Metallurgical apparatus – Means treating solid metal
Reexamination Certificate
2002-04-16
2004-12-21
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means treating solid metal
C219S411000
Reexamination Certificate
active
06833107
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a furnace for heat-treating wafer substrates for forming MR (magnetoresistive) heads, GMR (giant magnetoresistive) heads, MRAM (magnetic random access memory), etc. in a magnetic field in their production processes, and a heat treatment method using such a furnace.
PRIOR ART
A magnetic head generally has a structure in which a plurality of ferromagnetic layers are laminated on a substrate. For instance, the GMR head has a structure comprising non-magnetic insulating layers between ferromagnetic layers. The MRAM head has a structure comprising antiferro magnetic layers, a pinned magnetic layer, a non-magnetic conductive layer and free magnetic layers in this order from the side of a substrate. The pinned magnetic layers are entirely magnetized in one direction.
To magnetize the pinned layer in one direction, it is necessary to heat-treat or anneal a substrate provided with thin magnetic layers in a magnetic field. An oriented magnetic field of 0.5 T (tesla) or more is usually necessary to be applied, and an oriented magnetic field of more than 1.0 T is necessary depending on the materials of the pinned layer. To apply an oriented magnetic field to wafer substrates, a vacuum heat-treating furnace as shown in
FIG. 15
has conventionally been used. This vacuum heat-treating furnace comprises a magnetic field-generating coil
113
equipped with a cooling pipe
112
, a high-frequency coil
114
disposed inside the coil
113
, and a vacuum container
106
for holding a plurality of wafer substrates
110
disposed inside the high-frequency coil
114
.
However, the magnetic field-generating means in this heat-treating furnace with a magnetic field is constituted by an electromagnet having a coil, to which as large electric current as 500-800 A should be supplied to generate a magnetic field of 1.0 T or more, unsatisfactory from the aspect of safety. It also needs a facility for using large electric power, taking large electricity cost for generating a magnetic field, and a large amount of cooling water should be used to remove heat generated by large electric current. Because of these requirements, it suffers from high treatment cost. Further, because there is an extremely large leaked magnetic flux in the above structure, a large vacant space should be kept in addition to a facility space for the sake of safely, and the apparatus should be enclosed by a magnetic body such as iron, permalloy, etc. to prevent influence on ambient electronic equipment, taking the danger to human bodies into consideration.
With a superconductive coil, a magnetic field can be generated without using a large amount of electric power. Though the consumption of exciting current can be made smaller when a superconductive coil is used than when the electromagnet is used, liquid nitrogen or helium should always be consumed to keep superconductivity, resulting in high operation cost. Also, in a system using a superconductive coil, the variation of a magnetic field turns superconductivity to normal conductivity locally, resulting in heat generation in the coil, and if this state were left to stand, the superconductivity of the entire apparatus would be destroyed. Though the superconductive coil can generate as strong a magnetic field as several teslas to several tens of teslas, the range of a strong leaked magnetic field expands in proportion to its magnetic field strength like the electromagnet. Accordingly, it suffers from the problem of a leaked magnetic field like the electromagnet.
What can properly change a magnetic field strength without using exciting current is a Halbach-type magnetic circuit constituted by a combination of a plurality of permanent magnet segments having substantially the same magnetic force with different magnetization directions. For instance, see
Journal of Applied Physics
, Vol. 86, No. 11, Dec. 1, 1999, and
Journal of Applied Physics
, Vol. 64, No. 10, Nov. 15, 1988, and Japanese Patent Laid-Open No. 6-224027.
FIG. 16
shows one example of a Halbach-type magnetic circuit. The circular, Halbach-type magnetic circuit shown in
FIG. 16
is constituted by an inner, ring-shaped, permanent magnet assembly
1
and an outer, ring-shaped, permanent magnet assembly
2
, which are rotatable to each other. When the inner, ring-shaped, permanent magnet assembly
1
and the outer, ring-shaped, permanent magnet assembly
2
are at positions shown in FIG.
16
(
a
), the magnetic field direction of the inner, ring-shaped, permanent magnet assembly
1
is the same as that of the outer, ring-shaped, permanent magnet assembly
2
. Accordingly, there is a synthesized magnetic field having strength and direction shown by the arrow, which is formed by combining a magnetic field generated from the inner, ring-shaped, permanent magnet assembly
1
and a magnetic field generated from the outer, ring-shaped, permanent magnet assembly
2
, in a center hole
20
of the inner, ring-shaped, permanent magnet assembly
1
.
On the other hand, in a state as shown in FIG.
16
(
b
) in which the outer, ring-shaped, permanent magnet assembly
2
has been rotated by 180° from the position of FIG.
16
(
a
), the magnetic field generated from the magnetic circuit of the inner, ring-shaped, permanent magnet assembly
1
is offsetting the magnetic field generated from the magnetic circuit of the outer, ring-shaped, permanent magnet assembly
2
because of opposite magnetization directions. Accordingly, there is substantially no magnetic field in the center hole
20
. Thus, the strength of the magnetic field can be adjusted from substantially zero to maximum by the rotation angle of both rings.
When the articles to be heat-treated are wafer substrates having magnetic resistance layers, as large a magnetic field as 1.0 T or more is usually needed to stably improve the magnetic resistance effect, and the magnetic field should be uniform and in parallel with the magnetization direction of the thin magnetic layers. However, a conventional heat-treating furnace comprising an electromagnet fails to generate a uniform magnetic field in parallel with the thin magnetic layers.
OBJECTS OF THE INVENTION
Accordingly, an object of the present invention is to provide a small, high-safety, high-accuracy, heat-treating furnace with a uniform parallel magnetic field and reduced magnetic field leakage.
Another object of the present invention is to provide a method for heat-treating articles in a magnetic field using such a heat-treating furnace.
SUMMARY OF THE INVENTION
The inventors have found that when a plurality of articles are heat-treated or annealed at a time in a magnetic field, permanent magnets can be used for a magnetic field-generating means by providing a cooling means around a means for heating the articles, and that by using a double-ring-type, Halbach-type magnetic circuit as the magnetic field-generating means, a high-accuracy, uniform parallel magnetic field can be applied to the articles in a radial direction during heat treatment. The present invention has been completed based on these findings.
The first heat-treating furnace with a magnetic field of the present invention comprises (a) a magnetic field-generating means constituted by one ring-shaped, permanent magnet assembly comprising a plurality of permanent magnet segments combined with their magnetization directions oriented such that a magnetic flux flows in a diameter direction; and (b) a heat treatment means disposed in a center hole of the ring-shaped, permanent magnet assembly and comprising a cooling means, a heating means, and a heat-treating container for containing heat-treating holder for holding a plurality of articles to be heat-treated in this order from outside.
The ring-shaped, permanent magnet assembly preferably has an inner diameter of 120 mm or more, an outer diameter of 300 mm or more, and an axial length of 100 mm or more. The ring-shaped, permanent magnet assembly has a shorter axial length as it goes outside in a radius direction.
Each permanent magnet segment constituting the ring-shape
Azuma Yasuyuki
Kuriyama Yoshihiko
Ushijima Makoto
Hitachi Metals Ltd.
Kastler Scott
Sughrue & Mion, PLLC
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