Electric heating – Metal heating – By arc
Reexamination Certificate
2002-05-15
2004-01-13
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06677552
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to laser machining and more particularly relates to laser machining of crystalline and poly-crystalline materials and to silicon integrated circuit manufacturing and testing.
2. Description of the Prior Art
Lasers have been used for marking and machining of materials since shortly after their invention. Established techniques include laser cutting, drilling, and welding. These processes have been applied to a wide range of materials including, metals, ceramics, polymers, and natural-products such as cotton and paper.
Many types of lasers, including both continuous wave (CW) and pulsed systems, are used for material modification. In general, pulsed systems provide higher peak powers and CW sources provide greater total power. For pulse widths greater than 10 picoseconds the modification process is dependant on the ability of the material to adsorb the light. (Pulse widths are measured at FWHM.) The modification process is therefore wavelength dependant at power levels sufficient to machine materials. Light sources across a large region of the electromagnetic spectrum are, therefore, used for machining. These include CO
2
lasers in the infrared and excimer lasers in the ultraviolet. It has been believed that wavelength is less important when pulse widths are shorter than approximately 10 picoseconds.
In contrast, the lengths of laser pulses are known to be important parameters in many instances. Fluence, or energy per unit area, is also a critical factor. Short and powerful pulses produce results markedly different from long and less powerful pulses.
The effects of fluence and pulse length are in part due to the different processes that can occur in laser machining. These include heating and melting, blasting, and plasma development. Heating and melting are the result of thermal effects wherein photon energy is absorbed by the target and converted into thermal energy. A thermally affected zone is one in which material has been heated to the extent that its chemical or electrical functionality are changed. For example, when laser machining occurs near an integrated circuit, the thermally affected zone is the volume within which a state or operation of the integrated circuit is changed or a property that affects a state of the integrated circuit is changed. As the material is heated it melts and a thermally affected zone often extends beyond the area on which light is incident. Blasting is the result of sudden thermal excitation wherein rapid expansion causes material to be blown off of the target. Plasma may also form if the material is heated rapidly. Plasmas imply that electrons have attained sufficient energy to escape from the protons they are normally associated with. Due to the presence of free electrons, energy can travel very rapidly in plasmas. Heating times and cooling mechanisms can also be very important to laser machining processes. The rate at which energy leaves the affected area helps determine the characteristics of the machined material. For example, it is well known that the smoothness of holes drilled in thin metal sheets is much greater for picosecond laser pulses than for nanosecond pulses. Femtosecond laser micro-machining is essentially a non-thermal machining process. Energy is transferred to the material lattice in a picosecond time scale, resulting in a rapid formation of a plasma that expands and expels the vaporized material from the surface.
It is known in the current art that various levels of laser fluence have different effects on a machined surface.
FIG. 1
illustrates these regions as expected on a metal surface. In a first region
110
laser pulses have no effect on the material. A second region
130
begins at a damage threshold
120
and continues until a fluence of roughly ten times threshold
140
is reached. In the second region
130
the material is modified but not removed. A third region
150
, above ten times threshold
140
, is characterized by significant material modification and removal. The difference between the two thresholds
120
and
140
varies widely as a function of the material and fluence of the light. Generally, it is believed that the polarization and wavelength of the light used is unimportant.
Laser machining is most commonly used for metals, ceramics, natural products, and polymers. Other materials such as crystals, polycrystals, or glasses have been described as “problem materials” in the field. Such materials can have very high melting points or not adsorb photons at the easily produced wavelengths. An example of such a material is silicon. Silicon easily adsorbs light but traditionally forms irregular surfaces when laser machined. Silicon has a damage threshold fluence of roughly 0.13 J/cm
2
for 795 nanometer light.
Silicon is a very important material in the electronics industry wherein wafers are used in the manufacture of integrated circuits. There is a significant need for machining silicon with the accuracy and precision required in the integrated circuit industry. Silicon is currently machined using techniques such as grinding or ion beam etching. Grinding is used to remove bulk material but can thermally damage any existing circuits. Ion beam etching on the other hand has the precision and accuracy required but is an extremely slow process.
There are a number of applications that would greatly benefit from improved silicon machining techniques. For example, to test circuits it is advantageous to insert a probe through the backside of the silicon substrate on which the circuit has been built. In current technology ion beam etching is used to drill a hole through the substrate to the circuit to be tested. This process can take many days or even weeks and, therefore, is not practical for frequent use. Lack of a more practical machining process also limits the ability to quickly make connections between integrated circuits on either side of a flat substrate.
FIG. 2
shows a block diagram of a prior art laser machining (or marking) system generally designated
200
and including a laser
210
that produces a light beam
220
. Beam
220
is manipulated by a wide variety of optional optics
230
and is finally directed at a target
240
.
FIG. 3
illustrates a hole, generally designated
300
, drilled using prior art ion beam etching methods. Hole
300
is made in the silicon substrate
310
using a beam of high-energy ions (not shown). The beam slowly sputters material from substrate
310
. Typical removal rates are on the order of 100 cubic microns per second. A 500 by 500 by 100 micron hole therefore requires about 3 days at 100 Hz. Hole
300
is made from a backside
312
of substrate
310
to a front side
314
. Front side
314
optionally supports various electronic components
320
. For example, in
FIG. 3
, hole
300
is located across from a circuit of interest
330
.
FIG. 4
is a micrograph of a hole generally designated
400
laser drilled in silicon using methods of the prior art. The hole has a diameter of about 70 microns. Hole
400
was drilled at a fluence of 4.6 times threshold
120
. Bright spots
410
in the center of the hole are peaks of spikes that can rise several microns above the original surface. These are believed to occur as a result of plasma condensation processes. At other fluences pits can occur in the bottom surface of the hole (not shown). These spikes and pits, such as illustrated in
FIG. 3
, have prevented laser machining from replacing conventional ion beam etching processes in many applications.
FIG. 5
is a cross-sectional illustration of the pertinent features of hole
400
shown in FIG.
4
. The hole
400
is made in a silicon substrate
310
on which integrated circuits
320
and
330
have been manufactured. The bright spots
410
(
FIG. 4
) are shown as spikes
530
and pits
540
are also visible. Spikes
530
and pits
540
prevent further machining without affecting the circuit of interest
330
because the bottom surface
510
of hole
400
has become too irregular.
There therefore exists a significant n
Fry Alan Reilly
Tulloch William Michael
Weston Jeremy
White William Eugene
Carr & Ferrell LLP
Elve M. Alexandra
Johnson Jonathan
Positive Light, Inc.
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