U.S. patent application number 10/752416 was filed with the patent office on 2005-07-07 for micromachining by chemical mechanical polishing.
This patent application is currently assigned to Cabot Microelectronics Corp.. Invention is credited to Mikolas, David G., Wylie, Ian W..
Application Number | 20050148289 10/752416 |
Document ID | / |
Family ID | 34711627 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050148289 |
Kind Code |
A1 |
Mikolas, David G. ; et
al. |
July 7, 2005 |
Micromachining by chemical mechanical polishing
Abstract
Methods of micromachining structures on substrates using
chemical mechanical polishing techniques.
Inventors: |
Mikolas, David G.; (Clinton,
IA) ; Wylie, Ian W.; (Naperville, IL) |
Correspondence
Address: |
STEVEN D WESEMAN, ASSOCIATE GENERAL COUNSEL, IP
CABOT MICROELECTRONICS CORPORATION
870 NORTH COMMONS DRIVE
AURORA
IL
60504
US
|
Assignee: |
Cabot Microelectronics
Corp.
|
Family ID: |
34711627 |
Appl. No.: |
10/752416 |
Filed: |
January 6, 2004 |
Current U.S.
Class: |
451/41 ;
451/47 |
Current CPC
Class: |
B24B 37/042
20130101 |
Class at
Publication: |
451/041 ;
451/047 |
International
Class: |
B24B 001/00 |
Claims
1. A method for micromachining a structure, said method comprising
selectively removing at least a portion of the structure by
chemical mechanical polishing, wherein the structure thus formed is
at least partially non-planar and wherein the structure includes a
highest point and a lowest point, and has a height differential
between the highest point and the lowest point of 0.5 microns or
greater.
2. The method of claim 1 wherein the structure is formed on an
essentially planar substrate.
3. The method of claim 1 wherein said chemical mechanical polishing
step is conducted using a chemical mechanical polishing apparatus
that includes a polishing pad.
4. The method of claim 3 wherein said removal is by a combination
of chemical etch and mechanical polishing.
5. The method of claim 4 wherein said mechanical polishing is
controlled by varying at least one characteristic of the polishing
pad.
6. The method of claim 5 wherein said characteristic of the
polishing pad is stiffness.
7. The method of claim 6 wherein said stiffness is manipulated by
downforce on the pad, rotational velocity of the pad, acceleration
velocity of the pad, local curvature of the pad, or combinations
thereof.
8. The method of claim 1 wherein a concave structure is formed.
9. The method of claim 1 wherein a convex structure is formed.
10. The method of claim 1 wherein a rounded structure is
formed.
11. The method of claim 7 wherein local curvature on the pad is
provided be by pre-shaped asperities.
12. The method of claim 7 wherein local curvature on the pad is
provided by bumps under the pad.
13. (canceled)
14. The method of claim 1 wherein the height differential between
the highest point and the lowest point is 1 micron or greater.
15. The method of claim 1 wherein the height differential between
the highest point and the lowest point is 2 microns or greater.
16. A partially non-planar structure fabricated by the method of
claim 1.
17. A microlens array fabricated by the method of claim 1.
18. An optical fiber array connector fabricated by the method of
claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to micromachining techniques. More
particularly, the invention relates to methods for micromachining
substrates into defined structures using chemical mechanical
polishing.
[0003] 2. Description of the Related Art
[0004] Chemical mechanical polishing (CMP) is used in the
integrated circuit industry for planarizing substrates. In a
typical CMP process, the substrate is placed in direct contact with
a rotating polishing pad. A carrier applies pressure against the
backside of the substrate. During the polishing process, the pad
and table are rotated while a downward force is maintained against
the substrate back. An abrasive and chemically reactive solution,
commonly referred to as a "slurry," is applied to the pad during
polishing. The slurry initiates the polishing process by chemically
reacting with the film being polished. The polishing process is
facilitated by the rotational movement of the pad relative to the
substrate as slurry is provided to the substrate/pad interface.
Polishing is continued in this manner until the desired film is
removed.
[0005] A substantial amount of effort is directed in the industry
at maximizing the planarity of substrates being polished by CMP,
and minimizing the formation of non-planar features, such as corner
rounding, and dishing. As a result, CMP has not been considered by
the industry as a micromachining tool.
[0006] Current micromachining methods have undesirable attributes.
For instance, binary optics prepared by current methods typically
radiate substantial power in undesired orders, and have
polarization dependent phase shifts. Consequently, new and improved
micromachining methods are needed for optics fabrication and
fabrication of other structures, such as mechanical devices,
imagers, and displays.
SUMMARY OF THE INVENTION
[0007] The invention relates to micromachining methods using CMP.
Thus, in a first aspect the invention provides a method for forming
a substrate into a defined structure, said method comprising
selectively removing at least a portion of the substrate by
chemical mechanical polishing to provide the defined structure,
wherein the defined structure is at least partially non-planar.
[0008] The invention also provides structures and devices formed by
the methods described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates the formation of a beginning structure
using a deformed polishing pad.
[0010] FIG. 2 illustrates a polishing sequence that includes an
initial step of high pad acceleration followed by subsequent steps
of constant acceleration.
[0011] FIG. 3 illustrates the polishing of a feature under
conditions of varying downforce.
[0012] FIG. 4 illustrates polishing of a feature by a pad having
local curvature.
[0013] FIG. 5 illustrates polishing of a feature by a pad having
asperities on its surface.
[0014] FIG. 6 illustrates the use of a buffer material in a
polishing process.
DESCRIPTION OF THE CURRENT EMBODIMENT
[0015] This invention relates to a process for micromachining
substrates into defined structures, using chemical mechanical
polishing (CMP). Specifically, CMP is used to form structures that
are at least partially non-planar. Examples of such partially
non-planar structures include, but are not limited to, rounded
structures, convex and concave shapes, and combinations thereof, as
well as more complex structures. Such non-planar structures are
generally functional and find use in many applications, including
photonics, such as microlens arrays and optical fiber array
connectors, mechanical devices, imagers, displays, and so on. In
preferred aspects, the height differential between the highest
point and the lowest point on the surface of non-planar structures
manufactured according to this invention are 0.5 microns or
greater. In other preferred aspects, the height differential is 1
micron or greater. In further preferred aspects, the height
differential is 2 microns or greater.
[0016] CMP is a polishing process that exploits both chemical and
mechanical polishing techniques for the polishing of a surface.
Generally, the substrate is placed in direct contact with a
polishing pad, and the pad, substrate, or both, are rotated while a
downward force is maintained against the substrate back. A slurry,
representing the chemical aspect of CMP, is applied to the pad
during polishing. Alternatively, an abrasive containing polishing
pad may, be used in which case the chemical ingredients of the
polishing composition are applied to the pad during polishing.
[0017] In the invention, the characteristics of the polishing pad
are varied such that the CMP polish, including the optional use of
a chemical slurry, provides structures that are at least partially
non-planar. The polishing pad characteristics that can be varied
include the stiffness of the pad. Pad stiffness can be manipulated
by several properties of the CMP apparatus, including downforce on
the polishing pad; rotational velocity of the polishing pad; and
acceleration velocity of the polishing pad. In addition, stiffness
is also a function of the pad material. Other pad characteristics
that are advantageously utilized in the invention to form partially
non-planar structures include areas of local curvature on the pad,
areas of local increased or decreased downforce on the pad, and the
presence of asperities on the pad. In addition, the position of the
polishing pad relative to the substrate being polished, can be
exploited to provide structures that are partially non-planar.
[0018] A substrate to be micromachined according to the invention
preferably includes one or more beginning features upon which the
polishing pad acts to effect a change in shape. Such a beginning
feature can readily be prepared by semiconductor techniques known
in the art. The choice of beginning feature will depend in part on
the final geometry that is desired. One example of a useful
beginning feature is a stepped structure. Alternatively, or in
addition, the substrate may include two or more materials in the
same plane that have different properties, such as different
polishing rates, to allow CMP to effect a change in shape.
[0019] The characteristics of stiffness of the polishing pad can be
manipulated to provide rounding, such as corner rounding, of a
feature. To achieve large amounts of corner rounding, it is
desirable to have a pad that is not too stiff because it will
deform more at the edges of a feature. The centrifugal force
experienced by the pad at any point on its surface will determine
the increase in stiffness of the pad (centrifugal force is
controllable by the velocity and/or acceleration of the pad). Since
the centrifugal force on any point on a disk is proportional to the
rotational velocity times the distance from its axis of rotation,
the stiffness will increase as the distance from the axis
increases. The increase in stiffness is approximately proportional
to the square root of the increase in centrifugal force.
[0020] If a deformable pad, such as one made of a polymer with a
significant elastic modulus, and which is not pinned at the back so
it is free to deform, rotates at a small angular velocity it will
tend not to be stiffened. In such a case, the degree of comer
rounding will be great (see FIG. 1, illustrating a beginning
structure 10 being polished by a deformed polishing pad 20). If the
same pad is rotated at a significant rotational velocity, causing
stiffening, the amount of comer rounding will be less. An
adjustment of the relative distance of the feature being polished
from the rotational axis of the pad can achieve the same result as
adjusting the rotational speed of the pad because of the effect
noted above. FIG. 2 illustrates a beginning structure 10 that is
formed, for example, by a polishing sequence that includes an
initial step of high pad acceleration followed by subsequent steps
of constant acceleration.
[0021] The deformity of the pad can also be affected by the
downforce on the pad. If the relative downforce being applied to
the pad is greater it will tend to bend the pad to a greater degree
than if the force being applied is of a lesser amount. A stiffer
pad will deform less for a given applied downforce. FIG. 3
illustrates the polishing of a feature 10 under conditions of
varying downforce. "F=0, F=1" vs. "F=0, F=0.01" refer to the
relative values of "Force" being applied by the relative stiffness
in the pad and the downforce applied during polishing. As discussed
above, the more the pad is deformed, the greater the extent of
corner rounding.
[0022] A further pad characteristic that can be exploited to
produce controlled topography in the polishing surface, is
localized non-planarities, bumps, grooves or other surface features
on the pad itself. In a typical polishing operation, the polishing
surface is moved over the surface of the pad in a circular motion.
Therefore, any surface feature in the pad will affect the material
in a circular fashion. A circular ridge or groove on the pad
surface will tend to produce a linear feature on the polishing
surface. A large ridge or large groove on the pad will tend to
produce a corresponding large ridge or groove on the polishing
surface. Using intermittent ridges or grooves disposed in a
circular manner on pad surface will tend to reduce the rate at
which the material was being removed but will tend to produce the
same feature shape.
[0023] One advantage of pad surface features is the flexibility
that it provides to produce a high degree of local curvature in the
polishing surface. The deliberate juxtaposition of an existing
surface feature on the pad and on the polishing surface can be
utilized to produce a high degree of local curvature and
counter-curvature (concave and convex curves on the surface in
proximity). A cross-section of a surface feature being polished to
produce both of these curves is shown in FIG. 4.
[0024] When material is removed from a step-shaped feature with a
pad it is preferentially removed from the raised comers producing a
bevel or convex shape. As long as the material has a finite removal
rate outside the step shape, which can be increased through the use
of a very compliant pad, there will be a tendency to produce a
counter-curve or concave shape right beside the convex shape. After
the removal of a significant fraction of the material forming the
initial step (probably more than 50%), the total length of the
concave-curved surface will begin to approach the length of the
convex-curved surface.
[0025] It is possible to produce this curve and counter-curve
without a surface feature being present on the pad. However, it is
easier to produce curves with a higher degree of curvature with the
two sets of features on the opposing surfaces being in close
proximity. The surface features in the pad which help to produce
the surface features on the polishing surface should be of the
appropriate size to produce the desired feature dimension.
[0026] Additional layers of material, such as buffer layers, may be
used to underfill or overfill spaces between features on a
substrate, thus further enhancing the CMP process. Such layers are
described in published patent application number U.S. 2003/0136759,
which is incorporated herein by reference in its entirety.
[0027] When used, a buffer layer can assist in the formation
various shapes on a feature, including convex and concave shapes.
FIG. 6a provides one example of the use of a buffer material 50 for
the formation of concave surface on structure 10. When used for the
formation of a concave surface on structure 10, the buffer material
50 should be a material that is removed by polishing at a rate
slower than the rate at which structure 10 material is removed
during the same polishing procedure. The preferential removal of
structure 10 in comparison to buffer material 50 causes the
formation of a concave surface to structure 10 during the polishing
step.
[0028] Alternatively, buffer material can be used for the formation
of a concave surface on a structure. In this embodiment, the buffer
material should be a material that is removed by polishing at a
rate faster than the rate at which the underlying structure
material is removed during the same polishing procedure. The
preferential removal of buffer material in comparison to structure
material causes the formation of a convex surface to the structure
during the polishing step. The use of the two types of buffer
materials (those that are polished more quickly and those that are
removed more slowly) on the surface at the same time can be
utilized to produce very complex shapes in polished materials.
[0029] A further example of the advantageous use of buffer material
for the formation of partially non planar structures is depicted in
FIG. 6B. In this embodiment, the buffer material acts as a stop
layer, protecting any underlying structures from polishing. Thus in
this embodiment, the buffer material 50 should be a material that
is removed by polishing at a rate slower than the rate at which
structure 10 material is removed during the same polishing
procedure.
[0030] In the above embodiments, any buffer material remaining on
the substrate following polishing can be removed by well known
techniques such as chemical etching.
[0031] As noted earlier, it is preferred that all polishing steps
described herein be conducted in the presence of a chemical
polishing composition or slurry. The choice of polishing
composition or slurry is an important factor in the CMP step.
Depending on the choice of ingredients such as oxidizing agents,
film forming agents, acids, bases, surfactants, complexing agents,
abrasives, and other useful additives, the polishing slurry can be
tailored to provide effective polishing of the substrate layer(s)
at desired polishing rates while minimizing surface imperfections,
defects and corrosion and erosion. Furthermore, the polishing
composition may be selected to provide controlled polishing
selectivities to other thin-film materials used in substrate
manufacturing.
[0032] Examples of CMP polishing compositions and slurries are
disclosed, in U.S. Pat. Nos. 6,068,787, 6,063,306, 6,033,596,
6,039,891, 6,015,506, 5,954,997, 5,993,686, 5,783,489, 5,244,523,
5,209,816, 5,340,370, 4,789,648, 5,391,258, 5,476,606, 5,527,423,
5,354,490, 5,157,876, 5,137,544, 4,956,313, the specifications of
each of which are incorporated herein by reference.
[0033] While the present invention has been described by means of
specific embodiments, it will be understood that modifications may
be made without departing from the spirit of the invention. The
scope of the invention is not to be considered as limited by the
description of the invention set forth in the specification and
examples, but rather as defined by the following claims.
* * * * *