Etching a substrate: processes – Masking of a substrate using material resistant to an etchant
Reexamination Certificate
1998-09-21
2002-05-21
Mills, Gregory (Department: 1763)
Etching a substrate: processes
Masking of a substrate using material resistant to an etchant
C216S051000, C216S067000, C216S074000, C216S066000, C438S712000
Reexamination Certificate
active
06391216
ABSTRACT:
FIELD OF THE INVENTON
The present invention relates to a mask for reactive ion etching and an apparatus used therefor More specifically, the invention relates to a novel mask for reactive ion etching and an apparatus used therefor, which is characterized by a dry etching apparatus for a magnetic material, which is useful for production of a magnetic head for read-write of a magnetic disk, a microtransformer or a microinductor installed in a magnetic integrated circuit, a magnetic sensor, various quantum magnetic devices, such as a spin scattering magnetoresistance effect device, a spin valve device, a ferromagnetic tunneling device, a spin diode and a spin transistor, and a constitutional part of a micromachine, such as a thin film magnet and a magnetostrictive actuator.
BACKGROUND OF THE INVENTION
A microsemiconductor devices such as a VLSI and a micromagnatic device, are generally produced by a combination of two processes, i.e., a lithography technique and an etching technique.
The lithograph technique is a technique of producing fine patterns on a photosensitive film such as a resist film coated on a surface of a material to be processed, such as a thin film of a semiconductor and a thin film of a magnetic material, which includes a photolithography technique where exposure is conducted with an ultraviolet ray, an electron beam lithography technique where exposure is conducted with an electron beam, and an ion beam lithography technique where exposure is conducted with an ion beam.
The etching technique is a technique of producing a device by transferring the resist pattern produced by the lithography to the material to be processed, such as a thin film of a semiconductor and a thin film of a magnetic material.
The etching technique includes a wet etching method, an argon ion milling method and a reactive ion etching method. Among these etching methods, the reactive ion etching method is the best method because the pattern produced by the lithography can be transferred in the most precise manner, it is most suitable for fine working, and the etching rate is the largest. LSIs of semiconductors and semiconductor memory devices are actually produced by this method.
In the reactive ion etching method, a material to be processed is placed in a plasma of a reactive gas with applying an electric field, and atoms on the surface of the material to be processed are chemically and physically removed by an ion beam incident normally onto the surface of the material to be processed, by which an anisotropic working is possible, where a part not covered with the mask is vertically cut along the edge of the mask. Accordingly, a fine and sharp feature can be transferred by the reactive ion etching method. In the reactive ion etching method, chemical active species such as an ion and a radical of the reactive gas generated in the plasma are adsorbed on the surface of the material to be processed, to chemically react with the material to be processed, and a surface reactive layer having a lower bond energy. The surface of the material to be processed is exposed to the impact of cations accelerated by the electric field in the plasma, and thus the surface reactive layer having a lower bond energy is removed by the sputtering effect by ions or the evaporation effect of itself. That is, the reactive ion etching method is a process, which proceeds with a chemical action and a physical action simultaneously. As a result, the selectivity of etching only a specific material, and the anisotropy of vertically etching the surface of the material to be processed can be realized.
However, an effective reactive ion etching method has not been developed for a magnetic material. The wet etching method or the argon ion milling is actually used for a magnetic material, to produce a thin film magnetic head, a magnetic sensor and a microtransformer.
Under the circumstances with respect to a magnetic material, the tendencies of the miniaturization and the high density integration of a magnetic material is considerably delayed, which becomes a difficulty of development of these devices.
The reason why the reactive ion etching of a magnetic material is difficult is as follows. While the magnetic material mainly composed of a transition metal element can react with most of etching gases having been developed for etching semiconductor materials (e.g., CF
4
, CCl
4
, CCl
2
F
2
, CClF
3
, CBrF
3
, Cl
2
, C
2
F
6
, C
3
F
8
, C
4
F
10
, CHF
3
, C
2
H
2
, SF
6
, SiF
4
, BCl
3
, PCl
3
, SiCl
4
, HCl and CHClF
2
), only a reaction product having a far larger bond energy than the reaction product of the semiconductor material is produced. Therefore, it cannot be removed by the sputtering or the evaporation, and etching cannot proceed.
Under the circumstances, an investigation of a novel reactive ion etching process has been investigated that is not analogical inference of the existing technique for semiconductors, and recently a method using a plasma of a mixed gas of carbon monoxide (CO) and ammonia gas (NH
3
) has been developed by the inventor of the invention. The principal of this method is that a carbonyl compound of a transition metal (e.g., Fe(CO)
5
, Ni(CO)
4
, CO
2
(CO)
8
, Mn
2
(CO)
10
, Cr(CO)
6
, V (CO)
6
, Mo(CO)
6
and W(CO)
6
) is formed on the surface of the magnetic material mainly composed of a transition metal element as a material to be processed by an active radial of CO, which is than removed by the evaporation or the sputtering of an ion in vacuum, to proceed etching. The carbonyl compound of a transition metal is the compound having the smallest bond energy among the transition metal compounds. However, since CO is decomposed to CO
2
and C through a disproportionation reaction, an introduced CO gas does not contribute to the reaction, and the free C atom reacts with the transition metal element to form a stable transition metal carbide, and therefore an etching reaction does not proceed in general. The NH
3
gas plays a role of delaying the disproportionation reaction in the presence of the transition metal element, and the objective reactive ion etching can proceed in the plasma of a gas obtained by mixing the substantially same amounts of CO gas and NH
3
gas.
By a method according to this principal, it has been confirmed that magnetic materials, such as a permalloy (Fe—Ni alloy), a Co—Cr alloy and Fe, can be subjected to the reactive ion etching. As a result of the development of such an excellent reactive ion etching method for a magnetic material, further technical development of the method has been expected. However, this method involves a problem in that the etching rate is not so large, for example, 34 nm/min, although working of fine patterns and anisotropic shapes can be realized.
Furthermore, in the conventional etching method using the CO—NH
3
mixed gas plasma, an SiO
2
film produced by a sputtering method has been used as a mask material that is difficult to suffer the etching reaction, and there is a problem in that the working precision and productivity of the SiO
2
film are limited.
FIG. 2
shows a schematic flow diagram of the conventional process.
In step (a) of
FIG. 2
, on a substrate material such as Corning
7059
glass substrate
1
, a ferromagnetic thin film to be processed such as a permalloy (Fe—Ni alloy)
2
is formed by a sputtering method, a quartz (SiO
2
) thin film
3
as a mask material and a conductive material such as an amorphous carbon film
4
are formed in this order by a sputtering method, and a resist
5
as an electron beam-sensitive film is coated, for example, by a spin coating method. The amorphous carbon film
4
is a conductive layer necessary for not charging the material to be processed on electron beam exposure, which becomes necessary since the quartz (SiO
2
) film
3
is an insulating material. In step (b) of
FIG. 2
, a desired pattern is formed on the resist by electron beam writing and development. The amorphous carbon layer is etched by oxygen-ion etching with using the resist pattern as a mask, to expose the SiO
2
film along with the p
Hassanzadeh P.
Mills Gregory
National Research Institute for Metals
Wenderoth , Lind & Ponack, L.L.P.
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