U.S. patent application number 15/446631 was filed with the patent office on 2018-09-06 for wide lipseal for electroplating.
This patent application is currently assigned to LAM RESEARCH CORPORATION. The applicant listed for this patent is LAM RESEARCH CORPORATION. Invention is credited to Aaron Berke, Bryan Buckalew, Lee Peng Chua, Santosh Kumar, Robert Rash, Kari Thorkelsson.
Application Number | 20180251907 15/446631 |
Document ID | / |
Family ID | 63357279 |
Filed Date | 2018-09-06 |
United States Patent
Application |
20180251907 |
Kind Code |
A1 |
Thorkelsson; Kari ; et
al. |
September 6, 2018 |
WIDE LIPSEAL FOR ELECTROPLATING
Abstract
A lipseal is designed for use in a lipseal assembly of an
electroplating apparatus wherein a clamshell engages and supplies
electrical current to a semiconductor substrate during
electroplating. The lipseal includes an elastomeric body having an
outer portion configured to engage a cup of the lipseal assembly
and an inner portion configured to engage a peripheral region of
the semiconductor substrate. The inner portion includes a
protrusion having a width in a radial direction sufficient to
provide a contact area with the semiconductor substrate which
inhibits diffusion of acid in an electroplating solution used
during the electroplating. The protrusion is located at an inner
periphery of the lipseal.
Inventors: |
Thorkelsson; Kari;
(Portland, OR) ; Berke; Aaron; (Portland, OR)
; Kumar; Santosh; (Beaverton, OR) ; Rash;
Robert; (West Linn, OR) ; Chua; Lee Peng;
(Beaverton, OR) ; Buckalew; Bryan; (Tualatin,
OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LAM RESEARCH CORPORATION |
Fremont |
CA |
US |
|
|
Assignee: |
LAM RESEARCH CORPORATION
Fremont
CA
|
Family ID: |
63357279 |
Appl. No.: |
15/446631 |
Filed: |
March 1, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 17/001 20130101;
C25D 17/004 20130101; C25D 7/123 20130101 |
International
Class: |
C25D 17/00 20060101
C25D017/00; C25D 7/12 20060101 C25D007/12 |
Claims
1. A lipseal for use in a lipseal assembly of an electroplating
clamshell which engages and supplies electrical current to a
semiconductor substrate during electroplating, the lipseal
comprising an elastomeric body having an outer portion configured
to engage a cup of the lipseal assembly and an inner portion
configured to engage a peripheral region of the semiconductor
substrate, the inner portion including a protrusion having a width
in a radial direction sufficient to inhibit diffusion of acid in an
electroplating solution used during the electroplating, the
protrusion comprising an annular rim which extends completely
around an inner periphery of the lipseal.
2. The lipseal of claim 1, wherein the width is between inner and
outer walls of the protrusion and the width is at least about 0.032
inch.
3. The lipseal of claim 2, wherein the width is about 0.034
inch.
4. The lipseal of claim 1, wherein the outer portion includes a
downwardly extending rim configured to be received in a recess of
the cup.
5. The lipseal of claim 1, wherein an inner surface of the
projection defines an inner diameter of the lipseal.
6. A method of electroplating a semiconductor substrate using the
lipseal of claim 1, comprising supporting a pre-wet semiconductor
substrate in an electroplating clamshell such that the protrusion
of the lipseal contacts an outer periphery of the semiconductor
substrate, and contacting an exposed surface of the semiconductor
substrate inwardly of the protrusion with an electroplating
solution.
7. The method of claim 6, wherein the projection has a width of
about 0.034 inch.
Description
TECHNICAL FIELD
[0001] This invention relates to the formation of damascene
interconnects for integrated circuits, and electroplating
apparatuses which are used during integrated circuit
fabrication.
BACKGROUND
[0002] Electroplating is a common technique used in integrated
circuit (IC) fabrication to deposit one or more layers of
conductive metal. In some fabrication processes it is used to
deposit single or multiple levels of copper interconnects between
various substrate features. An apparatus for electroplating
typically includes an electroplating cell having a pool/bath of
electrolyte and a clamshell designed to hold a semiconductor
substrate during electroplating.
[0003] During operation of the electroplating apparatus, a
semiconductor substrate is submerged into the electrolyte pool such
that one surface of the substrate is exposed to electrolyte. One or
more electrical contacts established with the substrate surface are
employed to drive an electrical current through the electroplating
cell and deposit metal onto the substrate surface from metal ions
available in the electrolyte. Typically, the electrical contact
elements are used to form an electrical connection between the
substrate and a bus bar acting as a current source. However, in
some configurations, a conductive seed layer on the substrate
contacted by the electrical connections may become thinner towards
the edge of the substrate, making it more difficult to establish an
optimal electrical connection with the substrate.
[0004] Another issue arising in electroplating is the potentially
corrosive properties of the electroplating solution. Therefore, in
many electroplating apparatus a lipseal is used at the interface of
the clamshell and substrate for the purpose of preventing leakage
of electrolyte and its contact with elements of the electroplating
apparatus other than the inside of the electroplating cell and the
side of the substrate designated for electroplating.
SUMMARY
[0005] Disclosed herein is a lipseal for use in a lipseal assembly
of an electroplating clamshell which engages and supplies
electrical current to a semiconductor substrate during
electroplating. The lipseal comprises an elastomeric body having an
outer portion configured to engage a cup of the lipseal assembly
and an inner portion configured to engage a peripheral region of
the semiconductor substrate. The inner portion includes a
protrusion having a width in a radial direction sufficient to
inhibit diffusion of acid in an electroplating solution used during
the electroplating. The protrusion comprises an annular rim which
extends completely around an inner periphery of the lipseal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 shows an electroplating apparatus in which a lipseal
as described herein may be used to prevent acid from reaching
contact elements.
[0007] FIG. 2 shows details of a lipseal assembly which can be used
in the apparatus shown in FIG. 1.
[0008] FIG. 3 shows details of the lipseal assembly shown in FIG.
2.
[0009] FIG. 4 shows details of the lipseal assembly shown in FIG.
3.
[0010] FIG. 5 is a graph of acid concentration in lipseal area
versus lipseal width.
[0011] FIGS. 6a-c are photos of the copper seed layer after
electroplating with FIG. 6a showing the copper seed layer after
electroplating a dry wafer, FIG. 6b showing severe corrosion after
electroplating a wet wafer using a 0.028 inch wide lipseal, and
FIG. 6c showing minor corrosion of the copper seed layer after
electroplating a wet wafer using a 0.034 inch wide lipseal.
DETAILED DESCRIPTION
[0012] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
presented concepts. The presented concepts may be practiced without
some or all of these specific details. In other instances, well
known process operations have not been described in detail so as to
not unnecessarily obscure the described concepts. While some
concepts will be described in conjunction with specific
embodiments, it will be understood that these embodiments are not
intended to be limiting.
[0013] An exemplary electroplating apparatus is presented in FIG. 1
in order to provide some context for the various lipseal and
contact element embodiments disclosed herein. Specifically, FIG. 1
presents a perspective view of a wafer holding and positioning
apparatus 100 for electrochemically treating semiconductor wafers.
The apparatus 100 includes wafer-engaging components, which are
sometimes referred to as "clamshell components," or a "clamshell
assembly," or just as a "clamshell." The clamshell assembly
comprises a cup 101 and a cone 103. As will be shown in subsequent
figures, the cup 101 holds a wafer and the cone 103 clamps the
wafer securely in the cup. Other cup and cone designs beyond those
specifically depicted here can be used. A common feature is that a
cup that has an interior region in which the wafer resides and a
cone that presses the wafer against the cup to hold it in
place.
[0014] In the depicted embodiment, the clamshell assembly (which
includes the cup 101 and the cone 103) is supported by struts 104,
which are connected to a top plate 105. This assembly (101, 103,
104, and 105) is driven by a motor 107 via a spindle 106 connected
to the top plate 105. The motor 107 is attached to a mounting
bracket (not shown). The spindle 106 transmits torque (from the
motor 107) to the clamshell assembly causing rotation of a wafer
(not shown in this figure) held therein during plating. An air
cylinder (not shown) within the spindle 106 also provides a
vertical force for engaging the cup 101 with the cone 103. When the
clamshell is disengaged (not shown), a robot with an end effector
arm can insert a wafer in between the cup 101 and the cone 103.
After a wafer is inserted, the cone 103 is engaged with the cup
101, which immobilizes the wafer within apparatus 100 leaving a
working surface on one side of the wafer (but not the other)
exposed for contact with the electrolyte solution.
[0015] In certain embodiments, the clamshell assembly includes a
spray skirt 109 that protects the cone 103 from splashing
electrolyte. In the depicted embodiment, the spray skirt 109
includes a vertical circumferential sleeve and a circular cap
portion. A spacing member 110 maintains separation between the
spray skirt 109 and the cone 103.
[0016] For the purposes of this discussion, the assembly including
components 101-110 is collectively referred to as a "wafer holder"
(or "substrate holder") 111. Note however, that the concept of a
"wafer holder"/"substrate holder" extends generally to various
combinations and sub-combinations of components that engage a
wafer/substrate and allow its movement and positioning.
[0017] A tilting assembly (not shown) may be connected to the wafer
holder to permit angled immersion (as opposed to flat horizontal
immersion) of the wafer into a plating solution. A drive mechanism
and arrangement of plates and pivot joints are used in some
embodiments to move wafer the holder 111 along an arced path (not
shown) and, as a result, tilt the proximal end of wafer holder 111
(i.e., the cup and cone assembly).
[0018] Further, the entire wafer holder 111 is lifted vertically
either up or down to immerse the proximal end of wafer holder into
a plating solution via an actuator (not shown). Thus, a
two-component positioning mechanism provides both vertical movement
along a trajectory perpendicular to an electrolyte surface and a
tilting movement allowing deviation from a horizontal orientation
(i.e., parallel to the electrolyte surface) for the wafer
(angled-wafer immersion capability).
[0019] Note that the wafer holder 111 is used with a plating cell
115 having a plating chamber 117 which houses an anode chamber 157
and a plating solution. The chamber 157 holds an anode 119 (e.g., a
copper anode) and may include membranes or other separators
designed to maintain different electrolyte chemistries in the anode
compartment and a cathode compartment. In the depicted embodiment,
a diffuser 153 is employed for directing electrolyte upward toward
the rotating wafer in a uniform front. In certain embodiments, the
flow diffuser is a high resistance virtual anode (HRVA) plate,
which is made of a solid piece of insulating material (e.g.
plastic), having a large number (e.g. 4,000-15,000) of one
dimensional small holes (0.01 to 0.050 inches in diameter) and
connected to the cathode chamber above the plate. The total
cross-section area of the holes is less than about 5 percent of the
total projected area, and, therefore, introduces substantial flow
resistance in the plating cell helping to improve the plating
uniformity of the system. Additional description of a high
resistance virtual anode plate and a corresponding apparatus for
electrochemically treating semiconductor wafers is provided in U.S.
Published Patent Application No. 2010/0032310, which is hereby
incorporated by reference herein in its entirety for all purposes.
The plating cell may also include a separate membrane for
controlling and creating separate electrolyte flow patterns. In
another embodiment, a membrane is employed to define an anode
chamber, which contains electrolyte that is substantially free of
suppressors, accelerators, or other organic plating additives.
[0020] The plating cell 115 may also include plumbing or plumbing
contacts for circulating electrolyte through the plating cell--and
against the work piece being plated. For example, the plating cell
115 includes an electrolyte inlet tube 131 that extends vertically
into the center of anode chamber 157 through a hole in the center
of anode 119. In other embodiments, the cell includes an
electrolyte inlet manifold that introduces fluid into the cathode
chamber below the diffuser/HRVA plate at the peripheral wall of the
chamber (not shown). In some cases, the inlet tube 131 includes
outlet nozzles on both sides (the anode side and the cathode side)
of the membrane 153. This arrangement delivers electrolyte to both
the anode chamber and the cathode chamber. In other embodiments,
the anode and cathode chamber are separated by a flow resistant
membrane 153, and each chamber has a separate flow cycle of
separated electrolyte. As shown in the embodiment of FIG. 1, an
inlet nozzle 155 provides electrolyte to the anode-side of membrane
153.
[0021] In addition, plating cell 115 includes a rinse drain line
159 and a plating solution return line 161, each connected directly
to the plating chamber 117. Also, a rinse nozzle 163 delivers
deionized rinse water to clean the wafer and/or cup during normal
operation. Plating solution normally fills much of the chamber 117.
To mitigate splashing and generation of bubbles, the chamber 117
includes an inner weir 165 for plating solution return and an outer
weir 167 for rinse water return. In the depicted embodiment, these
weirs are circumferential vertical slots in the wall of the plating
chamber 117.
[0022] As stated above, an electroplating clamshell typically
includes a lipseal and one or more contact elements to provide
sealing and electrical connection functions. A lipseal may be made
from an elastomeric material. The lipseal forms a seal with the
surface of the semiconductor substrate and excludes the electrolyte
from a peripheral region of the substrate. No deposition occurs in
this peripheral region and it is not used for forming IC devices,
i.e., the peripheral region is not a part of the working surface.
Sometimes, this region is also referred to as an edge exclusion
area because the electrolyte is excluded from the area. The
peripheral region is used for supporting and sealing the substrate
during processing, as well as for making electrical connection with
the contact elements. Since it is generally desirable to increase
the working surface, the peripheral region needs to be as small as
possible while maintaining the functions described above. In
certain embodiments, the peripheral region is between about 0.5
millimeters and 3 millimeters from the edge of the substrate.
[0023] During installation, the lipseal and contact elements are
assembled together with other components of the clamshell. One
having ordinary skill in the art would appreciate the difficultly
of this operation, particularly, when the peripheral region is
small. An overall opening provided by this clamshell is comparable
to the size of the substrate (e.g., an opening for accommodating
200 mm wafers, 300 mm wafers, 450 mm wafers, etc.). Furthermore,
substrates have their own size tolerances (e.g., +/-0.2 millimeters
for a typical 300 mm wafer according to the SEMI specification). A
particularly difficult task is alignment of the elastomeric lipseal
and contact elements, since both are made from relatively flexible
materials. These two components need to have very precise relative
locations. When a sealing edge of the lipseal and contact elements
are positioned too far away from each other, insufficient or no
electrical connection may be formed between the contacts and
substrate during operation of the clamshell. At the same time, when
the sealing edge is positioned too close to the contacts, the
contacts may interfere with the seal and cause leakage into the
peripheral region. For example, conventional contact rings are
often made with multiple flexible "fingers" that are pressed in a
spring-like action onto the substrate to establish an electrical
connection as shown in the clamshell assembly of FIG. 2 (note cup
201, cone 203, and lipseal 212). Not only are these flexible
fingers 208 very difficult to align with respect to the lipseal
212, they are also easily damaged during installation and difficult
to clean if and when electrolyte gets into the periphery
region.
[0024] As explained above, in an electroplating cell, electrical
contact is made to the wafer around the wafer edge, and
electroplating is carried out on the rest of the wafer. However, if
the plating solution reaches the contacts, acid in the plating
solution can corrode the metal seed layer on the wafer in the
contact area, resulting in increased resistance irregularly
distributed around the wafer and correspondingly lowering plating
performance and increasing within wafer non-uniformity. Metal ions
in solution can also plate out onto the contacts, reducing plating
efficiency. In order to prevent seed corrosion and plating on the
contacts, the area where contact is made is separated from the
plating solution by a lipseal. Previously, it was thought that,
unless there was gross damage to the lipseal (cracking, tearing,
etc.), this was sufficient to completely isolate the contacts from
the plating solution. However, recent investigation has shown that,
when a wet wafer is placed onto the lipseal (for example, as in a
Sabre 3D advanced pretreatment process), a thin water layer remains
between the lipseal and the wafer that acid in the plating solution
can diffuse through to reach the contact area. At high temperatures
and/or long plating times, this diffusion can occur to the extent
that enough acid reaches the contact area to cause corrosion of the
metal seed layer (seed corrosion). To address this problem, a wider
lipseal has been designed to increase the distance the acid must
diffuse over and correspondingly slow the rate of acid reaching the
contact area. In this way, seed corrosion at the edge of the wafer
is reduced, and plating uniformity is improved.
[0025] In the electroplating cell, the wafer to be plated is held
in a cup, which makes electrical contact with the edge of the wafer
in an area enclosed by a lipseal while exposing the rest of the
wafer to the plating solution. The cup is partially immersed in the
plating solution in the plating cell during plating. However, as
explained above, acid can diffuse across a liquid film between the
wafer and the lipseal fast enough to damage the metal seed layer on
the wafer in the contact area.
[0026] According to an embodiment, a hardware design change has
been implemented to increase the width of the lipseal (the width of
a protrusion on the lipseal which seals against a wafer) to reduce
the rate of acid diffusion through a liquid layer between the wafer
and the lipseal. The increased width of the lipseal increases the
diffusion distance and results in less acid reaching the contact
area and thus less etching of the metal seed layer.
[0027] Lipseal width can be increased by either increasing the
outer diameter of the protrusion of the lipseal, or by decreasing
the inner diameter of the protrusion. The preferred implementation
is to increase the outer diameter of the protrusion, as this does
not reduce the area available for plating.
[0028] The shape of the protrusion on the lipseal can be extended
to form a lipseal with a similarly-shaped cross section as previous
designs, wherein the protrusion is simply elongated in the radial
dimension. This is the preferred implementation. The lipseal can
include a single contact surface in the form of an annular rim with
cylindrical walls and a flat or angled surface which contacts a
wafer.
[0029] Previously, the lipseal design for the Sabre 3D did not
prevent appreciable acid diffusion into the contact area at higher
temperatures or higher plating solution acid concentrations than
present during past standard operating conditions (less than or
equal to 35.degree. C., less than or equal to 140 grams per liter
acid). An advantage of the wider lipseal is that adequate sealing
can be provided in more demanding operating conditions such above
35.degree. C. and/or at higher acid concentrations than 140 grams
per liter acid.
[0030] In processing a wafer, when a wet wafer is placed on a
lipseal, a thin water layer remains and acid can diffuse through
this layer and reach the contact area and attack the metal seed
layer on the wafer. To avoid this problem, the lipseal is
configured to provide a longer diffusion path for the acid and thus
substantially increase the amount of time needed for etching to
occur in the contact area. A longer diffusion path can be achieved
through a wider lipseal (longer linear length).
[0031] Diffusion past the lipseal can be modeled as 1D diffusion
with a constant source, which has the formula: C/C.sub.s=erfc(z/2
{square root over (Dt)}), wherein z is the width of the lipseal, D
is the diffusion constant of the acid, t is time, C.sub.s is the
concentration of the acid at the source, C is the concentration of
the acid at z, and erfc is the complementary error function. Thus,
2 {square root over (Dt)}, the diffusion length, can be estimated
by knowing C, C.sub.s, and z for a given condition, and can be used
to find C as a function of z and C.sub.s and 2 {square root over
(Dt)} can be expected to remain approximately constant as long as
time, temperature, and the diffusing species remains the same.
[0032] For example, consider the case with z=0.020'' and
C.sub.s=180 g/L sulfuric acid: Under these conditions, C is
estimated to be approx. 8-9 g/L after a 1.5 h plating time,
yielding 2 {square root over (Dt)}.apprxeq.0.014'' and 2 {square
root over (Dt)} can be used to estimate post-plating sulfuric acid
concentration at other lipseal widths, as shown in the graph
depicted in FIG. 5. C reaches 1 g/L (approximate level at which
negligible corrosion occurs over 1.5 h) at approximately 0.028 inch
lipseal width, and falls rapidly after that. A preferred lipseal
width is at least 0.032 inch and more preferably at least about
0.034 inch.
[0033] FIG. 3 shows an embodiment of the lipseal 212 mounted on a
cup 201 with electrical contacts 208 engaging an underside of a
semiconductor substrate such as a wafer W. As shown in FIG. 4, the
lipseal 212 includes an inner portion 218 having a protrusion 220
having an upper surface 220a in contact with an underside of the
wafer W and an outer portion 230 having a rim 232 which engages a
recess 201a in the cup 201. The protrusion 220 extends axially
upward and has a width (measured radially between inner and outer
cylindrical walls of the protrusion) sufficient to inhibit
diffusion of acid in the plating solution from reaching the point
of contact between the electrical contacts 208 and the wafer W. For
processing a 300 mm diameter wafer, the width of the protrusion 220
can be at least about 0.032 inch, preferably at least about 0.034
inch. The lipseal 212 is preferably an integral piece made entirely
of elastomeric material which is configured to mate with the cup
201. Thus, the lipseal 212 is a separate consumable part which can
be easily replaced when desired.
[0034] FIGS. 6a-c are photos of the outer edges of wafers which
were processed under different conditions. FIG. 6a shows a wafer
which was plated without pre-wetting and because the lipseal
provided an adequate seal which prevented acid from diffusion past
it, the copper seed layer at the wafer's edge is not corroded. A
scratch is visible on the copper seed layer which resulted from the
electrical contact used during electroplating. FIG. 6b shows a
wafer which was plated with pre-wetting and a 0.028 inch wide
lipseal which did not provide an adequate seal. The pre-wetting
formed a water film between the wafer and lipseal which allowed
acid to diffuse past it and corrode the copper seed layer at the
wafer's edge. In FIG. 6b, the copper seed layer was severely
corroded and only thick copper oxide (black) and tantalum barrier
layer (silver) are visible. FIG. 6c shows a wafer which was plated
with pre-wetting and a 0.034 inch wide lipseal which provided an
adequate seal. Due to the wider lipseal which created a longer
diffusion path, the copper seed layer suffered only minor corrosion
as the copper seed layer is visible and a very thin layer of copper
oxide on the surface of the copper seed layer creates a minor
discoloration in the image.
[0035] In this specification, the word "about" is often used in
connection with numerical values to indicate that mathematical
precision of such values is not intended. Accordingly, it is
intended that where "about" is used with a numerical value, a
tolerance of .+-.10% is contemplated for that numerical value.
[0036] Although illustrative embodiments and applications of this
invention are shown and described herein, many variations and
modifications are possible which remain within the concept, scope,
and spirit of the invention, and these variations would become
clear to those of ordinary skill in the art after perusal of this
application. Accordingly, the present embodiments are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein, but may be
modified within the scope and equivalents of the appended
claims.
* * * * *