Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
2001-05-10
2004-11-23
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S314000, C430S319000, C430S296000, C029S603070, C029S603180
Reexamination Certificate
active
06821715
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetoresistive (MR) sensors. More particularly, it relates to a method of making MR sensors having a trackwidth narrower than 0.2 micron.
BACKGROUND ART
Magnetoresistive (MR) sensors for detecting and measuring magnetic fields find many scientific and industrial applications. Prior MR sensors include anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive (GMR) sensors, in which a sense current flows along, or parallel to, planes of the ferromagnetic elements. Prior MR sensors also include magnetoresistive tunnel junction (MTJ) sensors, in which a sense current flows perpendicular to the planes of the ferromagnetic elements through a dielectric barrier. The resistance of a MR sensor depends on the magnetization direction of the sensor. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the sensor, which in turn causes a change in resistance in the sensor and a corresponding change in the sense current or voltage.
The increasing areal density of magnetic storage media requires that the magnetic recording read/write heads be able to operate at ever-decreasing track widths (TW). Both the write element and the magnetic readback sensor of the recording head must be made smaller in order to achieve narrower data tracks. For example, in the highest areal density (~20 Gbit/in
2
) commercial products, the sensor TW, which is defined by optical lithography and ion beam milling, is approaching 0.3 micron. It is envisaged that in order to make heads suitable for recording densities of 100 Gbits/in
2
, the sensor TW will need to be around 0.13 micron.
At present, magnetoresistive (MR) heads are typically made by photolithographically defining the MR sensor from a continuous multilayer thin film. The MR sensor is often defined in two steps, one photolithographic step to define the TW dimension, and one lapping step to define the so-called “stripe height” (SH) dimension.
In the photolithographic patterning of the TW, an undercut resist scheme is necessary for the formation of high quality junctions. The best MR sensors are fabricated using an optical lithography, bilayer resist pedestal technique.
FIGS. 1
a
-
1
e
illustrate the fabrication of contiguous junction hard bias MR sensors using this prior art bilayer resist pedestal technique. As shown in
FIG. 1
a
, a bilayer resist pedestal structure includes an image resist layer
106
on top of an undercut polymer layer
104
. For fabricating a GMR sensor, the bilayer resist structure stands on a GMR layer structure
102
. The bilayer resist structure masks the active sensor region of the GMR layer structure
102
during an ion milling step which defines the sensor trackwidth edges as shown in
FIG. 1
b
. The bilayer resist structure then serves as a liftoff mask for depositing the hard bias layers
108
and leads
110
, which contact the edges of the sensor
102
as shown in
FIGS. 1
c
-
1
d
. As shown in
FIG. 1
d
, a quantity of hard bias material
108
′ and lead material
110
′ is also deposited on the sidewalls and top of resist layer
106
. However, this quantity of material is removed along with the resist layer
106
in a liftoff process described in a later step.
The undercut nature of the bilayer resist pedestal structure facilitates liftoff of the hard bias layers
108
and leads
110
. The undercut also allows superior junctions to be formed between the hard bias layers
108
and the sensor
102
(by minimizing shadow effects from hard bias material
108
′ deposited onto the resist
106
sidewalls and by eliminating the redeposition of milled material from the GMR structure
102
onto the resist
106
sidewalls).
FIG. 1
e
shows the sensor
102
with contiguous hard bias layers
108
and leads
110
after a liftoff process for removing the bilayer resist pedestal structure.
Undercut bilayer resist systems of the type depicted in
FIGS. 1
a
-
1
e
can be fabricated using e-beam lithography rather than photolithography. The present sensor trackwidths of 0.3 micron are already beginning to push the resolution limits of I-line photolithography. Fundamental constraints such as the diffraction limit of light make photolithographically patterning sub-0.2 micron TW sensors with I-line radiation practically impossible. Electron beam lithography has no such resolution limits, which make it an attractive (but by no means the only) choice for patterning ultra-narrow trackwidth MR sensors.
FIGS. 2
a
-
2
b
are schematic diagrams illustrating the top and side views of a bilayer resist pedestal using an e-beam resist chemistry technique. An e-beam sensitive image resist layer
206
is deposited on a resist layer
204
, which cannot be seen in
FIG. 2
a
. The open regions
202
on the image resist layer
206
are formed by exposing those regions to an electron beam and then dissolving the exposed resist in a suitable developer. The undercut is then formed by using an appropriate developer to dissolve the bottom resist layer, where the undercut distance is determined by the develop time.
Despite the high resolution of e-beam lithography, the bilayer resist pedestal technique described above becomes intractable for achieving trackwidths narrower than 0.2 micron. One reason for this is that forming such narrow pedestals requires controlling the resist undercut to a precision of hundredths of a micron. More fundamentally, the bilayer resist pedestal cannot be extended below 0.2 micron because the top resist layer would collapse unless the amount of undercut used in the present bilayer resist pedestal structure were significantly reduced. This is not an option because reducing the undercut would adversely affect the liftoff process and the junction quality. One might imagine that those difficulties could be circumvented by reducing the thickness of the GMR layer, which would allow the thickness and width of the bilayer resist pedestal to be scaled accordingly. This is not an option, though, because significant reduction of the GMR layer thickness is not possible.
U.S. Pat. No. 5,079,035 issued to Krounbi et al. on Jan. 7, 1992, discloses a method for fabricating a magnetoresistive transducer with contiguous junctions between a MR layer and hard bias layers using a bilayer resist pedestal structure as described above. As stated above, the method disclosed by Krounbi et al. cannot fabricate a MR sensor with a trackwidth narrower than 0.2 micron.
A bridge structure is described in an article entitled “Offset masks for lift-off photoprocessing” by G. J. Dolan published on Jun. 21, 1977 in
Applied Physics Letters
. Using photolithography, Dolan fabricated micron-scale, suspended resist structures with micron dimensions in bridge width, bridge height, and in bridge separation from the substrate surface. By using this bridge as a mask for oblique angle thin-film deposition, small-area Josephson Junctions could be fabricated. However, the width of the bridge formed by this technique is 1.5 micron, which is far too large to be used for making MR sensors with narrow trackwidths.
There is a need, therefore, for a resist structure suitable for lithographically patterning MR sensors with trackwidths narrower than 0.2 micron.
SUMMARY
According to an exemplary embodiment of the present invention, a fully undercut resist bridge structure to pattern MR sensors is formed by totally removing the bottom resist layer of a bilayer resist structure in the trackwidth region.
The fully undercut resist bridge structure is formed by using two polymer layers, with only the top polymer layer being sensitive to electron beam exposure and to the e-beam developer. Alternatively, short wavelength radiation (DUV, X-ray, and the like) could also be used to pattern the top polymer layer. In a preferred embodiment, the top polymer layer is made of an e-beam sensitive resist such as polymethyl methacrylate (PMMA). However, this imaging layer could be virt
Emilio Santini Hugo Alberto
Fontana, Jr. Robert Edward
Katine Jordan A.
Liu Jennifer
MacDonald Scott A.
International Business Machines - Corporation
Lumen Intellectual Property Services Inc.
McPherson John A.
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