U.S. patent application number 14/885610 was filed with the patent office on 2016-02-11 for getter structure and method for forming such structure.
This patent application is currently assigned to RAYTHEON COMPANY. The applicant listed for this patent is RAYTHEON COMPANY. Invention is credited to Stephen H. Black, Buu Diep, Roland Gooch, Adam M. Kennedy, Thomas Allan Kocian.
Application Number | 20160040282 14/885610 |
Document ID | / |
Family ID | 52682906 |
Filed Date | 2016-02-11 |
United States Patent
Application |
20160040282 |
Kind Code |
A1 |
Gooch; Roland ; et
al. |
February 11, 2016 |
GETTER STRUCTURE AND METHOD FOR FORMING SUCH STRUCTURE
Abstract
A getter structure and method wherein a layer of seed material
is deposited on a predetermined region of a surface of a structure
under conditions to form a plurality of nucleation sites on a
surface of the structure. The nucleation sites have an average
height over the surface area of the predetermined region of less
than one molecule thick. Subsequently a getter material is
deposited over the surface to form a plurality of getter material
members projecting outwardly from the nucleation sites.
Inventors: |
Gooch; Roland; (Dallas,
TX) ; Kennedy; Adam M.; (Santa Barbara, CA) ;
Black; Stephen H.; (Buellton, CA) ; Kocian; Thomas
Allan; (Dallas, TX) ; Diep; Buu; (Murphy,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RAYTHEON COMPANY |
Waltham |
MA |
US |
|
|
Assignee: |
RAYTHEON COMPANY
Waltham
MA
|
Family ID: |
52682906 |
Appl. No.: |
14/885610 |
Filed: |
October 16, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14193437 |
Feb 28, 2014 |
9196556 |
|
|
14885610 |
|
|
|
|
Current U.S.
Class: |
427/597 ;
427/250 |
Current CPC
Class: |
H01L 23/26 20130101;
B32B 2457/00 20130101; Y10T 428/24802 20150115; B32B 2255/205
20130101; B32B 3/10 20130101; C23C 14/28 20130101; G01J 5/045
20130101; B32B 15/04 20130101; C23C 14/06 20130101; H01L 2924/0002
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
International
Class: |
C23C 14/28 20060101
C23C014/28; C23C 14/06 20060101 C23C014/06 |
Claims
1. A method for forming a getter structure, comprising: forming a
plurality of nucleation sites of a seed material on a surface of
the structure; and forming a plurality of getter material members
projecting outwardly from the nucleation sites.
2. The method recited in claim 1 wherein the seed material is
formed on a region of the surface having a predetermined surface
area and wherein the nucleation sites have an average height over
the predetermined surface of less than one molecule thick.
3. The method recited in claim 2 wherein the getter material is
titanium.
4. A method for forming a getter structure, comprising: depositing
a layer of seed material on a surface of a structure under
conditions to form a plurality of nucleation sites on a surface of
the structure; and subsequently depositing a getter material over
the surface to form a plurality of getter material members
projecting outwardly from the nucleation sites.
5. The method recited in claim 4 wherein the deposition is flash
evaporation or electron -beam deposition.
6. The method recited in claim 4 including oxidizing the nucleation
sites prior to the getter material deposition.
7. The method recited. in claim 4 wherein the seed material is
formed on a region of the surface having a predetermined surface
area and wherein the nucleation sites have an average height over
the predetermined surface of less than one molecule thick.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a divisional application of application Ser. No.
14/193,437 filed Feb. 28, 2014 which application is hereby
incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] This disclosure relates generally to getter structures for
vacuum packaged electronic devices and method for forming such
structures.
BACKGROUND
[0003] As is known in the art, in order to maintain a high degree
of vacuum in a sealed vacuum container, such as for example in a so
called a Dewar assembly, a getter has been used to trap gas
molecules that slowly leak through the Dewar assembly seal or
outgas from the container material. Widely used getter materials
include titanium alone, and mixtures of titanium, zirconium,
vanadium, iron, and other reactive metals, which permanently
capture various gas molecules such as oxygen, hydrogen, nitrogen,
methane, carbon monoxide and carbon dioxide that are typically
found in an outgassed vacuum-sealed Dewar assembly. The getter
materials react with these gases to form oxides, carbides, hydrides
and nitrides which are stable at room temperature. Therefore, the
reactions are irreversible and do not involve the risk of future
gas release.
[0004] There are two categories of vacuum getters, Evaporable
getters and non-evaporable getters. Evaporable getters are flash
evaporated in place onto the interior Dewar surface after the Dewar
is sealed. A prime example is the shiny surface seen in a glass
radio or TV vacuum tube. If subsequently exposed to air, the getter
cannot be reactivated. A non-evaporable getter is installed or
deposited in the process of fabricating the device in which it will
serve, and activated by heating it to a high temperature for a
short time. The subject of this application is in the
non-evaporable category.
[0005] Trapping of residual gas molecules in a Dewar assembly has
been achieved by conventional externally fired getters, an example
of which is described in U.S. Pat. No. 5,111,049, inventors Romano
et al. A getter material such as a porous mixture of titanium and
molybdenum powders is placed within an Alloy 42 container, which is
welded onto a tube protruding from the Dewar body, The getter
material is activated by applying heat to the getter container at
about 800 degrees C. for about 10 minutes. However, the externally
fired getter is large and bulky, and must be fabricated external to
the Dewar body. To maintain a high degree of vacuum in a Dewar
assembly that contains a modern planar Infrared (IR) detector
array, which is typically rectangular with dimensions generally on
the order of 0.5 to 2 cm, the use of an externally fired getter
greatly increases the volume and weight of the assembly. Moreover,
the getter material must be located away from the IR detector
array, and external cooling must be applied to the Dewar body to
prevent thermal damage to the detector array and other Dewar
assembly components caused by the heat supplied to the getter. The
mechanical complexity of the getter assembly and the need for an
external cooler for the IR detector array increases the cost of the
IR detector.
[0006] A process for fabricating the vacuum-sealed Dewar assembly
is described in U.S. Pat. No. 5,433,639. However, since the surface
area of the deposited thin film getter is small, the amount of gas
that can be removed by the getter is limited. Because the IR
detectors preferably have a large fill factor which is the ratio of
the detector surface area to the total detector surface area to
increase the effectiveness of detection, the percentage of surface
area upon which the getter material can be deposited is therefore
relatively small.
[0007] As is also known in the art, a conventional uncooled IR
detector array is housed in a vacuum-sealed Dewar assembly with a
planar IR window, usually made of germanium and coated with a
surface coating to improve its IR transmittance. IR radiation
passes through the window and strikes the detector pixels in the
array. Uncooled IR detectors are typically silicon or Vanadium
Oxide microbolometers, which are temperature sensors that detect IR
radiation by heat sensing.
[0008] As is also known in the art, integrating a getter into a
wafer level vacuum packaged (WLVP) device that requires a large
area optical window is very limited in available area to place the
getter. In a wafer level packaged device the getter is usually
vacuum deposited by evaporation or sputtering the getter material
onto the device lid. In an optical device, such as an IR imaging
Focal Plane Array (FPA), the window occupies most of the available
area onto which the getter would be deposited.
[0009] One technique is described in U.S. Pat. No, 5,701,008. As
described therein, an increase in the surface area of getter is
achieved by etching a multitude of trenches to form column-like
protrusions in the cap wafer surface where the getter is to be
placed. The getter is deposited conformally on the convoluted
surface, thereby increasing its surface area by adding a third
dimension to the two-dimensional surface area. The getter is
deposited conformally by evaporation or sputtering onto the walls
of the column-like protrusions as well as the planar horizontal
surfaces. Other attempts involve methods to roughen the surface to
increase the area slightly before depositing a getter.
[0010] As is also known in the art, one method for forming a getter
is to sputter a film comprised of Zr, Ti, Fe and other metals
co-deposited on a substrate.
[0011] As is also known in the art, a deposited vacuum getter is a
structure (usually a thin layer) which is formed by evaporating or
sputtering a layer of material that can react chemically with
residual gas atoms in a vacuum environment to improve the vacuum
quality. The morphology of the getter film is important as it must
have as large an effective surface area as possible, onto which
reactive gas species will be trapped. The gettering area is not
only the geometrical area. Most of the active area is provided by
the voids at the grain boundaries. The growth of deposited films
has been studied extensively, resulting in the well-known Structure
Zone Models (SZMs) of Movchan and Demchishin, and Thornton, see
Handbook of Deposition. Technologies for Thin Films and Coatings,
P. M. Martin, Elsevier, 2009, ISBN 978-0-8155-2031-3. The SZM
models relate film structure to the homologous temperature, defined
as the ratio of film growth temperature to the melting temperature
of the deposited material. A critical factor in the film grain
growth is the mobility of the arriving atom on the substrate
surface. The mobility has a strong dependence on the arriving
energy and surface temperature. Atoms with high mobility (high
energy) will move and agglomerate on the surface and form large
grains. Atoms with low energy will stop sooner and form smaller
grains, resulting in a net larger void space than in a film with
large grains. Thus a film with many small grains is preferred over
one with large grains with void spaces between them. A fast
deposition rate also promotes smaller grains with void spaces in
between grains. The chemical and thermodynamic properties of the
material being deposited also will have an influence on the
resultant grain structure.
[0012] This can be illustrated in FIGS. 1A, 1A'-1C, 1C' for the
high mobility case with low deposition rate, and FIGS, 2A, 2A'-2C,
2C' for the lower mobility case with higher deposition rate. In
FIGS. 1A, 1A' atoms 4 arrive on a surface 3 and move around until
they lose enough energy to stop, or hit the edge of a cluster 6 of
atoms which are the basis for forming a grain. Resultant gains are
large as clusters grow sideways until they cover enough of the
surface 3 to intercept an increasing number of arriving atoms 4 and
start growing upward. The contact boundaries between grains 6
contains the void space 1 responsible for gettering action. In
FIGS. 2C-2C', atoms 4 arrive on surface 3 and move around until
they lose enough energy to stop, or hit the edge of a cluster.
Clusters start growing upward quickly as atoms 4 arrive fast enough
to pile up and quickly cover much of the surface and thus form
small grains with grain boundaries (void space) 1 between them.
[0013] As is also known in the art, the effectiveness of vacuum
deposited getters is strongly dependent upon the deposition method,
deposition conditions, and resultant film morphology and structure.
Vacuum getters for WLP and some other electronic packages consist
of a layer of metal deposited in the package in a way that the
grain structure forms tall columnar structures. The vertical
surfaces between the grains are many times the geometrical area of
the deposited getter and constitute most of the gettering
surface.
SUMMARY
[0014] In accordance with the present disclosure, a getter
structure is provided having: a substrate having a plurality of
nucleation sites formed of a seed material on a surface of the
substrate; and getter material members projecting outwardly from
the nucleation sites.
[0015] In one embodiment, a wafer level vacuum packaged (WLVP)
device is provided having: a first substrate having an array of
detectors thereon; a second substrate vacuum bonded to the first
substrate having a plurality of nucleation sites formed of a seed
material on a surface of the second substrate; and getter material
members projecting outwardly from the nucleation sites.
[0016] In one embodiment, a method is provided for forming a getter
structure. The method includes: forming a plurality of nucleation
sites of a seed material on a surface of the structure; and forming
getter material. members projecting outwardly from the nucleation
sites.
[0017] In one embodiment, a method is provided for forming a getter
structure. The method includes: depositing a layer of seed material
on a surface of a structure under conditions to form a plurality of
nucleation sites on a surface of the structure; and subsequently
depositing a getter material over the surface to form a plurality
of getter material members projecting outwardly from the nucleation
sites.
[0018] In one embodiment, the layer of seed material is deposited
over a region of the surface having a predetermined surface area
and wherein the nucleation sites having an average height over the
predetermined surface area less than one molecule thick.
[0019] In one embodiment, the deposition of the seed material is by
flash evaporation or electron-beam deposition.
[0020] In one embodiment, the method includes oxidizing the
nucleation sites prior to the getter material deposition.
DESCRIPTION OF DRAWINGS
[0021] FIGS. 1A, 1A'-1C, 1C' are sketches showing a process used to
form gettering material according to the PRIOR ART where a
relatively slow deposition is illustrated;
[0022] FIGS, 2A, 2A'-2C, 2C' are sketches showing a process used to
form gettering material according to the PRIOR ART where a
relatively high deposition is illustrated;
[0023] FIG. 3 is a simplified cutaway perspective view of a
wafer-level packaged Dewar assembly for an IR detector array in
accordance with the disclosure;
[0024] FIG. 4 is a simplified plan view of the IR detector array
used in the assembly of FIG. 3;
[0025] FIG. 5 is a flow chart of a process used to form gettering
material according to the disclosure;
[0026] FIG. 6 are sketches showing sequentially from top to bottom,
isometric sketches, on the right, and a side elevation views on the
left, of a process used to form a plurality of gettering material
structures, the structures being disposed on a plurality of
randomly formed nucleation sites according to the disclosure;
[0027] FIG. 6A is a cross section of an exemplary several of the
nucleation sites and the getter material thereof of FIG. 6.
[0028] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0029] Referring now to FIGS. 3 and 4, a Dewar assembly is shown
having a readout integrated circuit (ROIC) substrate 2 of a
semiconductor material, preferably silicon. An IR detector array 14
is positioned on the substrate 2 and includes a plurality of.
individual detector elements, also called pixels, 16. Although FIG.
4 shows only a 5.times.6 rectangular array of detector pixels 16,
it is understood that a typical IR integrated circuit generally
includes a planar IR detector array with up to several hundred by
several hundred pixels 16. In most commercial applications, IR
detectors are usually uncooled and detect the intensity of IR
radiation by sensing increases in temperature which result from the
heat imparted to the detectors by the IR radiation. A typical
example of an uncooled IR detector is a vanadium oxide (VOx)
microbolometer (MB), in which a plurality of individual detectors
are usually formed in an array on the ROIC substrate 2 by
conventional semiconductor manufacturing processes. The MB array
detects IR radiation by sensing the IR-generated heat, and is also
called a focal plane array (FPA) or a sensor chip assembly (SCA).
The substrate 2 is an integrated circuit used to process the signal
produced by the bolometers. In this case the bolometer is a
microbridge resistor that changes its resistance when its
temperature changes. The incoming radiation causes a change in the
temperature of the microbridge. Although other semiconductor
materials such as Si may be used, VOx is a commonly available and
cost effective material that is used in most commercial IR
detection applications.
[0030] The vacuum-sealed Dewar assembly includes a hermetic seal 8
surrounding the IR detector array to seal off the detector array
from the atmosphere. The seal 8 can be, for example, an indium,
gold-tin, or other solder, with the height of the seal precisely
controlled when it is deposited on the substrate 2 or preferably
wafer 10. The seal S supports a second substrate, a cap wafer, here
an IR transparent window 10, here for example, silicon so that with
wafer level packaging the window wafer 10 must have a compatible
thermal expansion coefficient with the FPA wafer which is also
silicon. The wafer 10 includes: a plurality of columnar gettering
material structures 21 (FIG. 6) formed on a predetermined region 20
of the surface of the wafer 10 having a predetermined surface area
in a manner to be described. The inner surface of the cap wafer 10
corresponds to the surface 3 in FIGS. 1 and 2. The location of the
getter area is shown generally in FIG. 3 as 20, and surrounds the
optically transparent IR window. When applied to non-optical WLP
packages, a much larger portion of the cap wafer may be covered
with getter 21.
[0031] More particularly, a flow diagram of a process used to form
the columns of getter material 21 is shown in FIG. 5. Briefly, and
referring also to FIG. 6 and FIG. 6A, the process includes: forming
a plurality of random nucleation sites 24 of a seed material on a
predetermined region on the surface of the cap wafer 10; and
forming a plurality of getter material members 21 projecting
outwardly from the nucleation sites 24. More particularly, the
wafer 10 is loaded into a vacuum deposition chamber. A very thin
first metal layer, for example, chromium, with an average thickness
of less than one molecule (less than a complete monolayer) over the
surface area of the predetermined region, is deposited on a surface
of the wafer 10 to form the nucleation sites 24 (FIG. 6, FIG. 6)
for a subsequent titanium (Ti) deposition. The Ti layer 21
preferentially grows from grain structure defined by the nucleation
sites 24. A requirement for the seed material is that it should
have low arrival energy and low surface mobility on a Si wafer 10
surface in order that it will form many small discontinuous clumps
of molecules; each clump corresponding to one of the nucleation
sites 24. Deposition could be by flash evaporation as by applying a
high current pulse through a wire causing it to melt and a portion
of it to evaporate, or by thermal evaporation of a measured volume
of evaporant, or by shutter controlled electron-beam (e-beam)
having a high shutter speed. Deposition by sputtering in a first
deposition chamber, then transferring to an electron-beam
deposition chamber for the getter deposition may also be used as
air exposure will oxidize the nucleation, sites, and the getter
material (for example, titanium) will adhere to the oxidized
sites.
[0032] The getter material 21 preferentially grows from grain
structure defined by the nucleation sites 24 to heights in the
range of, for example a few thousand Angstroms to a few
micrometers. The effectiveness of a getter material 21 is dependent
on its effective surface area. The grain boundaries increase the
effective area by many times over the geometrical surface and
increased area means improved activation under a given set of
time-temperature conditions. The gettering action works by reacting
Ti with molecules in the vacuum which diffuse into the grain
boundaries.
[0033] The use of materials for the seed material, with higher
melting points than the getter material 21 enhances the formation
of nucleation sites 24. Therefore, the best candidates for a seed
material should have a melting point close to or higher than that
of the melting point of the getter material although other metals
may be used. In one embodiment, the method includes deposition of
the seed material by sputtering, which opens the possibility of
using metals that are difficult to evaporate. If the subsequent
deposition of a titanium getter layer is done by evaporation, the
exposure to air in the transfer between sputtering chamber and
evaporation chamber will cause oxidation of the seed material. This
is not considered degrading to the getter as the Ti will adhere to
metal oxides. The seed material may be, for example, tungsten,
tantalum, titanium-tungsten, vanadium, zirconium, ruthenium,
molybdenum, hafnium, or chromium, Other possible elements include,
for example, silicon, or other metals. The seed material is not
directly involved in the gettering process in which gas atoms are
reactively removed from the vacuum environment,
[0034] Next, a layer of gettering material, such as for example,
titanium having a thickness from about 3000 to more than 10000
Angstroms thick is deposited in-situ over the first metal layer 24,
that is over the nucleation site metal. The titanium grows on. the
nucleation sites into columnar structures having an increasing
getter surface area along its sides.
[0035] After the formation of the getter material on the nucleation
sites, the wafer is removed from the chamber.
[0036] A number of embodiments of the disclosure have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the disclosure. For example, while two different methods
are described to achieve the deposition of the nucleation layer,
other methods may be used. Further, while the structure and method
have been described for a detector array, the method may be applied
to other electronic device structures, such as for example, MEMS
structures. Accordingly, other embodiments are within the scope of
the following claims.
* * * * *