Etching a substrate: processes – Gas phase etching of substrate – Application of energy to the gaseous etchant or to the...
Reexamination Certificate
1998-04-28
2002-02-26
Alanko, Anita (Department: 1746)
Etching a substrate: processes
Gas phase etching of substrate
Application of energy to the gaseous etchant or to the...
C216S067000, C438S709000, C438S725000
Reexamination Certificate
active
06350391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to U.V. laser surface treatment methods, particularly to the removal of foreign materials from substrate surfaces. More particularly, the invention relates to an improved chemistry for dry stripping process employing UV-laser radiation and reactive chemistry for the removal of photoresist from semiconductor wafers.
BACKGROUND OF THE INVENTION
In the manufacturing of various products it is necessary to apply a layer of protective material on a surface, which must be removed after a specified manufacturing step has been concluded. An example of such process is the so-called “photolithography” process, which is widely used in the manufacturing of integrated circuits. In this process, a pattern is created on a surface using a layer of protective material illuminated through a mask, and the surface is then treated with a developer which removes material from the unmasked portions of the surface, therefore leaving a predetermined pattern. The surface is then treated by ion implantation or by etching agents, which introduce the implanted species into the unmasked portions of the surface, or removes material from unmasked portions. Once these processes are completed, the role of the protecting mask ends and it must be removed. The process is conventional and well known in the art, and is described, e.g., in R. K. Watts, “Lithography”, VLSI Technology, S. M. Sze, ed., McGraw-Hill, N.Y., 1988, Chpt. 4.
Two main photoresist stripping methods exist in the modern VLSI/ULSI (Very/Ultra Large Scale Integration) circuits industry (D. L. Flamm, “Dry PlasmaResist Stripping”, Parts 1, 2 and 3; Solid State Technology, August, September and October 1992):
1) Wet stripping which uses acids or organic solvents;
2) Dry stripping, which uses plasma, O
3
, O
3
/N
2
O or U.V./O
3
-based stripping.
Both methods are problematic and far from being complete, especially when taking into consideration the future miniaturization in the VLSI/ULSI industry. The current technology is capable of dealing with devices having feature sizes of about 0.5 &mgr;m, but before the end of the century the expectation is that the workable size of the devices is expected to be reduced to 0.25 &mgr;m. The expected size changes require considerable changes in the manufacturing technology, particularly in the stripping stage. The prior art photoresist stripping techniques described above will be unsuitable for future devices, as explained hereinafter.
Utilizing only the wet stripping method is not a perfect solution, as it cannot completely strip photoresist after tough processes that change the chemical and physical properties of the photoresist in a way that it makes its removal very difficult. Such processes include, e.g., High Dose Implantation (HDI), reactive Ion Etching (RIE), deep U.V. curing and high temperatures post-bake. After HDI or RIE the side walls of the implanted patterns or of the etched walls are the most difficult to remove.
In addition, the wet method has some other problems: the strength of stripping solution changes with time, the accumulated contamination in solution can be a source of particles which adversely affect the performance of the wafer, the corrosive and toxic content of stripping chemicals imposes high handling and disposal costs, and liquid phase surface tension and mass transport tend to make photoresist removal uneven and difficult.
The dry method also suffers from some major drawbacks, especially from metallic and particulate contamination, damage due to plasma: charges, currents, electric fields and plasma-induced U.V. radiation, as well as temperature-induced damage, and, last but not least, from incomplete removal. During various fabrication stages, as discussed above, the photoresist undergoes chemical and physical changes which harden it, and this makes the stripping processes of the prior art extremely difficult to carry out. Usually a plurality of sequential steps, involving wet and dry processes are needed to remove completely the photoresist.
The art has addressed this problem in many ways, and commercial photoresist dry removal apparatus is available, which uses different technologies. For instance, UV ozone ashers are sold, e.g. by Hitachi, Japan (UA-3150A), dry chemical ashers are also available, e.g., by Fusion Semiconductor Systems, U.S.A., which utilize nitrous oxide and ozone to remove the photoresist by chemical ashing at elevated temperatures, microwave plasma ashing is also effected, e.g., as in the UNA-200 Asher (ULVAC Japan Ltd.). Also plasma photoresist removal is employed and is commercially available, e.g., as in the Aspen apparatus (Mattson Technology, U.S.A.), and in the AURA 200 (GASONICS IPC, U.S.A.).
More recently, photoresist removal has been achieved by ablation, using laser UV radiation, in an oxydizing environment, as described in U.S. Pat. No. 5,114,834. The ablation process is caused due to strong absorption of the laser pulse energy by the photoresist. The process is a localized ejection of the photoresist layer to the ambient gas, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating. The partly gasified and partly fragmented photoresist is blown upwards away from the surface, and instantly heats the ambient gas. Fast combustion of the ablation products occurs, due to the blast wave and may also be due to the photochemical reaction of the UV laser radiation and the process gases. The main essence of the process is laser ablation with combustion of the ablated photoresist which occurs in a reactive gas flowing through an irradiation zone. The combination of laser radiation and fast combustion provides simultaneous lowering of the ablation threshold of hard parts of the photoresist (side walls). The combusted ablation products are then removed by vacuum suction, or by gas sweeping leaving a completely clean surface.
While U.S. Pat. No. 5,114,834 provides an important novel process, it still does not provide a high throughput, which is industrially convenient, viz., an industrially acceptable number of wafers that can be stripped during a given time. The laser stripping throughput is determined by the stripping rate or by the number of laser pulses needed for providing complete stripping of a unit area of the photoresist per unit of time.
While reference will be made throughout this specification to the ablation of photoresist from semiconductor wafers, this will be done for the sake of simplicity, and because it represents a well known and widely approached problem. It should be understood, however, that the invention described hereinafter is by no means limited to the stripping of photoresist from wafers, but it applies, mutatis mutandis, to many other applications, such as stripping and cleaning of photoresist from Flat Panel Displays (FPD) or removal of residues from different objects, such as lenses, semiconductor wafers, or photomasks.
SUMMARY OF THE INVENTION
It has now surprisingly been found, and this is an object of the invention, that it is possible to accelerate a given laser removal process by reducing the number of pulses while carrying out the stripping process in the presence of a specific gas composition consisting of O
2
/O
3
and N
x
O
y
, wherein N
x
O
y
indicates one or more nitrogen oxides, x and y having the appropriate values for the given oxide or mixtures of oxides.
Four different gas compositions are preferred according to the present invention:
1) O
2
:O
3
(Comparative)
2) O
2
/O
3
:N
x
O
y
3) O
2
:N
x
O
y
4) N
x
O
y
This surprising result is obtained by providing additive amounts of N
x
O
y
gases in the reactive gas stream which oxidizes the ablated photoresist and enhances laser etching of hard side-walls. The effect of N
x
O
y
is significant, since contents of less than 5% of the total gas flow are capable of doubling the stripping throughput. Gas percentages given herein are given by volume, unless otherwise specified.
DETAILED DESCRIPTION OF THE INVENTION
The method of improving the throughput of stripped substrates in a
Genut Menachem
Livshits Boris
Tehar-Zahav Ofer
Alanko Anita
Cowan Liebowitz & Latman P.C.
Dippert William H.
Oramir Semiconductor Equipment Ltd.
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