Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-04-19
2004-04-06
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120, C204S298040, C204S298110, C118S504000, C427S569000, C427S248100
Reexamination Certificate
active
06716322
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to depositing a thin film on a substrate. More particularly, the preferred embodiment relates to controlling the deposition of thin films on a substrate during film deposition and controlling outboard shadowing and thus inboard-outboard asymmetry.
BACKGROUND
A variety of deposition techniques are known for depositing thin film material. Such techniques include sputter deposition, ion beam sputter deposition (IBD), and long-throw physical vapor deposition (PVD) systems. PVD is a thin film deposition process in the gas phase in which source material is physically transferred in a vacuum to a substrate without any chemical reaction involved. PVD includes both thermal and e-beam evaporation and sputtering. Additionally, thin films can be deposited using low pressure chemical vapor deposition in which chemical vapor deposition is performed at a pressure below atmospheric pressure.
Many of these deposition processes require deposition of thin films on substrates having particular topographical features that affect the distribution and properties of deposited material across the substrate. For example, lift off deposition processes are used in many important thin film feature fabrication processes, such as in the manufacture of magnetic heads and semiconductor devices. An exemplary substrate
10
, i.e., a wafer, showing layout features
12
thereon is illustrated in FIG.
1
. Notably, layout features
12
are typically fabricated from photoresist, which is selectively removed according to the written pattern after a lift off step. Lift off deposition processes allow definition of a pattern on a wafer surface without etching, and are typically used to define geometry of hard to etch metals, such as gold. In such processes, metal is lifted off in selected areas by dissolving underlying resist.
In a typical IBD process, for example, the substrate
10
is rotated during am deposition about a central axis or center
44
(FIG.
3
). Features
12
on the substrate
10
have an inboard side
22
, which is the side facing toward the center
44
, and an outboard side
24
, which is the side facing away from the center
44
, these sides being illustrated in
FIGS. 2A
,
2
B, and
2
C. As discussed in further detail below, control of the deposition profile on the inboard/outboard sides of a feature is often critical to device performance.
IBD is particularly well suited for lift off processes due to some unique features IBD possesses. The low process pressures and directional deposition are chief among them. These enable the lift-off step to be extremely clean and repeatable down to very small critical dimensions, e.g., for example, less than 0.5 &mgr;m.
In recent years, IBD has become the method of choice for deposition of stabilization layers for thin film magnetic heads because such an application requires a lift off step subsequent to the deposition of the stabilizing material.
In addition to good lift-off properties, IBD films have extremely good magnetic properties. Additionally, in IBD processes it can be very convenient to position system components to optimize the properties of the deposited film and to rotate the substrate to average out certain non-uniformities introduced by the tilting and other process steps.
For most applications, control of the deposited material onto the substrate is needed. In the fabrication of structures in which one axis is much longer than the other, e.g., in optical cross connect micro-electro mechanical systems (MEMS) where there is a very long vertical flap inside a wide trench, deposition control is critical. In particular, without sufficient control of the deposited optically reflecting metal coating, the flap can buckle due to the stress imbalance on the opposite sides of the flap. More generally, various standards relating to material deposition have been developed for the fabrication of semiconductor devices.
Next, variations in the thickness of the thin layer is a common problem in thin film deposition. As known in the art, these variations are exacerbated when, for example, photoresist masks are used in the lift-off steps. Techniques have been developed to control the overall thickness of layers of deposited materials onto the substrate. For example, a flux regulator has been used to help control the overall thickness of deposited thin layers by impeding the path of portions of the sputtered beam.
However, flux regulators have not been used to address problems associated with asymmetry in sidewall profiles. It is desirable to have symmetric profiles of the deposited material across the sidewall of device features on a substrate because otherwise device performance can be severely compromised. For example, in the manufacture of magnetic heads, the symmetry of the profile of the deposited material obtained after the lift-off step is imperative for stable performance of the device. Therefore, ideally, the deposition is controlled to maintain an appropriate profile.
A drawback of previous thin film deposition processes is that they cannot adequately control the profiles on either side of the photoresist, even when known flux regulators are used. One cause of this is the so-called “inboard-outboard” effect. This means that one side of a feature is more heavily coated than the other side, thus creating an asymmetric profile. This effect is a result of the fact that an off-center point on the substrate is bombarded by more atoms incident from the inboard side of the feature than the outboard side, for example, when the center axes of the target and substrate are collinear. This asymmetry is usually most pronounced at the edge of the substrate.
The source of this problem is related to the divergence of the deposition flux. Based on the geometry of the set-up, this divergence causes variations in the beam that impinges upon the substrate. As a result, asymmetric shadowing of the features occurs and creates an asymmetric profile of the deposited material, as shown is in the prior art depictions in
FIGS. 2A-2C
.
FIG. 2A
shows asymmetric deposition
20
and
20
′ on an inboard side
22
and an outboard side
24
, respectively, of a lift-off photoresist feature
12
on a substrate
10
. In this case, the slope of the profile at the inboard side
22
is significantly steeper than the slope of the profile at the outboard side
24
, which can substantially compromise device performance. Again, ideally, these sidewalls are not sloped, i.e., the sidewalls are perfectly vertical for optimum device performance.
FIGS. 2B and 2C
show basic elements that represent actual device features that may be more complicated, e.g., with multiple layers, more complicated topography. The step feature
112
of
FIG. 2B
represents, for example, the contact formed by the leads and the permanent magnet layers on the walls of the MR sensor shown in
FIG. 9
of U.S. Pat. No. 6,139,906 to Hedge et al., the entirety of which is incorporated by reference herein. This is actually just as critical for the device performance as the slope of the deposited film formed with the lift-off mask that is discussed above. Alternatively, the step feature
112
represents the long vertical flap of an optical cross connect MEMS device, in which case without sufficient control of the deposited optically reflecting metal coating. Alternatively, the device may be a laser bar or integrated laser device on a wafer, in which case the sidewalls of feature
112
would reflect the laser facets, and the coating would be a reflective or antireflective coating. Control of such coating thicknesses are critical to the laser performance.
FIG. 2B
depicts asymmetric deposition
120
and
120
′ on an inboard side
122
and an outboard side
124
, respectively, of a permanent photoresist feature
112
on a substrate
110
. In the prior art, typically, the inboard side
122
of the step feature
112
has more material deposited thereon.
The trench feature
212
of
FIG. 2C
is one that is commonly found in microelectronic device manufacturing. When a certain
Devasahayam Adrian J.
Druz Boris
Hayes Alan V.
Hedge Hari
Kanarov Viktor
Veeco Instruments Inc.
VerSteeg Steven H.
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