT)
shaping, using a laser operating other than TEMoo, and adjusting
the Rayleigh range, the depth of focus or beam waist.
[0054] The use of a mask is illustrated in FIGS. 6A and B. The
basic method consists of placing a mask in the beam path or on the
target itself. If is desired to block a portion of the beam, the
mask should be made of an opaque material and be suspended in the
beam path (FIG. 6A), the mask may be placed on the target and be
absorptive so as to contour the target to the shape of the mask
(FIG. 6B).
[0055] The varying spot size is accomplished by varying the laster
f7#, i.e., varying the focal length of the lens or input beam size
to the lens as adjustable diaphragm, in other than the TEMoo mode
means that higher order transverse modes could be used. This
affects the beam and material as follows: the beam need not be
circular or gaussian in intensity. The material will be ablated
corresponding to the beam shape.
[0056] The Rayleigh range (Z axis) may be adjusted by varying the
beam diameter, where the focal plane is in the x-y axis.
Example 2
Transparent Material
[0057] A series of tests were performed on an SiO2 (glass) sample
to determine the laser induced breakdown (LIB) threshold as a
function of pulse width between 150 fs-7 ns, using a CPA laser
system. The short pulse laser used was a 10 Hz Ti:Sapphire
oscillator amplifier system based on the CPA technique. The laser
pulse was focused by an f=25 cm lens inside the SiO2 sample. The
Rayleigh length of the beam is .about.2 mm. The focused spot size
was measured in-situ by a microscope objective lens. The measured
spot size FWHM (full at half max) was 26 urn in diameter in a
gaussian mode. The fused silica samples were made from Corning
7940, with a thickness of 0.15 mm. They were optically polished on
both sides with a scratch/dig of 20-10. Each sample was cleaned by
methanol before the. Thin samples were used in order to avoid the
complications of self-focusing of the laser pulses in the bulk. The
SiO2 sample mounted on a computer controlled motorized X-Y
translation stage. Each location on the sample was illuminated by
the laser only once.
[0058] Two diagnostics were used to determine the breakdown
threshold Fth. First, the plasma emission from the focal region was
collected by a lens to a photomultiplier tube with appropriate
filters. Second, the change of transmission through the sample was
measured with an energy meter. (See FIG. 1) Visual inspection was
performed to confirm the breakdown at a nanosecond pulse duration.
FIG. 7 shows typical plasma emission and transmitted light signal
versus incident laser energy plots, at a laser pulse width of
.tau.p=300 fs. It is worth noting that the transmission changed
slowly at around Fth. This can be explained by the temporal and
spatial behavior of the breakdown with ultrashort pulses. Due to
the spatial variation of the intensity, the breakdown will reach
threshold at the center of the focus, and because of the short
pulse duration, the generated plasma will stay localized. The
decrease in transmitted light is due to the reflection, scattering,
and absorption by the plasma. By assuming a gaussian profile in
both time and space for the laser intensity, and further assuming
that the avalanche takes the entire pulse duration to reach
threshold, one can show that the transmitted laser energy Ut as a
function of the input energy U is given by Ut=kU, U<=Uth
Ut=kUth[l+ln(U/Uth)], U>Uth where k is the linear transmission
coefficient. The solid curve in FIG. 7 is plotted using Eq. (1),
with Uth as a fitting parameter. In contrast, breakdown caused by
nanosecond laser pulses cuts off the transmitted beam near the peak
of the pulses, indicating a different temporal and spatial
behavior.
[0059] FIG. 8 shows the fluence breakdown threshold Fth as a
function of laser pulse width. From 7 ns to about 10 ps, the
breakdown threshold the scaling in the relatively long pulse width
regime (triangles and squares) are also shown as a comparison--it
can be seen that the present data is consistent with earlier work
only in the higher pulse width portion of the curve. When the pulse
width becomes shorter than a few picoseconds, the threshold starts
to increase. As noted earlier with respect to opaque material
(metal), this increased precision at shorter pulse widths is
surprising. A large increase in damage threshold accuracy is
observed, consistent with the multiphoton avalanche breakdown
theory. (See FIGS. 8 and 9.) It is possible to make features
smaller than spot size in the x-y focal plane and smaller than the
Rayleigh range (depth of focus) in the longitudinal direction or Z
axis. These elements are essential to making features smaller than
spot size or Rayleigh range.
Example 3
Tissue
[0060] A series of experiments was performed to determine the
breakdown threshold of cornea as a function of laser pulse width
between 150 ns, using a CPA laser system. As noted earlier, in this
CPA laser system, laser pulse width can be varied while all other
experimental parameters (spot size, wavelength, energy, etc.)
remain unchanged. The laser was focused to a spot size (FWHM) of 26
Am in diameter. The plasma emission was recorded as a function of
pulse energy in order to determine the tissue damage threshold.
Histologic damage was also assessed.
[0061] Breakdown thresholds calculated from plasma emission data
revealed deviations from the scaling law, Fth .alpha. T1/2, as in
the case of and glass. As shown in FIG. 9, the scaling law of the
fluence threshold is true to about 10 ps, and fail when the pulse
shortens to less than a few picoseconds. As shown in FIGS. 10 and
11, the ablation or LIB threshold varies dramatically at high
(long) pulse width. It is very precise at short pulse width. These
results were obtained at 770 nm wavelengths. The standard deviation
of breakdown threshold measurements decreased markedly with shorter
pulses. Analysis also revealed less adjacent histological damage
with pulses less than 10 ps.
[0062] The breakdown threshold for ultrashort pulses (<10 ps) is
less than longer pulses and has smaller standard deviations.
Reduced adjacent histological damage to tissue results from the
ultrashort laser pulses.
[0063] In summary, it has been demonstrated that sub-wavelength
holes can be machined into metal surfaces using femtosecond laser
pulses. The effect is physically understood in terms of the thermal
diffusion length, over the time period of the pulse deposition,
being less than the absorption depth of the incident radiation. The
interpretation is further based on the hole diameter being
determined by the lateral gaussian distribution of the pulse in
relation to the threshold for vaporization and ablation.
[0064] Laser induced optical breakdown dielectrics consists of
three general steps: free electron generation and multiplication,
plasma heating and material deformation or breakdown. Avalanche
ionization and multiphoton ionization are the two processes
responsible for the breakdown. The laser induced breakdown
threshold in dielectric material depends on the pulse width of the
laser pulses. An empirical scaling law of the fluence breakdown
threshold as a function of the pulse width is given by Fth .alpha.
.sqroot.tau.p, or alternatively, the intensity breakdown threshold,
Ith=FtW.tau.p. Although this scaling law applies in the pulse width
regime from nanosecond to tens of picoseconds, the invention takes
advantage of the heretofore unknown regime where breakdown
threshold does not follow the scaling law when suitably short laser
pulses are used, such as shorter than 7 picoseconds for gold and 10
picoseconds for SiO2.
[0065] While not wishing to be held to any particular theory, it is
thought that the ionization process of a solid dielectric
illuminated by an intense laser pulse can be described by the
general equation
dne(t)/dt=.eta.(E)ne(t)+(dne(t)/dt)PI-(dne(t)/dt)loss where ne (t)
is the free electron (plasma) density, .eta.(E) is the avalanche
coefficient, and E is the electric field strength. The second term
on the right hand side is the photoionization contribution, and the
third term is the loss due to electron diffusion, recombination,
etc. When the pulse width is in the picosecond regime, the loss of
the electron is negligible during the duration of the short
pulse.
[0066] Photoionization contribution can be estimated by the
tunneling rate. For short pulses, E-108 V/cm, the tunneling rate is
estimated to be w.about.4.times.109 sec-1, which is small compared
to that of avalanche, which is derived below. However,
photoionization can provide the initial electrons needed for the
avalanche processes at short pulse widths. For example, the data
shows at 1 ps, the rms field threshold is about 5><107 V/cm.
The field will reach a value of 3.5.times.107 V/cm (rms) at 0.5 ps
before the peak of the pulse, and w.about.100 sec-1. During a
DELTA.t.about.100 fs period the electron density can reach
ne.about.nt [1-exp(-w.DELTA.t)].about.1011 cm-3, where nt-1022 is
the total initial valence band electron density.
[0067] Neglecting the last two terms there is the case of an
electron avalanche process, with impact ionization by primary
electrons driven by the laser field. The electron density is then
given by ne (t)=no xexp(n(E)t), where no is the initial free
electron density. These initial electrons may be generated through
thermal ionization of shallow traps or photoionization. When
assisted by photoionization at short pulse regime, the breakdown is
more statistical. According to the condition that breakdown occurs
when the electron density exceeds nth .congruent. 1018 cm-3 and an
initial density of no congruent. 1010 cm-3, the breakdown condition
is then given by .eta.tau.p congruent. 18. For the experiment, it
is more appropriate to use nth .congruent. 1.6.times.1021 cm-3, the
plasma critical density, hence the threshold is reached when
eta.tau.p .congruent.30. There is some arbitrariness in the
definition of plasma density relating to the breakdown threshold.
However, the particular choice of plasma density does not change
the dependence of threshold as function of pulse duration (the
scaling law).
[0068] In the experiment, the applied electric field is on the
order of a few tens of MY/cm and higher. Under such a high field,
the electrons have an average energy of .about.5 eV, and the
electron collision time is less than 0.4 fs for electrons with
energy U.gtoreq.gt;=5-6 eV. Electrons will make more than one
collision during one period of the electric oscillation. Hence the
electric field is essentially a dc field to those high energy
electrons. The breakdown field at optical frequencies has been
shown to correspond to dc breakdown field by the relationship
Erm.kappa.th (w)=Edcth (1+w2.tau.2)1/2, where w is the optical
frequency and .tau. is the collision time.
[0069] In dc breakdown, the ionization rate per unit length,
.alpha., is used to describe the avalanche process, with
.eta.=.alpha.(E)vdrift, where vdrift is the drift velocity of
electrons. When the electric field is as high as a few MV/cm, the
drift velocity of free electrons is saturated and independent of
the laser electric field, vdrift .congruent.2><107 cm/s.
[0070] The ionization rate per unit length of an electron is just
eE/Ui times the probability, P(E), that the electron has an energy
.gtoreq.gt;=Ui, or .alpha.(E)=(eE/Ui)P(E). Denoting EkT,E p, and Ei
as threshold fields for electrons to overcome the decelerating
effects of thermal, phonon, and ionization scattering,
respectively. Then the electric field is negligible, EkT, so the
distribution is essentially thermal, P(E) is simply exp(-Ui/kT). It
has been suggested: P(E).about.exp(-const/E) for EkT p;
P(E).about.exp(-const/E2) at higher fields (E>Ep). Combining the
three cases the expression that satisfies both low and high field
limits: .alpha.(E)=(eE/Ui)exp(-Ei/(E(1+E/Ep)+EKT).
[0071] This leads to Fth .alpha. E2.tau.p.about.1/.tau.p, i.e., the
fluence threshold will increase for ultrashort laser pulses when
E>.sqroot.Ep Ei is satisfied.
[0072] FIG. 12 is a plot of .alpha, as a function of the electric
field, E. From experimental data, calculated according to
.eta..tau.p=30 and eta.=avdrift. The solid curve is calculated from
the above equation, using Ei=30 MV/cm, Ep=3.2 MV/cm, and EkT=0.01
MV/cm.
[0073] These parameters are calculated from U=eEl, where U is the
appropriate thermal, phonon, and ionization energy, and 1 is the
correspondent energy relation length (lkT=lp .about.5.ANG., the
atomic spacing, and li .congruent.30.ANG.). It shows the same
saturation as the experimental data. The dashed line is corrected
by a factor of 1.7, which results in an excellent fit with the
experimental data. This factor of 1.7 is of relatively minor
importance, as it can be due to a systematic correction, or because
breakdown occurred on the surface first, which could have a lower
threshold. The uncertainty of the saturation value of vdrift also
can be a factor. The most important aspect is that the shape
(slope) of the curve given by the equation provides excellent
agreement with the experimental data. Thus, the mechanism of laser
induced breakdown in fused silica (Example 2), using pulses as
short as 150 fs and wavelength at 780 nm, is likely still dominated
by the avalanche process.
[0074] Opaque and transparent materials have common characteristics
in the curves of FIGS. 3, 8, and 9 each begins with Fth versus T1/2
behavior but then distinct change from that behavior is evident.
From the point of deviation, each curve is not necessarily the same
since the materials differ. The physical characteristics of each
material differ requiring a material specific analysis. In the case
of SiO2 FIG. 8) the energy deposition mechanism is by dielectric
breakdown. The optical radiation is releasing electrons by
multiphoton ionization (M PI) that are tightly bound and then
accelerating them to higher energies by high field of the laser. It
is thought that only a small amount of relatively high energy
electrons exist prior to the laser action. The electrons in turn
collide with other bound electrons and release them in the
avalanching process. In the case of metal, free electrons are
available and instantly absorbing and redistributing energy. For
any material, as the pulses get shorter laser induced breakdown
(LIB) or ablation occurs only in the area where the laser intensity
exceeds LIB or ablation threshold. There is essentially
insufficient time for the surrounding area to react thermally. As
pulses get shorter, vapor from the ablated material comes off after
the deposition of the pulse, rather than during deposition, because
the pulse duration is so short. In summary, by the method of the
invention, laser induced breakdown of a material causes
thermal-physical changes through ionization, free electron
multiplication, dielectric breakdown, plasma formation, other
thermal-physical changes in state, such as melting and
vaporization, leading to an irreversible change in the material. It
was also observed that the laser intensity also varies along the
propagation axis (FIG. 13). The beam intensity as a function of R
and Z expressed as: I((Z,R)=Io/(1+Z/ZR)2.multidot.exp(-2R2AV2z)
where ZR is the Rayleigh range and is equal to [Figure] Wo is the
beam size at the waist (Z=0).
[0075] We can see that the highest value of the field is at Z=R=0
at the center of the waist. If the threshold is precisely defined
it is possible to damage the material precisely at the waist and
have a damaged volume representing only a fraction of the waist in
the R direction or in the Z direction. It is very important to
control precisely the damage threshold or the laser intensity
fluctuation.
[0076] For example, if the damage threshold or the laser
fluctuations known within 10% that means that on the axis (R=0)
I(0,Z)/Io=1/(1=(Z/ZR)2=0.9 damaged volume can be produced at a
distance ZR/3 where ZR again is the Rayleigh range. For a beam
waist of Wo=.lambda, then Figure] and the d distance between hole
can [Figure] as shown in FIG. 13.
[0077] The maximum intensity is exactly at the center of the beam
waist (Z=0, R=0). For a sharp threshold it is possible to damage
transparent, dielectric material in a small volume centered around
the origin point (Z=0, R=0). The damage would be much smaller than
the beam waist in the R direction. Small cavities, holes, or damage
can have dimensions smaller than the Rayleigh range (ZR) in the
volume of the transparent, dielectric material. In another
variation, the lens can be moved to increase the size of the hole
or cavity in the Z dimension. In this case, the focal point is
essentially moved along the Z axis to increase the longitudinal
dimension of the hole or cavity. These features are important to
the applications described above and to related applications such
as micro machining, integrated circuit manufacture, and encoding
data in data storage media.
[0078] Advantageously, the invention identifies the regime where
breakdown threshold fluence does not follow the scaling law and
makes use of such regime to provide greater precision of laser
induced breakdown, and to induce breakdown in a preselected pattern
in a material or on a material. The invention makes it possible to
operate the laser where the breakdown or ablation threshold becomes
essentially accurate. The accuracy can be clearly seen by the
I-bars along the curves of FIGS. 8 and 9. The I-bars consistently
show lesser deviation and correspondingly greater accuracy in the
regime at or below the predetermined pulse width.
[0079] While this invention has been described in terms of certain
embodiment thereof, it is not intended that it be limited to the
above description, but rather only to the extent set forth in the
following claims. The embodiments of the invention in which an
exclusive property or privilege is claimed are defined in the
appended claims.
[0080] The teaching of the following references are incorporated
herein by reference:
FOREIGN REFERENCES
[0081] TABLE-US-00001 Publication Number Country Date IPC Class
Germany December 1992 DE04119024A1 WO08908529 World Intellectual
March 1989 Property Organization (WIPO)
OTHER REFERENCES
[0082] C. V. Shank, R. Yen, and C. Hirlimann, "Time-Resolved
Reflectivity Measures of Femtosecond-Optical-Pulse-Induced Phase
Transitions in Silicon", Physical Review Letters, vol. 50, No. 6,
454-457, Feb. 7, 1983. C. V Shank, R. Yen, and C. Hirlimann,
"Femtosecnd-Time-Resolved Surface Structural Dynamics of Optically
Excited Silicon", Physical Review Letters, vol. 51, No. 10,
900-902, Sep. 5, 1983. C. V. Shank and M. C. Downer, "Femtosecond
Dynamics of Highly Excited Semiconductors", Mat. Res. Soc. Symp.
Proc, vol. 51, 15-23, 1985. S. Kuper and M. Stuke, "Femtosecond uv
Excimer Laser Ablation", Applied Physics B, vol. 44, 199-204, 1987.
S. Preuss, M. Spath, Y. Zhang, and M. Stuke, "Time Resolved
Dynamics of Subpicosecond Laser Ablation", Applied Physics Letters,
vol. 62, No. 23, 3049-3051, Jun. 7, 1993. A. M. Malvezzi, N.
Bloembergen, and C. Y. Huang, "Time-Resolved Picosecond Optical
Measurements of Laser-Excited Graphite", Review Letters, vol. 57,
No. 1, 146-149, Jul. 7, 1986. D. H. Reitze, X. Wang, H. Ahn, and M.
C. Downer, "Femtosecond Laser Melting of Graphite", Physical Review
B, vol. 40, No. 17, Dec. 15, 1989. F. Muller, K. Mann, P. Simon, J.
S. Bernstein, and G. J. Zaal, "A Comparative Study of Decomposition
of Thin Films by Laser Induced PVD with Femtosecond and Nanosecond
Laser Pulses", SPIE, vol. 1858, 464-475, 1993. International Search
Report Form PCT/ISA/210 Dated 31 Jul. 1995 and Mailed 4 Aug. 1995.
M. W. Berns et al., "Laser Microsurgery in Cell and Developmental
Biology", Science, vol. 213, No. 31, 505-513, July 1981. G. L.
LeCarpentier et al., "Continuous Wave Laser Ablation of Tissue:
Analysis of Thermal and Mechanical Events", IEEE Transactions on
Biomedical Engineering, vol. 40, No. 2, 188-200, February 1993. C.
LeBlanc, "Realization and Characterization of a High Intensity
Femtosecond Laser System Based on all Titanium Doped Sapphire",
Annales de Physique, vol. 19, No. 1, Abstract, February 1994. R.
Birngruber, C. Puliafito, A. Gawande, W. Lin, R. Schoenlein, and J.
Fujimoto, "Femtosecond Laser-Tissue Interactions: Retinal Injury
Studies", IEEE Journal of Quantum Electronics, vol. QE-23, No. 10,
1836-1844, October 1987. B. Zysset, J. Fujimoto, and T. Deutsch,
"Time-Resolved Measurements of Picosecond Optical Breakdown",
Applied Physics B 48, 139-147 (1989). B. Zysset, J. Fujimoto, C.
Puliafito, R Birngruber, and T. Deutsch, "Picosecond Optical
Breakdown: Tissue Effects and Reduction of Collateral Damage",
Lasers in Surgery and Medicine 9:192-204 (1989). S. Watanabe, R.
Anderson, S. Brorson, G. Dalickas, J. Fujimoto, and T. Flotte,
"Comparative Studies of Femtosecond to Microsecond Laser Pulses on
Selective Pigmented Cell Injury in Skin", Photochemistry and
Photobiology vol. 53, No. 6, 757-762, 1991). N. Bloembergen,
"Laser-Induced Electric Breakdown in Solids", IEEE Journal of
Quantum Electronics, vol. QE-10, No. 3, (March 1974). R Birngruber,
C. Puliafito, A. Gawande, W. Lin, R. Schoenlein, and J. Fujimoto,
"Femtosecond Laser-Tissue Interactions: Retinal Injury Studies",
IEEE Log No. 8716039, (1987). D. Stern, R. Schoenlein, C.
Puliafito, E. Dobi, R. Birngruber, and J. Fujimoto, "Corneal
Ablation by Nanosecond, Picosecond, and Femtosecond Lasers at 532
and 625 nm", Arch Ophthalmol, vol. 107, (April 1989). J. Squier, F.
Salin, and G. Mourou, "100-fs Pulse Generation and Amplification in
Ti:Al2 03", Optics letters, vol. 16, No. 5, (March 1991). B. Frueh,
J. Bille, and S. Brown, "Intrastromal Relaxing Excisions in Rabbits
with a Picosecond Infrared Laser", Lasers and Light in
Ophthalmology, vol. 4, No. 3/4, (1992), 165-168 R. Remmel, C.
Dardenne, and J. Bille, "Intrastromal Tissue Removal Using an
Infrared Picosecond Nd:YLF Ophthalmic Laser Operating at 1053 nm",
Lasers and Light in Ophthalmology, vol. 4, No. 3/4, 169-173,
(1992). J. Squier and G. Mourou, "Tunable Solid-State Lasers Create
Ultrashort Pulses", Laser Focus World, (June 1992). M. H. Niemz, T.
P. Hoppeler, T. Juhasz, and J. Bille, "Intrastromal Ablations for
Refractive Corneal Surgery Using Picosecond Infrared Laser Pulses",
Lasers and Light in Ophthalmology, vol. 5, No. 3, pp. 149-155
(1993). H. Cooper, J. Schuman, C. Puliafito, D. McCarthy, W. Woods,
N. Friedman, N. Wang, and C. Lin, "Picosecond Neodymium: Yttrium
Lithium Fluoride Laser Sclerectomy", Am. Journal of Opth.
115:221-224, (February 1993). K. Frederickson, W. White, R.
Wheeland, and D. Slaughter, "Precise Ablation of Skin with Reduced
Collateral Damage Using the Femtosecond-Pulsed, Terawatt
Titanium-Sapphire Laser", Arch Dermatol, vol. 129, (August 1993).
H. Kapteyn and M. Murnane, "Femtosecond Lasers: The Next
Generation", Optics & Photonics News, (March 1994). G. Mourou,
A. Zewail, P. Barbara, and W. Knox, "New Generation of Ultrafast
Sources Marked by Higher Powers, Versality", Optics Photonics News,
(March 1994). D. Du, X. Liu, G. Korn, J. Squier, and G. Mourou,
"Laser-Induced Breakdown by Impact Ionization in SiO2 with Pulse
Widths from 7 ns to 150 fs", Appl. Phys. Lett 64 (23), (Jun. 6,
1994). Optics, Eugene Hecht et al., Addison-Wesley Publishing
Company, 1979.
[0083] The teachings of the following United States patents (US
patents); World Intellectual Property Organization patents (WO
Patents); Japanese patents (JP Patents); European Patent Office
patents (EP Patents) and patents of other foreign jurisdictions
(DD, DE, GB, RU, SU) are incorporated herein by reference. The
methods and apparatus described herein can be advantageously
utilized in the methods and apparatus of the incorporated
references to achieve the enhanced properties and features
described herein in the methods and apparatus of the incorporated
references. For example where the reference uses a radiation beam,
such as a light beam, in-particular a laser beam, the laser
techniques according to the present invention can be used. The
laser pulse of the present invention can be directed for example by
using a light pipe or a wave guide. Commonly available, such as
commercially available, light pipes and wave guides can be
used.
[0084] TABLE-US-00002 U.S. Pat. Nos. 6,251,102 6,238,386 6,231,568
6,231,567 6,224,589 6,213,998 6,200,311 6,190,377 6,152,919
6,066,127 5,951,543 5,893,828 5,868,731 5,860,426 5,783,798
5,725,523 5,688,263 5,658,275 5,611,797 5,607,420 5,571,098
5,562,658 5,549,600 5,534,000 5,454,808 5,403,306 5,397,327
5,366,456 5,346,489 5,312,396 5,290,279 5,269,778 5,224,942
5,219,347 5,195,541 5,194,712 5,180,378 5,163,935 5,154,708
5,125,923 5,098,427 5,092,864 5,074,861 5,004,338 4,963,143
4,950,268 4,939,336 4,862,886 4,846,171 4,832,979 4,812,613
4,791,927 4,791,926 4,790,310 4,788,975 4,736,743 4,729,373
4,702,245 4,693,244 4,669,465 4,658,817 4,638,800 4,633,872
4,627,435 4,597,380 4,592,353 4,580,557 4,573,466 4,573,465
4,566,453 4,564,012 4,550,240 4,532,400 4,519,390 4,517,973
4,503,854 4,497,319 4,491,131 4,478,217 4,473,074 4,470,414
4,469,098 4,459,986 4,408,602 4,273,109 4,270,845 4,266,549
4,266,548 4,266,547 4,249,533 4,240,431 4,233,493 4,185,633
4,174,154 4,144,888 4,143,660 4,141,362 3,910,276 3,906,953
3,865,114 3,865,113 3,858,577 3,123,066
[0085] TABLE-US-00003 WO patents 0,141,871 0,128,447 0,110,304
0,115,592 0,113,812 0,113,810 0,106,908 0,019,920 0,110,288
0,108,576 0,108,579 0,078,242 0,054,686 0,048,525 9,965,405
9,955,243 9,955,218 9,951,156 9,944,518 9,900,062 9,818,394
9,713,468 9,715,236 9,612,441 9,410,923 9,325,156 9,314,432
9,312,727 9,311,699 9,221,299 9,217,138 9,208,427 9,206,641
9,203,977 9,012,619 8,704,610 8,606,642 8,505,263 8,505,262
8,500,010
[0086] TABLE-US-00004 JP patents 2001054524 2001029359 2001008946
2001008945 11104147 10328196 10113354 08229049 09010221 09000537
08299352 08148737 07124169 06090959 05220167 05220166 05220101
05176938 04023339 04129545 03278489 03139346 03041943 02239857
02099048 01320050 01036650 01034347 63318935 60104902 59195891
58145901 58087887 58084887 56114390 55083011 55083010
[0087] TABLE-US-00005 EP patents 1097676 0717964 0669107 0423431
0458506 0391976 0372362 0327410 0341943 0297360 0292622 0069351
* * * * *