Radiant energy – Inspection of solids or liquids by charged particles – Methods
Reexamination Certificate
2001-01-10
2003-09-16
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Methods
Reexamination Certificate
active
06621081
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the use of focused ion beams (FIB) in the preparation of samples comprising discrete regions of heterogeneous materials for viewing with electron microscopy and, more particularly, to the preparation of sections of magnetic heads for viewing with a scanning electron microscope (SEM).
BACKGROUND OF THE INVENTION
A typical prior art head and disk system is illustrated in FIG.
1
. In operation the head
10
is supported by a suspension
13
as it flies above the disk
16
. The magnetic transducer
10
, usually called a “head,” is composed of elements that perform the task of writing magnetic transitions (the write head
23
) and reading the magnetic transitions (the read head
12
). The electrical signals to and from the read and write heads
12
,
23
travel along conductive paths
14
which are attached to or embedded in the suspension arm (not shown). Typically there are two electrical contact pads each (not shown) for the read and write heads
12
,
23
. Wires or leads (not shown) are connected to these pads and routed in the suspension
13
to the arm electronics (not shown). The disk
16
is attached to the spindle
22
that is driven by spindle motor
24
to rotate the disk. The disk
16
comprises a substrate
26
on which a plurality of thin films
21
are deposited. The thin films include ferromagnetic material that is used to record the magnetic transitions in which information is encoded.
The write head
23
portion of the transducer
10
is further illustrated in FIG.
2
.
FIG. 2
is a section view of the write head
23
taken parallel to the air bearing surface which is not shown. The write head
23
includes two pole pieces which are referred to as P
1
31
and P
2
32
and a coil (not shown). To decrease the side writing and, therefore, to reduce the track width, the pole pieces
31
,
32
are shaped into narrow tips at the gap
33
. To be effective the tip of P
1
31
should be very close to the same size as the tip of P
2
32
and should extend up from the larger body of P
1
31
pole piece about 1 to 1.5 times the gap
33
thickness. In one prior art method P
1
31
is deposited first and initially has a broad, flat tip that is subsequently ion milled using the tip of P
2
32
as a mask to form the tip of P
1
31
. U.S. Pat. No. 6,111,724 to Hugo Santini discusses a prior art process for making P
2
32
tips and describes an improvement using a zero—throat—height defining layer.
Regardless of the method used to form P
2
32
, the width of the track written by this type of inductive head
23
is largely determined by the width of the bottom of P
2
32
(P
2
b
). P
2
32
tends to be wider at the top (away from the gap
33
) which creates an additional complication in measuring the width of P
2
b.
It is important to be able to measure P
2
b
with some precision to monitor the manufacturing process. There are numerous variables in the process which affect the formation and shape of P
2
32
including those affecting the photolithography used to define the shapes, the plating process used for depositing the ferromagnetic material, the seed layer removal process and the ion milling used to shape P
1
31
using P
2
32
as a mask. These variables can change from time to time in the manufacturing process and may even vary across a single wafer (not shown).
One prior art method used to measure the width of P
2
b
uses a FIB to cut a section in the write head
23
to expose the tip of P
2
32
, the gap
33
and the tip of P
1
31
. In
FIG. 2
, a thin film layer of protective material
37
such as tungsten (W) or platinum (Pt) is deposited to preserve the P
2
32
outline while a hole (not shown) is being cut. The hole is cut with a perpendicular incidence to expose the section illustrated in
FIG. 2
as a substantially planar sidewall of the hole. An SEM beam is then used at an angle off of perpendicular to image the sidewall. The SEM image thus obtained will contain an image of the P
2
32
and P
1
31
tips. However, this prior art method is deficient in that it fails to yield truly planar side walls since the different materials that make up the sample of the pole piece tips have significantly different FIB etch rates. For example, the head fabrication process typically creates a thin redep layer (not shown) on the sides of P
2
32
which has a higher etch rate than the NiFe which is commonly used for pole piece tips and the tungsten protective material has a lower etch rate than the NiFe. Voids in the tungsten can also contribute to variations in etch rate. Since the FIB has a very small diameter and is rastered, the higher etch rate materials will be cut deeper and the surface will not be planar. This topography can contribute to undesirable contrast. In the samples of pole piece tips, the top of the P
2
32
is several microns higher than the surrounding field region. Since the beam begins to cut on the field at the same time as the top of P
2
, the field region will be greatly recessed relative to the P
2
32
. Therefore, the ending surface will have nonplanarity which follows the P
2
pattern. The nonplanarity of the cut surface obscures contrast in the SEM image at the boundary between the different materials (for example, NiFe and the W) that define the critical dimension to be measured. The nonplanarity introduces undesirable contrast that makes it difficult to measure contrast due to the boundaries between the NiFe P
2
32
and the tungsten coating
37
in the SEM image. Since the width of the NiFe P
2
32
is a critical dimension, it is important to be able to measure it precisely.
Thus, there is a need for an improved process for preparing the pole piece tip samples for imaging.
SUMMARY OF THE INVENTION
The improved process according to the invention prepares samples for imaging by directing the FIB beam so that its incident angle is not parallel to the planar boundaries between various materials including boundary between the protective material deposited over the structure and outside material of the structure. This has the effect of evening out the etch rate, since the majority of the key beamlets cut more than one type of material. The resulting sample surface is easier to interpret than one produced by the prior art since the obscuring effect of curtaining is reduced. This allows greater accuracy of measurement from the image obtained by an SEM. A method according to a preferred embodiment of the invention is used to prepare a P
2
tip so that the bottom width may be more accurately measured. A sample magnetic transducer with the P
2
tip exposed is prepared for imaging by first depositing a protective material such as tungsten or platinum over the P
2
tip (which is a ferromagnetic material such as NiFe). The sides of P
2
tip and the protective material have contact planes which are perpendicular to the general plane of the upper surface of the sample. The sample is positioned in relation to the focused-ion beam so that most of the key beamlets cut at least two materials, for example, tungsten and NiFe, in the planar contact region to reduce the curtaining effect caused by the unequal rates of etching. The focused-ion beam is then used to etch away material to expose a new surface on which the cross section of the P
2
tip is exposed for subsequent imaging.
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patent: 6042736 (2000-03-01), Chung
patent: 6067703 (2000-05-01), Takahashi et al.
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patent: 6111749 (1994-04-01), None
patent: 9186210 (1997-07-01), None
McCaffry and Barna, “Preparation of Cross-sectional TEM Samples”, Microscopy Research and Technique, 1997, 36:362.
R. Alani, “Recent Advances in Ion Milling Techn
Anderson Bruce
International Business Machines - Corporation
Knight G. Marlin
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