U.S. patent application number 09/738118 was filed with the patent office on 2002-06-20 for micro-electromechanical structure resonator frequency adjustment using radient energy trimming and laser/focused ion beam assisted deposition.
Invention is credited to Cheng, Peng, Ma, Qing, Rao, Valluri.
Application Number | 20020074897 09/738118 |
Document ID | / |
Family ID | 24966645 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074897 |
Kind Code |
A1 |
Ma, Qing ; et al. |
June 20, 2002 |
Micro-electromechanical structure resonator frequency adjustment
using radient energy trimming and laser/focused ion beam assisted
deposition
Abstract
The invention relates to a microbeam oscillator. Tuning of the
oscillator is carried out by addition or subtraction of material to
an oscillator member in order to change the mass of the oscillator
member.
Inventors: |
Ma, Qing; (San Jose, CA)
; Cheng, Peng; (Campbell, CA) ; Rao, Valluri;
(Saratoga, CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
24966645 |
Appl. No.: |
09/738118 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
310/311 |
Current CPC
Class: |
Y10T 29/4916 20150115;
Y10T 29/49105 20150115; H03H 2009/02511 20130101; B81C 2201/053
20130101; H03H 9/02393 20130101; B81C 1/0015 20130101; B81B
2201/0271 20130101; H03H 9/2405 20130101 |
Class at
Publication: |
310/311 |
International
Class: |
H02N 002/00 |
Claims
What is claimed is:
1. A process of forming an oscillator comprising: patterning a
plurality of spaced-apart stacks on an oscillator member; and
removing at least one of the spaced-apart stacks.
2. The process according to claim 1, before removing, further
comprising: determining a first resonant frequency of the
oscillator.
3. The process according to claim 1, before patterning further
comprising: forming a protective layer over the oscillator
member.
4. The process according to claim 1, before patterning further
comprising: forming a protective layer over the oscillator member;
and patterning the protective layer.
5. The process according to claim 1, before patterning, further
comprising: forming a protective layer over the oscillator member;
forming an ablative layer over the oscillator member; and
patterning to form a plurality of spaced-apart stacks.
6. The process according to claim 1, before patterning further
comprising: forming a protective layer over the oscillator member,
wherein the protective layer is selected from a refractory metal, a
refractory metal oxide, a refractory metal silicide, a refractory
metal nitride, and combinations thereof.
7. The process according to claim 1, before patterning further
comprising: forming a protective layer over the oscillator member,
wherein the protective layer is selected from a silicon-containing
composition.
8. The process according to claim 1, wherein removing further
comprises: directing a radiant energy source to at least one of the
spaced-apart stacks, wherein the radiant energy source is selected
from a laser, an ion beam, and combinations thereof.
9. The process according to claim 1, wherein removing is repeated
until an empirical removal pattern is established, further
comprising: determining a second resonant frequency of the
oscillator; and forming the empirical removal pattern upon a second
oscillator.
10. The process according to claim 1, wherein removing further
comprises: selecting at least one spaced-apart stack for removal
based upon a first resonant frequency of the oscillator member and
based upon a respective position of each at least one spaced-apart
stack along the oscillator member, under conditions to approach a
second resonant frequency.
11. The process according to claim 1, further comprising: providing
the oscillator member, wherein the oscillator member is a beam and
wherein the oscillator member has a mass in the range from about
0.1.times.10.sup.-7 gram to about 10.times.10.sup.-7 gram.
12. The process according to claim 1, wherein patterning further
comprises: forming a plurality of spaced-apart stacks, wherein each
of the spaced-apart stacks has a mass in a range from about 0.02%
the mass of the oscillator member to about 2% the mass of the
oscillator member.
13. The process according to claim 1, further comprising:
determining first resonant frequency of the oscillator member; and
after removing, further comprising: determining a second resonant
frequency of the oscillator.
14. The process according to claim 1, wherein the oscillator member
is oscillated while removing.
15. The process according to claim 1, wherein patterning comprises
forming a bulk material on the oscillator member with deposition of
a vapor.
13. A process of forming an oscillator comprising: providing an
oscillator member; determining a first resonant frequency of the
oscillator member; patterning at least one structure on the
oscillator member; and determining a second resonant frequency of
the oscillator member.
14. The process according to claim 13, before patterning further
comprising: forming a protective layer over the oscillator
member.
15. The process according to claim 13, wherein patterning, further
comprising: directing radiant energy at the oscillator member.
16. The process according to claim 13, wherein patterning, further
comprising: directing radiant energy at the oscillator member; and
removing at least one structure from the oscillator member.
17. The process according to claim 13, wherein patterning, further
comprising: directing radiant energy at the oscillator member; and
precipitating a vapor on the oscillator member.
18. The process according to claim 13, wherein the radiant energy
source is selected from a focused ion beam and a laser.
19. The process according to claim 13, wherein patterning further
comprises: continuously monitoring the resonant frequency from the
first frequency to the second frequency by vibrating the oscillator
member.
20. The process according to claim 13, wherein patterning is
repeated to form an empirical spaced-apart stack pattern, further
comprising: determining the second resonant frequency of the
oscillator member; and forming the empirical spaced-apart stack
pattern upon a second oscillator member.
21. A micro resonator comprising: an oscillator member disposed
upon an oscillator pedestal; and at least one structure disposed
upon the oscillator member.
22. The micro resonator according to claim 21, wherein the at least
one structure comprises: a pattern of spaced-apart stacks disposed
upon the oscillator member, wherein the oscillator member has a
mass in a range from about 0.1.times.10.sup.-7 gram to about
10.times.10.sup.-7 gram.
23. The micro resonator according to claim 22, the spaced-apart
stacks further comprising: a protective layer disposed upon the
oscillator member, wherein the protective layer is selected from a
refractory metal, a refractory metal oxide, a refractory metal
silicide, a refractory metal nitride, and combinations thereof.
24. The micro resonator according to claim 22, the spaced-apart
stacks further comprising: a protective pad selected from aluminum,
an aluminum alloy, silver, a silver alloy, indium, an indium
alloy.
25. The micro resonator according to claim 22, wherein the
oscillator member is made of a material selected from polysilicon,
a metal, a metal nitride, a metal oxide, a metal silicide, and
combinations thereof.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to micro
electromechanical structure (MEMS) fabrication and, more
specifically, the present invention relates to the fabrication of a
high frequency beam resonator. In particular, the present invention
relates to frequency adjustment of the high frequency beam
resonator.
[0003] 2. Description of Related Art
[0004] As microelectronic technology progresses, the need has
arisen for smaller and higher frequency resonators for both signal
filtering and signal generating purposes among others. The prior
state of the art used discrete crystals or devices that generate a
surface acoustical wave (SAW) for their desired functions. As
miniaturization of devices progresses, the discrete crystals and
SAW generating devices become relatively larger and therefore much
more difficult to package. For example, discrete devices limit the
size of the overall system to larger configurations and they are
more expensive to produce and to install.
[0005] Once a resonator is fabricated, process variances may cause
a given resonator to have a frequency that is not within preferred
range for a given application. For such out-of-range resonators, if
another use therefor cannot be found, the resonator must be
discarded as a yield loss.
[0006] What is needed is a MEMS resonator that overcomes the
problems in the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] In order that the manner in which the above-recited and
other advantages of the invention are obtained, a more particular
description of the invention briefly described above will be
rendered by reference to specific embodiments thereof, which are
illustrated, in the appended drawings. Understanding that these
drawings depict only typical embodiments of the invention that are
not necessarily drawn to scale and are not therefore to be
considered to be limiting of its scope, the invention will be
described and explained with additional specificity and detail
through the use of the accompanying drawings in which:
[0008] FIG. 1 is an elevational cross-section view that depicts
preliminary fabrication of a MEMS resonator beam according to the
present invention;
[0009] FIG. 2 is an elevational cross-section view of the resonator
beam structure depicted in FIG. 1 after further processing;
[0010] FIG. 3 illustrates further processing of the structure
depicted in FIG. 2;
[0011] FIG. 4 illustrates further processing of the structure
depicted in FIG. 3;
[0012] FIG. 5 illustrates further processing of the structure
depicted in FIG. 4;
[0013] FIG. 6 illustrates further processing of the structure
depicted in FIG. 5;
[0014] FIG. 7 illustrates further processing of the structure
depicted in FIG. 6 after formation of a oscillator member
layer;
[0015] FIG. 8 illustrates a top plan view of the structure depicted
in FIG. 7;
[0016] FIG. 9 illustrates an elevational cross section view of a
cantilever oscillator with patterning for forming spaced apart
stacks;
[0017] FIG. 10 is an elevational cross-section view the structure
depicted in FIG. 9 after the patterning of the protective layer and
an ablation layer;
[0018] FIG. 11 is a top plan view of the inventive structure after
patterning of the protective layer and an ablation layer;
[0019] FIG. 12 is a top plan view of the structure depicted in FIG.
11 after selective removal of a number of the spaced-apart
stacks;
[0020] FIG. 13 is an elevational cross-section view of the
structure depicted in FIG. 12, taken along the cross-section line
13-13 to illustrate the inventive process;
[0021] FIG. 14 is an elevational cross-section view that depicts
alternative processing;
[0022] FIG. 15 is an elevational cross-section view that depicts
alternative processing; and
[0023] FIG. 16 is a process flow chart according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The following description includes terms, such as upper,
lower, first, second, etc. that are used for descriptive purposes
only and are not to be construed as limiting. The embodiments of an
apparatus or article of the present invention described herein can
be manufactured, used, or shipped in a number of positions and
orientation.
[0025] Reference will now be made to the drawings wherein like
structures will be provided with like reference designations. In
order to show the structures of the present invention most clearly,
the drawings included herein are diagrammatic representations of
integrated circuit structures. Thus, the actual appearance of the
fabricated structures, for example in a photomicrograph, may appear
different while still incorporating the essential structures of the
present invention. Moreover, the drawings show only the structures
necessary to understand the present invention. Additional
structures known in the art have not been included to maintain the
clarity of the drawings.
[0026] In a first embodiment, a process of forming a resonator is
carried out by removing discrete amounts of material until a
preferred resonant frequency is established. FIG. 1 is an
elevational cross-section view that depicts preliminary fabrication
of a micro electromechanical system (MEMS) resonator beam according
to the present invention. A substrate 10 is depicted that, in one
non-limiting example is a P-type silicon substrate that has a high
sheet resistance as is known in the art. Upon substrate 10 a pad
oxide 12 is formed that may have a thickness in a range from about
5,000 .ANG. to about 15,000 .ANG., and preferably about 10,000
.ANG. according to this embodiment. Upon pad oxide 12 a silicon
nitride layer 14 is formed. Silicon nitride layer 14 may be
Si.sub.xN.sub.y such as Si.sub.3N.sub.4 or it may be in other
stoichiometric or solid solution ratios. In this embodiment,
silicon nitride layer 14 may be in a thickness range from about 500
.ANG. to about 1,500 .ANG., preferably about 1,000 .ANG.. Silicon
nitride layer 14 may be formed by deposition such as physical vapor
deposition (PVD) or by chemical vapor deposition (CVD). Preferably,
silicon nitride layer 14 is formed by low pressure CVD (LPCVD)
under conditions that are known in the art. Upon silicon nitride
layer 14 a first polysilicon layer 16 is formed. First polysilicon
layer 16 may be formed by CVD, preferably LPCVD under conditions
that are known in the art. First polysilicon layer 16 may be in a
thickness range from about 2,000 .ANG. to about 4,000 .ANG.,
preferably about 3,000 .ANG. according to this embodiment.
Electrical conductivity in first polysilicon layer 16 may be
achieved by ion implantation in order to obtain a preferred sheet
resistance. Alternatively, doping may be in situ during CVD or
LPCVD formation of first polysilicon layer 16.
[0027] FIG. 2 illustrates the result of a first mask process to
define a bottom electrode. First polysilicon layer 16 has been
segmented into pedestals 18 and a bottom electrode 20, also
referred to as the drive electrode 20. Where the first mask process
uses an organic resist, removal of the resist may be carried out by
use of an aqueous sulfuric acid (H.sub.2SO.sub.4) and hydrogen
peroxide (H.sub.2O.sub.2) solution as is known in the art.
[0028] FIG. 3 illustrates the formation of a sacrificial oxide
layer 22. Sacrificial oxide layer 22 acts to support what will be
an oscillator member. A deposition process such as the
decomposition of tetra ethyl ortho silicate (TEOS) may be used, or
other oxide depositions known in the art. In this embodiment the
thickness of sacrificial oxide layer 22 may be in a range from
about 50 .ANG. to about 1,000 .ANG..
[0029] FIG. 4 illustrates the effect of patterning with a second
mask. This process exposes part of pedestal 18 that is used as
anchorage to what will become an oscillator member. In one
variation of this embodiment, where sacrificial oxide layer is
about 100 .ANG., an oxide dry etch is carried out to expose an
upper surface 24 of pedestal 18. In another variation of this
embodiment, where sacrificial oxide layer is about 300 .ANG., an
oxide dry etch is carried out to expose an upper surface 24 of
pedestal 18. Where the second mask process uses an organic resist,
removal of the resist may be carried out by use of an aqueous
sulfuric acid (H.sub.2SO.sub.4) and hydrogen peroxide
(H.sub.2O.sub.2) solution as is known in the art.
[0030] FIG. 5 illustrates the effect of a process that forms a
second polysilicon layer 26 that deposits conformably over any
topology that exists upon substrate 10. Second polysilicon layer 26
may be formed by CVD, preferably LPCVD. The thickness of second
polysilicon layer will be selected based upon a preferred target
frequency of the future oscillator member. In one variation of this
embodiment, second polysilicon layer 26 may have a thickness in a
range from about 500 .ANG. to about 1,500 .ANG., and preferably
about 1,000 .ANG.. In another variation of this embodiment, second
polysilicon layer 26 may have a thickness in a range from about
1,500 .ANG. to about 4,500 .ANG., and preferably about 3,000 .ANG..
In a manner similar to the ion implantation of first polysilicon
layer 16, second polysilicon layer may be doped to a preferred
sheet resistance that will be selected according to a specific
application. Alternatively, doping may in situ during CVD or LPCVD
formation of second polysilicon layer 26.
[0031] During the process flow, it may be preferred to activate any
doping by a thermal treatment. In addition to dopant activation,
stress relief may be achieved in the polysilicon structures.
Thermal treatment may include an anneal as known in the art for
doped and undoped polysilicon structures, or a faster, rapid
thermal anneal (RTA) as known in the art for polysilicon
structures. The specific thermal treatment may be selected
according to a specific oscillator quality, both as to resistivity
and to stiffness.
[0032] FIG. 6 illustrates the effect of processing with a third
mask. The oscillator that is to be formed is patterned from second
polysilicon layer 26. Second polysilicon layer 26 in this
non-limiting embodiment, has been formed by a substantial blanket
deposition of polysilicon. FIG. 6 illustrates the patterning of
second polysilicon layer 26 to remove all but the oscillator member
portion and the pedestal anchorage portion of second polysilicon
layer 26. Accordingly, what may be referred to as an oscillator
member 28 or a top electrode 28 is formed according to a process
that will be further illustrated herein. Etching of second
polysilicon layer 26 may be carried out under conditions known in
the art. One condition is a dry anisotropic polysilicon etch that
may be time dependent and/or that stops on subjacent structures
such as sacrificial layer 22. Where the third mask process uses an
organic resist, removal of the resist may be carried out by use of
an aqueous sulfuric acid (H.sub.2SO.sub.4) and hydrogen peroxide
(H.sub.2O.sub.2) solution as is known in the art.
[0033] After the removal of the third mask, sacrificial oxide layer
22 may be removed as depicted in FIG. 7. In one embodiment,
sacrificial oxide layer 22 is wet etched in an aqueous hydrofluoric
acid (HF) system. Accordingly, the HF system is selective to the
polysilicon structures. Thereafter, the oscillator and substrate
are allowed to dry. Drying may be thermally assisted or it may be
vacuum assisted, or both as is known in the art.
[0034] FIG. 8 is a top plan view of an oscillator bridge 30
according to the present invention. Top electrode 28 is an
oscillator member that spans between two pedestals 18. It can be
seen that drive electrode 20 may have a span beneath top electrode
28 that may vary in size within the dashed area. Additionally,
electrical connection 32 to drive electrode comprises a segment of
first polysilicon layer (FIG. 1).
[0035] According to the present invention, laser tuning of the
inventive oscillator may be accomplished by forming at least one
structure on the oscillator. For example the at least one structure
may be a plurality of spaced-apart stacks. FIG. 9 is an
illustration of a cantilever beam oscillator that may be
manufactured according to the present invention.
[0036] The structures of oscillator pedestal 18 and top electrode
28 may comprise an electrically conductive material. One example of
an electrically conductive material is polysilicon according to the
embodiment set forth herein. The polysilicon is selected from
undoped polysilicon and doped polysilicon, either p-doped or
n-doped. Another example of an electrically conductive material is
a metal such as metals that are typically used in the fabrication
of metallization layers. The metal may be selected from aluminum,
copper, silver, gold, and the like. The metal may also be selected
from titanium, niobium, zirconium, hafnium, and the like. The metal
may also be selected from tungsten, cobalt, nickel, scandium and
others known in the art. Another example of an electrically
conductive material is refractory metal nitrides selected from
titanium nitride, tantalum nitride, tungsten nitride, aluminum
nitride, combinations thereof, and the like.
[0037] According to one embodiment, after the formation of top
electrode 28, and preferably before the removal of sacrificial
oxide layer 22, the entire structure may be treated to make the
resonator structure an integral unit. Where pedestal 18 and top
electrode 28 are polysilicon, treatment may be a rapid thermal
process (RTP) such a heating in an inert environment over a
temperature increase range from about 100.degree. C. to about
2,000.degree. C. and for a process time from about 10 seconds to
about 5 minutes. In order to provide a microfine-grained,
substantially homogenous polysilicon resonator structure that will
resist disintegration during field use, it is preferable to use a
polysilicon composition that has a grain size in a range from about
0.1 micron to about 10 micron and an aspect ratio from about 1:1 to
about 4:1, preferably from about 1.1:1 to about 2:1. Preferably,
the polysilicon is doped by implanting doping elements at the
borders between individual homogenous phases of the
polysilicon.
[0038] Where top electrode 28 and pedestal 18 are made of a metal,
fabrication may be preferably carried out by sputtering. An RTP may
also be carried out to anneal the composite structure. In any
event, the resonant frequency of a beam, bridge or a plate/membrane
is a function of both resonator stiffness and resonator mass.
Accordingly, a preferred resonant frequency, a preferred
oscillation frequency or the like may be achieved in part by
selecting a material according to its known stiffness.
[0039] After the formation of top electrode 28, a protective layer
30 and an ablative layer 32 are formed over oscillator member 28 as
depicted in FIG. 9. A fourth mask 34 is patterned over ablative
layer 32 in preparation for the formation of spaced-apart stacks
that may be selectively removed for oscillator tuning. Protective
layer 30 may act as a diffusion barrier that may be made from
materials such as titanium (Ti), chromium (Cr), silicon (Si),
thorium (Th), cerium (Ce), alloys thereof, combination thereof, and
the like. Metal oxide compounds may be also used such as titania,
chromia, silica, thoria, and ceria. Metal nitride compounds may
also be used such as Ti.sub.xN.sub.y, Cr.sub.xN.sub.y,
Si.sub.xN.sub.y, Th.sub.xN.sub.y, Ce.sub.xN.sub.y, and the like.
Metal silicide compounds may also be used such as Ti.sub.xSi.sub.y,
Cr.sub.xSi.sub.y, Th.sub.xSi.sub.y, Ce.sub.xSi.sub.y, and the like.
In any event the metal oxide, the metal nitride, and the metal
silicide compounds may be provided in both stoichiometric and solid
solution ratios.
[0040] FIG. 10 illustrates cantilever beam oscillator 100 after
further processing. Other oscillator structures may be used such as
microbridge resonators and the like as illustrated in FIG. 8 or the
like, membrane resonators and the like, and other resonators. In
the present invention a cantilever beam oscillator 100 is used to
illustrate the inventive method. FIG. 10 illustrates cantilever
beam oscillator 100 after further processing in which sacrificial
oxide layer 22 has been removed. The removal process may be done by
isotropic etching, preferably by wet etching. Etch selectivity in
the preferable isotropic wet etch is configured to make the etch
recipe less selective to sacrificial oxide layer 22, than to any
and all of substrate 10, drive electrode 20, oscillator pedestal
18, and top electrode 28. The etch recipe selectivity is above
about 20:1, preferably below about 100:1, more preferably below
about 1000:1, and most preferably below about 5000:1. After the
removal process, it is observed that top electrode 28 is disposed
spaced apart from drive electrode 20. Optionally, the removal of
sacrificial oxide layer 22 may precede formation of protective
layer 30 and ablative layer 32, or following removal of ablative
structure 40.
[0041] A plurality of spaced-apart stacks 36 include a protective
pad 38 that is formed from protective layer 30, and ablative
structure 40 that is formed from ablative layer 32. The
spaced-apart stacks 36 are patterned upon a first surface 42 of
oscillator 100. As illustrated in FIG. 10, protective pad 38 was
simultaneously patterned out of protective layer 30, while ablative
structure 40 was patterned out of ablative layer 32. Ablative
structure 40 is preferably made from a material that will vaporize
at the intensities of a focused ion beam (FIB) or a laser.
Protective pad 38 acts to resist damage to upper surface 42 of
oscillator member 28 during removal the ablative structure 40 of
selected spaced-apart stacks 36. The material of protective pad 38
may be selected from a refractory metal, a refractory metal
silicide, a refractory metal nitride, and combinations thereof. For
example a refractory metal silicide may be Ti.sub.xSi.sub.y,
wherein x and y are configure for both stoichiometric and other
solid solution combinations. Alternatively, protective pad 38 may
be selected from a silicon-based composition such as polysilicon
and the like for both doped and undoped polysilicon. Other
silicon-based compositions may include silicon oxide such as
Si.sub.xO.sub.y such as stoichiometric silica and the like in both
stoichiometric and other solid solution combinations. Other
silicon-based compositions may include silicon nitride such as
Si.sub.xN.sub.y for example Si.sub.3N.sub.4 and the like in both
stoichiometric and other solid solution combinations.
[0042] Optionally, protective pad 38 may be patterned through a
negative mask by patterning the mask with a plurality of recesses,
and by successively lining the recesses with protective pad 38,
followed by second filling the recesses with ablative material 40.
Thereafter, a planarization such as chemical mechanical
planarization (CMP) or the like, or a plasma etchback or the like
may be carried out. In order to achieve a structure similar to that
depicted in FIG. 10, the formation of protective pad 38 is
preferably carried out by collimated physical vapor deposition
(PVD). Alternatively, protective layer 30 may be unpatterned such
that the mass thereof is figured into the ultimate frequency of
oscillator 100.
[0043] Removal of selected spaced-apart stacks 36 is carried out by
determining a first resonant frequency of top electrode 28 and
removing at least one of the spaced-apart stacks 36 with a radiant
energy source. The radiant energy source is selected from a laser
and the like, an ion beam and the like, and combinations thereof.
Preferably, the radiant energy source is a laser that may be used
for laser ablation. By removal of the spaced-apart stack 36, it is
meant that ablative structure 40 is removed according to the
present invention, and that protective pad 38 may or may not be
removed in whole or in part.
[0044] In one embodiment of the present invention, the removal of
selected spaced-apart stacks 36, or one of them, is carried out in
a passive or static implementation. In this embodiment, a first
resonant frequency is determined, at least one spaced-apart stack
36 is removed, and second resonant frequency is determined by
vibrating the oscillator 100 after removal of at least one
spaced-apart stack 36. In another embodiment of the present
invention, the removal of selected spaced-apart stacks 36, or one
of them, is carried out in an active or dynamic implementation. In
this embodiment, a second resonant frequency is determined by
monitoring any change in resonant frequency while simultaneously
removing selected spaced-apart stacks 36, one of them, or a portion
thereof where radiant energy controls may be sufficiently
sensitive.
[0045] FIG. 11 is a top plan view of the oscillator 100 depicted in
FIG. 10 to illustrate a pattern formed of the ablative layer 32 and
of spaced-apart stacks 36. During removal of selected spaced-apart
stacks 36, or one of them, an empirical removal pattern is
established upon oscillator 100. FIG. 12 is an illustration of a
removal pattern 44 that may arise from combination of empirical
and/or academic knowledge of a preferred configuration of
spaced-apart stacks 36 that are selected for removal based upon a
delta in the first resonant frequency and a preferred second
resonant frequency. Empirical and/or academic knowledge may then be
applied to a second resonator in the same process batch.
Alternatively, the second resonator may be located in a region in a
second wafer or the like that is likely to have similar process
results. Alternatively, a second resonator may be located on a
second wafer that may have had similar process conditions as the
first resonator. Additionally, a combination of a similar process
wafer and a similar region of a wafer may be combined to select the
second resonator. Additionally, a progressive stepping across a
given wafer may be carried out under conditions that allow for
finite difference tracking of changes in the first resonant
frequency, and stack removal may be adjusted in response previous
empirical data obtained for the given wafer or for a previous wafer
that may have been processed under similar conditions.
[0046] The final frequency of oscillator 100 is based upon the mass
of remaining spaced-apart stacks 36, and a respective position of
each at least one spaced-apart stack 36 along the top electrode 28
that is not removed, under conditions to approach a second resonant
frequency. FIG. 13 is an elevational cross-section view taken along
the line 13-13 from FIG. 12. FIG. 13 depicts the structure of
oscillator 100 after removal of the ablative structure 40 of at
least one spaced-apart stack 36. Preferably, removal of a spaced
apart stack 36 is carried out by directing a radiant energy source
toward a selected spaced-apart stack 36. Because the ablative layer
is now configured as a plurality of discrete ablative structures 40
that make up spaced-apart stacks 36, a radiant energy beam that is
directed toward a selected spaced-apart stack 36 has sufficient
margin for a radiant beam overlap error that is limited to the area
immediately surrounding a given spaced-apart stack 36, without
impinging upon an adjacent spaced-apart stack 36. In this way, a
substantially discrete amount of material, that is a single
ablative structure 40, is removable from upper surface 42 such that
substantially discrete tuning of oscillator 100 may be carried out.
A preferred source of radiant energy is a laser. In the present
invention, the duration and intensity of the radiant energy source
is less effective to remove a discrete amount of material, compared
to the removal of a discrete spaced-apart stack 36 or the ablative
structure 40 portion of a spaced-apart stack 36.
[0047] In one embodiment, oscillator 100 is a beam such as a
cantilever beam. For some applications such as a hand-held
telecommunications use by way of non-limiting example, the mass of
oscillator 100 is in the range from about 0.1.times.10.sup.-7 gram
to about 10.times.10.sup.-7 gram. The process may be carried out in
this embodiment wherein each of the spaced-apart stacks 36 has a
mass in a range from about 0.02% the mass of the oscillator 100 to
about 2% the mass of oscillator 100.
[0048] In one embodiment of the present invention, sacrificial
oxide layer 22 is removed before the selective removal of at least
one spaced-apart stack 36. In a first alternative of this
embodiment, selective removal of at least one spaced-apart stack 36
is carried out in the passive or static mode wherein oscillator 100
is not being tested in motion. In a second alternative of this
embodiment, selective removal of at least one spaced-apart stack 36
is carried out in the active or dynamic mode wherein oscillator 100
is being tested in motion. In either embodiment, intermittent
testing of oscillator 100 may be carried out to achieve a preferred
resonant frequency. In another embodiment, sacrificial oxide layer
22 is removed after the selective removal of at least one
spaced-apart stack 36.
[0049] FIG. 14 illustrates another embodiment of the present
invention. In this embodiment, an oscillator 200 includes a
sacrificial oxide layer 22 to support the oscillator member 28.
This embodiment represents a passive or static tuning of oscillator
200. Bulk material 46 is added to oscillator 200 by the use of a
radiant energy source 48 such as a laser or a focused ion beam
(FIB) in the presence of a deposition vapor. Bulk material 46 acts
to deposit upon upper surface 42 at the conjunction of the
deposition vapor, the radiant energy source 48, and upper surface
42. In this manner, bulk material 46 is added by directing radiant
energy source 48 over a preferred amount of upper surface 42. Bulk
material 46 may be a compound such at SiO.sub.2 formed from the
thermal decomposition of tetraethyl ortho silicate (TEOS). It may
also be a metal such as tungsten (W), chromium (Cr), cobalt (Co),
nickel (Ni), platinum (Pt), alloys thereof, combinations thereof,
and the like. Although not depicted, it is understood that where
necessary to protect oscillator member 28 during the formation of
bulk material 46, a protective layer such as protective layer 30
may be formed upon upper surface 42.
[0050] In another embodiment, the active or dynamic tuning of
oscillator 200 is carried out as illustrated in FIG. 15.
Sacrificial oxide layer 22 has been removed, oscillator 200 is in
motion 50, and radiant energy source 48 is building a bulk material
46 while the frequency of oscillator 200 is being monitored. As the
vapor impinges oscillator 200, the vapor forms condensate and/or a
precipitate that is deposited by such mechanisms as decomposition
of the vapor into an at least in part non-volatile portion. The
conditions that are sufficient to cause the impinging vapor to
deposit to form bulk material 46 may be practiced according to
known methods of laser or FIB deposition techniques. Such
conditions may also be selected from either the preferred CVD
processes or from PVD processes.
[0051] In any event, an empirical process may include determining a
first resonant frequency of oscillator 200, patterning at least one
structure such as ablative structure 40 (subtractive patterning) or
bulk material 46 (additive patterning) on oscillator upper surface
42, and then determining a second resonant frequency of oscillator
200. Alternatively, the inventive method may include continuously
monitoring the resonant frequency of oscillator 200 as it changes
from the first frequency to the second frequency by continuously
vibrating the oscillator 200 while patterning.
[0052] The inventive oscillator is typically a component that may
be placed in an electronic device such as a handheld and/or
wireless device. Such handheld and/or wireless devices may include
a personal data assistant (PDA), a cellular telephone, a notebook
computer, and the like. The inventive oscillator is also typically
placed in an electronic device such as a storage device including a
magnetic storage device and the like where the oscillator may be a
read/write structure.
[0053] FIG. 16 illustrates the inventive process 300. First, an
oscillator is provided 310 that includes an oscillator member. A
first resonant frequency of the oscillator member is determined
320. Next, at least one structure is patterned 330 on the
oscillator member. This patterning is either subtractive, additive
or both. Next, a second resonant frequency of the oscillator member
is determined 340.
[0054] In one embodiment of the present invention, because of both
passive and active patterning techniques, either or both of which
can be additive or subtractive, an oscillator may be tuned to meet
a preferred application. It will become clear that both subtractive
and additive techniques may be combined such that the subtractive
technique acts as a discrete stage tuning where a slight overshoot
may occur, and then the additive technique acts as a continuous
tuning to bring the preferred resonant frequency closer to the
preferred number. Control of the additive technique may be
dominated by the presence and physical state of the deposition
vapor where adjustment of a laser or an FIB may lack the needed
sensitivity to achieve a preferred resonant frequency. In other
words, the subtractive technique approaches a digital adjustment to
a preferred resonant frequency for an oscillator, and the additive
technique approaches an analog adjustment to the preferred resonant
frequency.
[0055] It will be readily understood to those skilled in the art
that various other changes in the details, material, and
arrangements of the parts and method stages which have been
described and illustrated in order to explain the nature of this
invention may be made without departing from the principles and
scope of the invention as expressed in the subjoined claims.
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