U.S. patent application number 09/726015 was filed with the patent office on 2001-06-21 for method for hermetically encapsulating microsystems in situ.
Invention is credited to ois Gueissaz, Fran?ccedil.
Application Number | 20010004085 09/726015 |
Document ID | / |
Family ID | 8239618 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004085 |
Kind Code |
A1 |
Gueissaz, Fran?ccedil;ois |
June 21, 2001 |
Method for hermetically encapsulating microsystems in situ
Abstract
The method for hermetically encapsulating microsystems in situ
consists, in a first phase, of mounting on a common substrate (1),
several microsystems (6) surrounded by a metal adhesion layer (4)
deposited on the substrate (1). In a second phase, in a common
deposition step a first metal layer (7) is deposited by
electrolytic means on each microsystem (6) and on an annular zone
(7a) of the adhesion layer (4) surrounding each microsystem (6), so
as to completely cover each microsystem by overlap. Subsequently a
second metal layer (9) is deposited by electrolytic means on the
first metal layer (7) and on the adhesion layer so as to cover most
of the first layer with the exception of at least one passage (10)
per microsystem (6), providing access to the first layer (7). The
metal of the first layer is different from the metals of the
adhesion layer, the second layer and the microsystem. The first
layer (7) is removed by selective chemical etching through the
passages (10) which are closed to obtain metal capsules
hermetically enclosing each microsystem.
Inventors: |
Gueissaz, Fran?ccedil;ois;
(Wavre, CH) |
Correspondence
Address: |
SUGHRUE,MION,ZINN,
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Family ID: |
8239618 |
Appl. No.: |
09/726015 |
Filed: |
November 30, 2000 |
Current U.S.
Class: |
228/124.6 ;
205/125; 216/17; 228/180.22; 228/256; 228/260; 257/E21.499;
257/E23.128; 257/E23.193; 427/96.2; 427/97.2 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 2924/09701 20130101; H01L 2924/16152 20130101; H01L 23/315
20130101; H01L 2924/01079 20130101; H01L 23/10 20130101; H01L
2924/3025 20130101; B81C 1/00293 20130101; H01L 21/50 20130101;
H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
228/124.6 ;
228/180.22; 228/256; 228/260; 427/96 |
International
Class: |
B23K 031/02; B05D
005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 1999 |
EP |
99125008.5 |
Claims
What is claimed is:
1. A method for hermetically encapsulating microsystems in situ
wherein, in a first phase, several microsystems are mounted on a
common substrate, said microsystems being surrounded by a metal
adhesion layer deposited on the substrate, the method wherein, in a
second phase, in a common deposition step a first metal layer is
deposited on each microsystem and on an annular zone of the
adhesion layer surrounding each microsystem, so as to completely
cover each microsystem by overlap, wherein a second metal layer is
deposited by electrolytic means on the first metal layer and on the
adhesion layer so as to cover the first layer over most of its
surface leaving at least one passage per microsystem in the second
layer to provide access to the first layer, the metal of the first
layer being different from the metals of the adhesion layer, the
second layer and the microsystem, wherein the first layer is
removed by selective chemical etching through each passage of the
second layer, and wherein said passages are then closed to obtain
metal capsules hermetically enclosing each microsystem.
2. A method according to claim 1, wherein the first layer is
deposited by electrolytic means.
3. A method according to claim 1, wherein the first layer coating
each microsystem includes at least two openings each arranged
between the microsystem and the corresponding passage of the second
layer, and wherein the second layer extends into each of the
openings as far as the adhesion layer so as to create a support
pillar for the second layer between each passage and the
corresponding microsystem.
4. A method according to claim 1, wherein said passages are formed
by means of two extensions facing the first layer for each
microsystem, said extensions coming out outside the second layer,
the width of which does not vary between said extensions so as to
create a reinforcing part at the passages.
5. A method according to claim 1, wherein the second layer is
deposited so as to enclose the first layer with the exception of a
multitude of passages of reduced dimensions arranged on the top
portions of the second layer above each microsystem.
6. A method according to either of claims 3 and 5, wherein the
passages arranged on the top portions of the second layer are
closed by depositing a drop of solder to be solidified on each
passage or by causing a wave of liquid solder to be solidified to
close all said passages.
7. A method according to claim 1, wherein the passages are closed
by heating and compressing portions of the second layer around each
passage and by soldering them to the adhesion layer.
8. A method according to claim 7, wherein the adhesion layer
includes bumps of a soldering metal arranged on a metal base layer
of said adhesion layer and at the location of the passages of the
second layer so as to close said passages when the portions of the
second layer are thermocompressed.
9. A method according to claim 1, wherein the second layer rests on
the soldering metal bumps of the adhesion layer distributed over
the perimeter of each annular zone surrounding the corresponding
microsystem, and wherein a portion of the first layer is arranged
between neighbouring metal bumps to come out outside the second
layer and to define several passages of the second layer per
microsystem.
10. A method according to claim 9, wherein the passages for each
microsystem are closed by heating the soldering metal bumps to
cause the second layer to be lowered onto a base layer of the
adhesion layer and to hermetically seal the capsules onto the
microsystems.
11. A method according to claim 10, wherein guide elements for the
capsules are secured to the adhesion layer before being coated by
the first layer, said elements remaining after removal of the first
layer and being used to guide the capsules vertically when the
second layer is lowered to close the passages.
12. A method according to claim 1, wherein the metal of the first
layer is copper or a copper alloy, and in that the metal of the
second layer is gold or a gold alloy.
13. A method according to claim 1, wherein the metal base layer of
the adhesion layer is made of a first metal fixing layer on the
substrate made of titanium or chromium, a second metal layer made
of nickel or palladium or rhodium or ruthenium or molybdenum or
platinum as a diffusion barrier for the solder, and a third metal
layer made of gold as oxidation protection.
14. A method according to claim 1, wherein a third metal layer is
deposited on the second layer without closing the passages of the
second layer, wherein a fourth metal layer is deposited on the
third layer in order to completely enclose the third layer between
the second layer and the fourth layer closing obstructing the
passages of the second layer, the metal of the second layer being
the same as the metal of the fourth layer, and wherein the first
layer is removed by selective chemical etching through each
passage.
15. A method according to claim 1, wherein, prior to mounting the
microsystems, conductive strips for the external electric
connection of each microsystem are made on the substrate, wherein
an insulating layer is deposited on the median portion of the
length of the strips, leaving their ends free for an electric
connection, and wherein the adhesion layer is then deposited so as
to pass over the insulation of the strips and structured so as to
define metal paths connected to one of the ends of the conductive
strips, the other end of the strips being connected to the
corresponding microsystem.
16. A method according to claim 1, wherein, prior to mounting the
microsystems, a series of conductive holes are made through an
insulating part of the substrate for the external electric
connection of each microsystem, and wherein metal pads are
connected to the conductive holes on the surface of the substrate
opposite the microsystems.
17. A method according to claim 1, wherein after closing the
capsules, they are covered with a protective layer of resin.
18. A method according to claim 1, wherein after closing the metal
capsules, the substrate is cut or diced in order to separate each
encapsulated microsystem having metal contact pads which are
accessible from the exterior.
Description
[0001] The present invention concerns a method for hermetically
encapsulating microsystems in situ. At least one microsystem
mounted on a substrate is encapsulated under a metal capsule made
in situ. "Mounted" means either placing the microsystem, made
beforehand, on the substrate, or making the microsystem in situ on
the substrate. Preferably, several microsystems of micrometric
dimensions are manufactured together on the same substrate. The
encapsulation enclosing the microsystem must be sealed hermetically
and leave said microsystem free of movement inside the capsule.
[0002] "Microsystems" means three-dimensional structures, i.e.
microoptoelectromechanical devices (MOEMS) or
microelectromechanical devices (MEMS) such as reed contactors,
accelerometers, micromotors, sensors of micrometric size, which
need to be left free to move after encapsulation. The construction
of said microsystems can be made on an insulating substrate or on a
substrate comprising integrated circuits which have been made
beforehand. In this latter case, it is possible to use the metal
contact pads of the integrated circuit to begin depositing the
metal layers which will form part of the microsystem and to allow
it to be electrically connected to said circuit.
[0003] In Swiss Patent No. 688213 by the same Applicant, a reed
contactor or contactor with strips of micrometric size and the
manufacturing method thereof. The contactor comprises metal strips
at a distance from each other in the rest state which are made by
electrolytic means in several steps and are attached to a base
plane. The strips are formed of an iron and nickel alloy, deposited
by an electrolytic method. This alloy has the property of being
ferromagnetic so that the strips are able to be put in contact with
each other when a magnetic field passing through them creates a
force of attraction there between. This contactor is encapsulated
under a hollow cover which is fixed for example using an epoxy
adhesive material onto the base plane. The latter may be a glass
substrate or an insulating layer obtained by oxidising the surface
of a silicon substrate. The cover is formed of a glass plate in
which cavities are formed by chemical etching. This plate allows
each contactor to be enclosed in each of the etched cavities. The
plate may be bonded onto the base plane or soldered by an eutectic
or anodic solder. In a final operation, the multitude of contactors
thereby made and sealed are separated by cutting or dicing
operation.
[0004] In this type of embodiment, it is necessary to machine the
glass plate separately from the substrate on which the contactors
are manufactured. This constitutes a drawback. Moreover, the plate
has to be bonded precisely onto the base plane using an epoxy
adhesive material. The sealing is not hermetic over the long term,
since the epoxy resin absorbs water and degasses substances capable
of disturbing the operation of the contactor. In other embodiments,
an heat treatment for encapsulating the contactor can be
destructive.
[0005] In Swiss Patent No. 688213 it will also be noted that during
contact resistance measurements between the metal strips, prior to
encapsulation of the contactors, the contact resistance average of
all the contactors made on a same substrate was centred around 10
ohms. After said encapsulation, this contact resistance average was
measured rising to 10 to 60 ohms.
[0006] European Patent No. 0 302 165 discloses a sheet of tin which
is formed by stamping to act as the metal dome for an integrated
circuit. This stamped sheet is then bonded onto a base plate where
the integrated circuit is placed so as to close said circuit under
the dome. The whole assembly is subsequently coated with a layer of
polyethylene. The adhesive material, as explained above, can cause
contamination of the microsystem. Consequently it does not allow
hermetic encapsulation to be guaranteed. It is also not possible to
design the dome in situ by stamping. Moreover, making these stamped
sheets, which have to be individually placed on each microsystem,
complicates the encapsulation of several microsystems mounted on a
same substrate.
[0007] In the field of combined micromechanical and electronic
devices, the use of sacrificial layers is already known. One can
cite the case in which one wishes for example to make a metal
bridge between an integrated circuit and a sensor. On the other
hand in the case of making an hermetic metal encapsulation for
microsystems, the use of sacrificial layers is not known.
[0008] U.S. Pat. No. 5,798,283 discloses a method for manufacturing
at least one microelectromechanical device with an electronic
circuit. A cavity is etched in the substrate for example made of
silicon in order to house therein the micromechanical device. The
latter is constructed using different layers of polysilicon in
order to obtain elements able to be free of movement. The device
has to be protected using layers of silicon oxide or nitride so
that the subsequent steps for making the integrated circuit can be
performed. This protection of the micromechanical device is
necessary to protects it against dopant diffusion temperatures
(boron, phosphorus for example) which can be higher than
700.degree. C. Such temperature can partly destroy the elements of
said micromechanical device designed with certain metals with a low
melting point. Such protecting layers also allows to avoid doping
said elements if polysilicon is involved to be avoided.
[0009] Once the integrated circuit operations are finished, two
openings arranged in a protective layer disposed above layers of
SiO2 or Si3N4 allow said layers of SiO2 or Si3N4 to be partly
removed by chemical etching. That allows thus to release the
micromechanical device and to leave it free of movement. During
such removal, precautions must be taken to avoid too great a
lateral etching, because the integrated circuit is constructed
beside the micromechanical device.
[0010] Instead of making two openings in the protective layer, it
might have been envisaged to use only one layer of porous
polysilicon in order to remove the layers of SiO2 or Si3N4 by
chemical etching, in particular using fluorohydric acid, through
the polysilicon, and then to rinse with deionised water.
[0011] Several drawbacks of said method from this document can be
cited. Firstly, the encapsulation is made using non-metallic
layers. Moreover, a cavity has to be arranged beforehand in the
substrate to house therein the microsystem by etching techniques
similar to those used in the microelectronic field. The microsystem
has also to be protected while the corresponding integrated circuit
is being made with layers which can withstand high temperatures.
Consequently, there is no question of depositing metal layers in
particular by electrolytic means on said micromechanical device to
create an hermetic metal encapsulation.
[0012] European Patent No. 0 435 530 discloses an electronic system
hermetically sealed by metal layers one of which is deposited by
electrolytic means. The electronic system is an association of
different integrated circuits, with high density interconnection
(HDI). These circuits are housed and bonded using polymers in a
cavity micro-machined in a glass or ceramic substrate. A first
metal layer, in particular made of chromium or titanium, is
sputtered onto a dielectric layer which overhangs the
interconnections made for the different circuits. This first layer
allows to coat the entire structure and to come into contact with
the surface of the substrate. Subsequently, a second metal layer is
deposited by electrolytic means above the first layer in order to
create a thicker protective layer against various contaminating
elements able to disturb the circuits.
[0013] European Patent No. 0 435 530 provides no teaching for
making an encapsulation for microsystems, such as reed type
contactors. One drawback is that the polymers used to bond the
circuits, produce gases, i.e. degas. That thus creates defects
which will be noticeable as regards the proper operation of the
contactor. Moreover, it is to be noted that creating a metal
capsule via a sacrificial metal layer removed after deposition of a
subsequent metal layer forming the capsule, is not envisaged.
[0014] One object of said invention is to provide an hermetic
encapsulation in situ for microsystems which overcomes the
drawbacks of the aforecited prior art.
[0015] Another object of the present invention is to be able to
make a metal capsule via electrodeposition of metal layers for
encapsulating microsystems at temperatures lower than 350.degree.
C. maximum. This overcomes the drawbacks of methods of prior art
wherein, in particular, the diffusion of phosphorus or boron for
integrated circuits occurs at temperatures exceeding 700.degree. C.
and able even to reach 1300.degree. C.
[0016] Another object of the invention is to avoid a large
dispersion of contact resistance values after hermetic
encapsulation. The microsystem can be a contactor which has to be
in an inert or reducing atmosphere.
[0017] These objects, in addition to others are achieved as a
result of the method for hermetically encapsulating microsystems in
situ wherein, in a first phase, several microsystems are mounted on
a common substrate, said microsystems being surrounded by a metal
adhesion layer deposited on the substrate, the method being
characterised in that, in a second phase, in a common deposition
operation a first metal layer is deposited on each microsystem and
on an annular zone of the adhesion layer surrounding each
microsystem so as to completely cover each microsystem by overlap,
in that a second metal layer is deposited by electrolytic means on
the first layer and on the adhesion layer so as to cover the first
layer over most of its surface leaving at least one passage per
microsystem in the second layer to provide access to the first
layer, the metal of the first layer being different from the metals
of the adhesion layer, the second layer and the microsystem, in
that the first layer is removed by selective chemical etching
through each passage arranged in the second layer, and in that each
passage in the second layer is closed or sealed to obtain metal
capsules hermetically enclosing each microsystem.
[0018] One advantage of the method of the invention consists in
making an hermetic metal encapsulation using means which allow
simultaneously processing of substrates on which several
microstructures have been mounted. The microstructures are made for
example in situ onto the substrate. However, they can be made too
beforehand and placed after onto the substrate.
[0019] Another advantage of the method of the invention lies in the
fact that the metal capsule made on the substrate and enclosing the
microsystem is held without the use of adhesive materials. Said
adhesive materials may contain polymers capable of degassing
contaminating elements inside the metal capsule, liable to disturb
the microsystem.
[0020] The creation of a metal encapsulation for microsystems using
depositions of metal layers has thus been envisaged. One of metal
layers acts as a sacrificial layer. Moreover, at least the final
metal layer is deposited by electrolytic means on a metal adhesion
layer which adheres well to the insulating surface of the
substrate.
[0021] In order to make this capsule, a first metal layer, called
the sacrificial layer is deposited, preferably by electrolytic
means, onto the whole of the microsystems and onto annular zones of
the adhesion layer surrounding each microsystem. The first layer
allows to completely cover each microsystem by overlapping. After
this first metal layer has been deposited, the covered microsystems
have a dome shaped appearance. A second metal layer is then
deposited by electrolytic means onto the first layer, said second
layer having passages providing access to the first layer.
[0022] The first metal layer is formed of a different metal to the
metals forming the second layer, the adhesion layer and also the
microsystem. This first layer is able to act as the sacrificial
layer to be removed selectively by chemical etching through at
least one passage made in the second metal layer in order to make
the metal capsule. In a final encapsulation step, it is necessary
to close or seal the passage or passages made in the second layer
in order to hermetically close the capsule while keeping inside the
capsule the microsystem in an inert or reducing atmosphere.
[0023] "Metal" also includes all the metal alloys depending on a
particular metal.
[0024] This electrodeposition technique allows high quality
encapsulation of microsystems at a low cost.
[0025] Another advantage of the method of the invention is that it
avoids having to protect the microsystem for the subsequent
manufacture of the integrated circuit arranged next to it as
described in U.S. Pat. No. 5,798,283. In the case for example of a
microcontactor, these encapsulation steps even occur at the ambient
temperature.
[0026] In a preliminary phase of the method, one may for example
form on a substrate, of which at least one surface is insulating,
conductive strips for the electric connection of the microsystem
with the exterior. An insulation of the median portion of the
strips is then made. Moreover, a surface metallisation connects one
end of the strips and also passes above the insulation of the
strips. Also in this first phase of the method, the microsystem to
be encapsulated is mounted on the substrate. In a second phase, the
metal capsule is formed with the closing of its orifices. The
substrate may be cut subsequently to obtain a multitude of
encapsulated microsystems.
[0027] The invention will be better understood with reference to
the drawings showing non limiting embodiment examples of the method
of the invention in which:
[0028] FIG. 1a shows the first step of the method according to the
invention with a portion of a substrate on which conductive strips
with insulation, an adhesion layer and a microsystem have been
made;
[0029] FIG. 1b shows the first step of the method according to the
invention with a portion of a substrate on which conductive strips
with insulation, an adhesion layer with solder bumps and a
microsystem have been made;
[0030] FIGS. 2a and 2b show a top view and a cross-section along
II-II of FIG. 2a after the deposition of a sacrificial metal layer
on the microsystem and on the adhesion layer according to a first
embodiment;
[0031] FIGS. 3a and 3b show a top view and a cross-section along
III-III of FIG. 3a after the deposition of a second metal layer
above the sacrificial layer which is formed of different metal and
according to a first embodiment;
[0032] FIGS. 4a and 4b show a top view and a cross-section along
IV-IV of FIG. 4a after the removal of the sacrificial layer by
chemical etching through passages of the capsule thereby made
according to a first embodiment;
[0033] FIGS. 5a, 5b and 5c show a top view and cross-section along
V-V of FIG. 5a after the closing of the passages of the metal
capsule so as to hermetically encapsulate the microsystem according
to a first embodiment;
[0034] FIGS. 6a and 6b show a top view and a cross-section along
VI-VI of FIG. 6a after the deposition of a sacrificial layer on the
microsystem and on the adhesion layer which includes solder bumps
according to a second embodiment;
[0035] FIGS. 7a, 7b and 7c show a top view and cross-section along
VII-VII and VIII-VIII of FIG. 7a after the deposition of a second
metal layer above the sacrificial layer which is formed of a
different metal according to a second embodiment;
[0036] FIG. 8 shows a cross-section along VII-VII of FIG. 7a after
removal of the sacrificial layer by chemical etching via the
passages of the second layer according to a second embodiment;
[0037] FIG. 9 shows a cross-section along VII-VII of FIG. 7a after
the closing of the passages of the metal capsule of the microsystem
according to a second embodiment;
[0038] FIG. 10 shows a vertical cross-section after the deposition
of a sacrificial layer on the microsystem and on the adhesion layer
according to a third embodiment;
[0039] FIG. 11 shows a vertical cross-section after the deposition
of a second and third metal layers on the sacrificial layer
according to a third embodiment;
[0040] FIG. 12 shows a vertical cross-section after the deposition
of a fourth metal layer of the same metal as the second layer on
the third metal layer according to a third embodiment;
[0041] FIG. 13 shows a vertical cross-section after the removal of
the sacrificial layer by chemical etching through the passages of
the second layer according to a third embodiment;
[0042] FIG. 14 shows a vertical cross-section prior to the addition
of drops of solder on the passages of the second layer for closing
the metal capsule according to a third embodiment;
[0043] FIG. 15 shows a vertical cross-section after the closing of
the metal capsule with the solidified drops of solder according to
a third embodiment;
[0044] FIGS. 16a, 16b and 16c show a top view and cross-section
along XVI-XVI and XVII-XVII of FIG. 16a after the deposition of a
sacrificial metal layer on the microsystem and on the adhesion
layer and passing around solder bumps of the adhesion layer
according to a fourth embodiment;
[0045] FIG. 17 shows a cross-section along XVI-XVI of FIG. 16a
after the deposition of a second metal layer on the sacrificial
layer and on the solder bumps of the adhesion layer according to a
fourth embodiment;
[0046] FIG. 18 shows a cross-section along XVI-XVI of FIG. 16a
after the removal of the sacrificial layer through the passages of
the second layer between the solder bumps according to a fourth
embodiment;
[0047] FIG. 19 shows a cross-section along XVI-XVI of FIG. 16a
after the closing of the metal capsule by heating the solder bumps
according to a fourth embodiment;
[0048] FIGS. 20a and 20b show a top view and a cross-section along
XX-XX of FIG. 20a after the removal of the sacrificial layer
through a multitude of passages in the second layer according to a
fifth embodiment; and
[0049] FIG. 21 shows a cross-section along XX-XX of FIG. 20a after
the closing of the metal capsule by a wave of liquid solder
according to a fifth embodiment.
[0050] FIGS. 1 to 5 show the different steps of the hermetic
encapsulation in situ of microsystems according to a first
embodiment of the method of the invention. For purpose of
simplification, a single microsystem is shown in said Figures,
whereas in reality, several microsystems are mounted on a common
substrate in order to be encapsulated simultaneously.
[0051] FIGS. 1a and 1b show a portion of a substrate 1, which may
be entirely insulating such as a glass or ceramic plate, or a
substrate, for example made of silicon, the surface of which is
oxidised to become insulating. The dimensions of the substrate may
be those of a silicon substrate on which integrated circuits are
made, for example of 6 inches (152.4 mm). This portion of the
substrate visible in FIGS. 1a and 1b corresponds to the dimensions
for one of the microsystems made in common on the same
substrate.
[0052] In a first phase of the method shown in FIGS. 1a and b, a
conductive layer is deposited first of all on the insulating
surface of substrate 1 and is structured so as to form conductive
strips 2. An insulating layer 3 is then deposited only on the
median part of conductive strips 2 to form thus insulated electric
paths. Finally, a metal adhesion layer 4 is deposited on the
substrate passing over insulating layer 3. This adhesion layer
gives a surface metallisation able to define electric terminals 5
connected only to one of the ends of the conductive strips for the
electric connection of the microsystem after the sawing or dicing
of the substrate. This adhesion layer is able to withstand
construction of the microsystem and the capsule. It forms finally a
conductive plane for the electrolytic deposition steps which allow
metal layers of significant thickness to be obtained.
[0053] The conductive layer forming conductive strips 2 must adhere
well to the substrate and allow the subsequent insulating layer 3
to adhere well. This conductive layer must also be compatible with
metal adhesion layer 4 and have low electric resistance at the
interface of the connection of the two metal layers. It is
important that the leading edges do not have a negative slope, or
form a cornice, in order that the insulating layer covers them
perfectly. Conductive strips 2 may be made of a material such as
aluminium, gold, titanium, copper, chromium, tungsten or
titanium-tungsten alloy. These strips are useful for the electric
connection of the microsystem with the exterior after its
encapsulation.
[0054] The insulating layer must adhere well to insulating
substrate 1 and to conductive strips 2, for example like a layer of
silicon or silicon nitride Si3N4. Moreover, it must contain little
internal stress, have a thermal expansion coefficient close to that
of the substrate and perfectly cover the leading edges of the
conductive strips.
[0055] Metal adhesion layer 4 must adhere well to substrate 1 and
insulating layer 3. It may be made as specified by Swiss Patent No.
688213, i.e. by depositing first of all titanium or chromium which
is then covered with gold which acts as a protection layer against
oxidation. This second metal layer acts as a metal base surface for
the electrodeposition of the subsequent metal layers. The chemical
etching products for structuring these first metal layers are known
and consequently will not be explained. An annular zone 7a is shown
in dotted lines in FIGS. 1a and 1b to show the location of the
deposition of a subsequent metal layer.
[0056] In the event that soldering is used on the adhesion layer,
it is necessary to provide a base layer of the adhesion layer, said
base layer being made of three metal levels. The first metal level
is formed of titanium or chromium and enables it to be fixed to the
substrate. The second metal level is formed of nickel or palladium
or rhodium or ruthenium or platinum or molybdenum or another
material in order to act as a diffusion barrier if there is a
solder. Finally the third metal level is formed of gold to act as a
protection layer against oxidation, in particular for the first
metal level.
[0057] In FIG. 1b, solder bumps 13 of gold and tin alloy (Au--Sn)
or tin and lead alloy (Sn--Pb) may also form part of the adhesion
layer at determined locations. These solder bumps are used as will
be seen hereinafter to better close the passages made in the metal
capsule during thermocompression of portions of the capsule onto
said bumps. The gold-tin alloy is formed of 20% tin and 80% gold in
weight, whereas the tin-lead alloy is formed of 60% tin and 40%
lead in weight.
[0058] In embodiments which are not shown in the Figures, instead
of conductive strips 2, one could have made conductive holes
passing through insulating parts of the substrate or through the
substrate if it is entirely insulating or insulated conductive
holes in a conductive substrate. An insulating substrate can be a
glass or ceramic plate. On one side of the substrate these holes
connect microsystem 6 and on the other side they are electrically
connected to metal pads allowing the microsystem to be connected to
the exterior once it is encapsulated.
[0059] It is clear that insulating step 3 of conductive strips 2 is
not taken into account if conductive holes have been made through
the substrate as in the embodiment indicated hereinabove.
[0060] In the event that conductive strips 2 are replaced by
conductive holes, in particular metallised holes, the adhesion
layer is not structured to define metal paths with metal electric
connection pads of the microsystem. These pads are made in this
case on the back of the substrate. This adhesion layer need only to
surround each microsystem for the deposition by electrolytic means
of the metal layers for making the capsule.
[0061] In the event that a silicon substrate is used as a support
for the microsystems, the conductive strips deposited on the
substrate may be replaced by conductive paths in the silicon. These
paths are made by a diffusion step with a p-type dopant in an
n-type substrate or with an n-type dopant in a p-type substrate.
The metal connection of each end of said conductive paths is
achieved through windows made on insulation layer made of silicon
oxide. One advantage of this embodiment is that it assures
electrostatic protection.
[0062] Microsystem 6, which may for example be a reed contactor, is
constructed or mounted without damaging the previously deposited
layers. For the construction of a contactor with its metal strips,
electrodeposition techniques are also used, for example by
structuring the metal levels in several steps using photoresist and
masks to expose them, as described in Swiss Patent No. 688 213. The
microsystem thereby formed is connected to one end of conductive
strips 2 or to conductive holes.
[0063] Instead of making said microsystems in situ, they may be
manufactured separately and then each fixed onto a same substrate
in electric connection at one end of the conductive strips or
conductive holes provided for this purpose.
[0064] In FIGS. 2a and 2b, a first sacrificial metal covering layer
7 is deposited in particular by electrolytic means onto the
microsystems and onto annular zones 7a, visible in FIGS. 1a and 1b,
around each microsystem. In this way, the first metal layer, which
is in particular made of copper or a copper alloy, completely
covers each microsystem.
[0065] The metal adhesion layer is not dissociated from one
microsystem to another. It can be thus used for the
electrodeposition of various parts of the first layer covering all
the microsystems. So one location of the adhesion layer on the
substrate is connected to a terminal of a power source. In this
embodiment, one or two openings 8 in each part of the first layer
are also provided inside each annular zone. Said openings provide
access to adhesion layer 4 to be used to form one or two metal
support pillars during deposition of the next metal layer.
[0066] This first sacrificial layer 7 is formed of a metal, such as
copper or a copper alloy, which can be selectively dissolved with
respect to the other metal layers which are formed of different
metals. It must contain little internal stress and have good
levelling properties.
[0067] For the electrodeposition of this sacrificial layer, the
microsystems and the adhesion layer are first of all covered with a
photoresist layer. The photoresist is exposed through a mask in
order to remove parts of the photoresist which have been exposed or
not exposed depending on the type of photoresist. The parts of
photoresist removed aim to free each microsystem and an annular
zone of the adhesion layer surrounding each microsystem. The first
metal layer can thus be deposited onto each microsystem and onto
the annular zone surrounding it. Subsequently, the rest of the
photoresist is removed to have access to the adhesion layer through
the openings 8 made in the first layer and around each covered
microsystem. One part of the annular zone of the adhesion layer is
situated above insulation 3 of the conductive strips. So, the
deposition of the sacrificial metal layer only short circuits the
ends of the strips connecting the microsystems.
[0068] In order to create these metal domes covering each
microsystem, the metal layer may be deposited by a different method
to electrolytic means. For example, said metal layer may be
deposited by thermal evaporation or by cathodic sputtering without
having to exceed the temperature limit of 350.degree. C. However,
these other methods are longer and thus more expensive.
[0069] Openings 8 of the first metal layer visible in FIGS. 2a and
2b are completely surrounded by the first layer. However, it is of
course understood that they could have been designed to start from
an edge of said first layer to give the appearance in a plane view
of a U-shaped opening. Those skilled in the art will know how to
find openings of all shapes allowing the creation of pillars or
reinforcing parts during deposition of the second metal layer.
[0070] In FIGS. 3a and 3b, the metal capsule is made by depositing
by electrolytic means a second metal layer 9 onto first sacrificial
metal layer 7 and onto adhesion layer 4 or onto annular zones of
the adhesion layer surrounding parts of the first layer. Said
second layer 9 is formed of another metal, such as preferably gold
or a gold alloy, or possibly chromium or a chromium alloy. One or
two facing passages 10 are provided in said second layer 4 to
provide access to first sacrificial layer 7 in order to dissolve it
selectively with respect to the other metal layers. Said passages
10 are shown with an oblong shape, but it is clear that they could
also be of circular or square shape.
[0071] Each pillar 14 or reinforcing part made by deposition of
said second layer 9 is arranged between one of passages 10 and the
corresponding microsystem 6. This capsule consequently has an
hermetic closing on its periphery with the exception of the two
passages 10. Moreover, the two support pillars 14 of the capsule
are able to contain any deformation which could be caused during
the final closing step of the passages of the capsule. Said metal
of the capsule must also be malleable and contain little internal
stress and have good coating properties and very low porosity.
[0072] The first sacrificial layer 7 surrounding each support
pillar 14 can be removed by a chemical etchant, as explained with
FIGS. 4a and 4b below. For the dissolution of the first layer, the
chemical etchant passes through passages 10 of each capsule and
around said pillars 14. Of course, only the first layer 7 need pass
at least on one side of each pillar or reinforcing part, as
explained hereinbefore, to be able to remove this layer during
chemical etching step.
[0073] Although only one passage 10 and a single support pillar 14
could be envisaged to make the capsule, it is preferable to have
two or more passages for removing sacrificial layer 7; by having
for example two facing passages 10, this facilitates the removal of
the sacrificial layer and cleaning of the inside of the capsule via
the flow of treatment solutions.
[0074] As explained with reference to FIGS. 2a and 2b regarding the
deposition of first metal layer 7, a photoresist layer (not shown)
is also used. This photoresist is exposed through a mask to be able
to remove parts of the photoresist in order to have access to the
first layer and to annular zones of the adhesion layer surrounding
each part of the first layer. The annular zones are located above
insulation layer 3 and have no contact with the end of conductive
strips 2 for the external electric connection of microsystems
6.
[0075] Of course, if conductive holes through substrate 1 are used
for the external connection of microsystems 6, the deposition of
second metal layer 9 may be made beyond the annular zones over the
entire surface of the substrate. However, passages 10 have to be
kept to provide access to sacrificial layer 7.
[0076] In FIGS. 4a and 4b, sacrificial covering layer 7 is
dissolved by chemical etching through the two passages 10
selectively without etching the metals of microsystem 6, for
example iron and nickel. The chemical etchant must not cause any
damage to microsystem 6 or metal capsule 9 whether by chemical
etching or by a violent reaction with sacrificial covering layer 7.
There must also not be any residue inside the metal capsule, likely
to degas after the final closing thereof.
[0077] In FIGS. 5a and 5b, the encapsulated microsystems 6 are
still secured to the substrate. In this step, passages 10 of the
metal capsule must be closed in an inert or reducing atmosphere. A
suitable tool 12, which sprays the inside of said capsule 9 with a
protective gas, is brought near. Once the capsule has been cleaned
of its original atmosphere, the tool compresses parts 11 around
each passage 10. Then the tool bonds parts 11 by thermocompression
or ultrasound onto the base layer of adhesion layer 4. After this
operation, the metal capsule is sealed in an impervious manner.
Support pillars 14, in this step, are used to prevent the
deformation propagating in the direction of microsystem 6. Metal
capsule 9 thus forms an hermetic protection above microsystem
6.
[0078] In FIG. 5c, to reduce the power necessary to close the
capsule, solder bumps 13, forming part of the adhesion layer as
described above, are provided. The thermocompression of parts 11
around passages 10 of metal layer 9 on said solder bumps assures
melting of said bumps and the impervious closing of passages
10.
[0079] The final step, not shown in the Figures, consists in
separating by cutting or dicing the multitude of encapsulated
microsystems from the substrate. The encapsulated microsystems thus
can be used for example in usual ambient conditions. It's even
possible to coat each microsystem with a resin layer before or
after cutting in order to assure better mechanical protection.
[0080] If final metal layer 9 was made of chromium, one could avoid
making support pillars. As chromium is not ductile, one must avoid
deforming it when passages 10 are being closed. In such case, it
would be possible to close each metal capsule in an impervious
manner by depositing on each passage a drop of solder to be
solidified. However, gold or a gold alloy is better suited to
making the capsule, since it is ductile and resists different
chemical etchants.
[0081] Microsystem 6 constructed on the plate or insulating
substrate, which may be a layer of silicon oxide made on a silicon
wafer, has a total height of the order of 50 .mu.m prior to its
final encapsulation. The total height, when the metal capsule is
finished, is of the order of 100 .mu.m or even 150 .mu.m maximum,
with a thickness of the metal of the capsule of the order of 15 to
20 .mu.m. Compact components are thus made by the method according
to the invention.
[0082] In the event that all the steps of the method are
implemented on a single face, one may also envisage reducing the
thickness of the substrate by chemical etching of the back of
substrate after encapsulation and before the substrate is cut or
diced. In order to do this, one must take the required precautions
so as to avoid damaging the side of the substrate bearing the
encapsulated microsystems. However, if the substrate is thin from
the start, this avoids having to reduce its thickness at the end of
the encapsulation method.
[0083] As a result of the electrodeposition technique, metal layers
of greater thickness can be deposited, which is difficult to
achieve with thermal evaporation or cathodic sputtering. This
electrodeposition technique allows less expensive and quicker
manufacturing for such thicknesses even if gold is used to make the
capsule. Comparatively, the design, according to the prior art, of
a glass plate independent of the substrate wherein recesses are
made to place or construct the microsystems and then enclose them
is more time consuming and expensive.
[0084] FIGS. 6 to 9 show steps of the hermetic encapsulation in
situ of microsystems according to a second embodiment of the method
of the invention. It is to be noted that the elements of these
Figures, which correspond to those of FIGS. 1 to 5, bear identical
reference numbers.
[0085] In FIGS. 6a and 6b in which the conductive strips and the
insulating layer have not been shown, a first sacrificial metal
layer 7, made in particular of copper or a copper alloy, is
deposited in particular by electrolytic means, on an annular zone
of adhesion layer 4 and on microsystem 6 to completely cover it.
Two extensions 15 of sacrificial layer 7, of smaller width than
that covering microsystem 6, pass above solder bumps 13 of adhesion
layer 4. These two extensions 15, used to create the passages of
the second metal layer which will be discussed hereinafter, are
disposed, as well as the two solder bumps 13, on two opposite sides
of sacrificial layer 7.
[0086] In FIGS. 7a and 7b, a second metal layer 9, made in
particular of gold or a gold alloy, is deposited by electrolytic
means onto sacrificial layer 7 and onto parts of the adhesion
layer. This layer 9 defines in a top view a rectangular shape
stopping at the end of each extension 15 in order to avoid covering
them completely. Passages 10 thus are created as a result of said
extensions 15 coming out of second layer 9.
[0087] FIG. 7c is a cross-section along the line VIl-VlI of FIG. 7a
and which shows the superposition of the various layers. On
insulating substrate 1, metal adhesion layer 4 includes solder
bumps 13 formed in particular of a gold and tin alloy. Extension 15
of the sacrificial layer passes above solder bump 13. Second metal
layer 9 passes above the sacrificial layer and is also connected on
each side of extension 15 to solder bump 13.
[0088] FIG. 8 shows the removal of the sacrificial layer using a
chemical etchant through passages 10. Said passages are obtained by
the extensions of the sacrificial layer coming out of the second
layer. After such removal, microsystem 6 is free inside metal
capsule 9.
[0089] FIG. 9 shows the closing of capsule 9 using a tool 12
pressing parts of second layer 9 located on solder bumps 13. During
the compression of these parts, solder bumps 13 are heated to be
melted and thus to seal passages 10. Given that the passages are of
smaller size on two reinforced sides of second layer 9, it is no
longer necessary to provide reinforcing pillars as for the first
embodiment. The compression of the parts delimiting passages 10
will not damage microsystem 6.
[0090] FIGS. 10 to 15 show steps for hermetically encapsulating
microsystems in situ according to a third embodiment of the method
of the invention. It is to be noted that the elements of these
Figures, which correspond to those of FIGS. 1 to 5, bear identical
reference numbers.
[0091] FIG. 10 shows the deposition in particular by electrolytic
means of a sacrificial metal layer 7, made in particular of copper
or a copper alloy, on an annular zone of the metal adhesion layer 4
surrounding microsystem 6 and on said microsystem in order to cover
it completely. Although electrically connected, sacrificial layer 7
deposited on microsystem 6 is not contiguous with the sacrificial
layer of a neighbouring microsystem on the same substrate 1, since
it is only deposited on a limited annular zone around the
respective microsystem.
[0092] FIG. 11 shows the successive depositions by electrolytic
means of a second metal layer 9, made in particular of gold or a
gold alloy, and a third metal layer 16, made in particular of
copper or a copper alloy like the sacrificial layer. These layers
are deposited above sacrificial layer 7 and on an annular zone
surrounding sacrificial layer 7. Two passages 10 are made in the
two layers 9 and 16 to provide access to sacrificial layer 7. The
shape of the passages could be oblong or circular or square.
[0093] The same photoresist layer is used for the two successive
metal depositions. The second metal layer 9 has a small thickness
of the order of 0.5 .mu.m whereas third metal layer 16 has a
thickness of the order of 20 .mu.m so that the final metal capsule
resists mechanical stress. This allows a sufficiently thick capsule
to be made as well as savings given that the second layer is
preferably made of gold or a gold alloy.
[0094] As third layer 16 is preferably made of the same metal as
the sacrificial layer in order to use the same electrolyte baths,
it is necessary to protect it from any chemical etchant. In order
to do this, as shown in FIG. 12, a fourth metal layer 17 of an
identical metal to the second layer is deposited on the third layer
and on an annular zone surrounding it. This fourth metal layer is
connected to the second layer while leaving passages 10 free. The
third layer is consequently entirely inserted between the second
and fourth metal layers and is thus protected from any chemical
etchant for the removal of sacrificial layer 7. The thickness of
the fourth layer is of the order of 0.5 .mu.m.
[0095] FIG. 13 shows the removal of sacrificial layer 7 by a
chemical etchant passing through passages 10, the third layer being
protected by the second and fourth layers.
[0096] FIGS. 14 and 15 show the structure after the removal of the
sacrificial layer, microsystem 6 being free of movement in the
capsule. For example in the case of a contactor, metal strips
thereof are free to move. Solder drops 18 are then brought by a
tool (not shown) onto each passage 10 in the direction of arrow f
and are solidified in order to seal the passages and to
hermetically close the capsule.
[0097] FIGS. 16 to 19 show steps for hermetically encapsulating
microsystems in situ according to a fourth embodiment of the method
of the invention. It is to be noted that the elements of these
Figures, which correspond to those of FIGS. 1 to 5, bear identical
reference numbers.
[0098] In FIGS. 16a, 16b and 16c, a series of solder bumps 13 of
adhesion layer 4 were made around microsystem 6 in a preceding step
of the method, as well as guide elements 20 placed in the direction
of the corners of the microsystem and inside the series of solder
bumps. These guide elements 20 are formed of a different metal to
solder bumps 13 and sacrificial layer 7 to withstand in particular
higher temperatures than solder bumps 13. They are used to guide
second layer 9 when the capsule is closed as discussed
hereinafter.
[0099] Said bumps can be regularly spaced over the entire periphery
of the microsystem without being in direct contact with said
microsystem 6. A sacrificial layer 7 is deposited by electrolytic
means on microsystem 6 and on an annular zone of adhesion layer 4
without passing above said solder bumps 13. For that, a photoresist
masking has been previously provided. However, parts 19 of the
sacrificial layer are disposed in the spaces between the solder
bumps in order to be able to create passages in the second metal
layer. One can see said passages in FIG. 16c which is a
cross-section along the line XVII-XVII of FIG. 16a.
[0100] In FIG. 17, a second metal layer 9 is deposited by
electrolytic means on sacrificial layer 7 and on solder bumps 13.
The second layer does not come into contact with the base layer of
adhesion layer 4, since it does not extend beyond the periphery of
sacrificial layer 7. Consequently it lets portions 19 of the
sacrificial layer emerge from second metal layer 9 in order to be
able to define passages 10, visible in FIG. 18, in the spaces
between solder bumps 13.
[0101] In FIG. 18, sacrificial layer 7 has been removed using a
chemical etchant through parts 19, i.e. through passages 10 of
second layer 9. This second layer 9 appears, after removal of the
sacrificial layer, like a roof resting on the series of solder
bumps 13 and sheltering microsystem 6.
[0102] The hermetic closing of the metal capsule is shown in FIG.
19. Substrate 1 with all the microsystems under their capsule is
placed in an oven to bring a heat wave 21 towards solder bumps 13
to make them melt. As soon as solder bumps 13 melt, capsule 9 is
lowered in direction v by its own weight and by capillarity to
close the microsystem hermetically by sealing all the passages.
Since when solder bumps 13 melt, the capsule no longer has a fixed
point of support, it may move in a horizontal direction and come
into contact with the microsystem. Guide elements 20 shown as being
four in number in FIGS. 16 to 19 are thus provided to prevent the
capsule moving too far in the horizontal direction and becoming
fixed to the adhesion layer using solder bumps 13 at a location
able to disturb the proper operation of microsystem 6.
[0103] The stick shape given by way of illustration in FIGS. 16 to
19 for guide elements 20 is not limiting, since these elements
could take other forms. For example, one could use only two guide
elements 20 arranged close to two opposite corners of microsystem
6. These two elements may be cylindrical or L-shaped. Of course,
the use of these guide elements is not obligatory, provided one
ensures that the descent of second layer 9 occurs exclusively in a
vertical manner.
[0104] Since solder bumps 13 are made of a gold and tin alloy or a
tin and lead alloy and second layer 9 resting on said bumps is made
of gold or a gold alloy, there is a risk of diffusion of the bump
alloy in the second layer when they are melted by heat wave 21.
Consequently, too large a quantity of melted material is liable to
no longer guarantee sufficient space for the microsystem. In order
to prevent such diffusion, those skilled in the art know how to
place a diffusion barrier between solder bumps 13 and second layer
9.
[0105] FIGS. 20 and 21 show two final steps for hermetically
encapsulating microsystems in situ according to a fifth embodiment
of the method of the invention. It is to be noted that the elements
of these Figures, which correspond to those of FIGS. 1 to 5, bear
identical reference numbers.
[0106] FIGS. 20a and 20b show the metal capsule formed by second
metal layer 9, made in particular of gold or a gold alloy, with the
multitude of passages 10 made over its top portion. The sacrificial
layer has been removed through the passages 10 using a selective
chemical etchant. This capsule has been deposited on an annular
zone of adhesion layer 4 around the microsystem and encloses
without contact said microsystem 6.
[0107] Passages 10 are of sufficiently small size to be able to be
placed on the top portion of the capsule and allow them to be
sealed by a liquid solder wave 23. Said solder wave is brought by a
rotating cylindrical tool 22 moving in a direction h above
substrate 1, as can be seen in FIG. 21, or by a continuous solder
wave. As a result of the capillarity effect, liquid solder 23 will
seal said passages 10 without risking coming into contact with
microsystem 6.
[0108] Cylindrical tool 22 includes inside one or more supply
conduits for liquid solder 23, not visible in FIG. 21. Liquid
solder opens out via close together orifices, made on the periphery
of the cylinder in order to create a wave of liquid solder 23. The
width of the tool is such that in a single passage over the
substrate, it allows all the passages 10 made in the second layer
of all the encapsulated microsystems to be sealed. It is to be
noted that the surface evenness of the top portions of the second
layer is of the order of more or less 10 .mu.m, so that the tool
can seal all the passages once without too much difficulty.
[0109] Instead of rotating tool 22 for sealing passages 10,
substrate 1 carrying all the microsystems 6 encapsulated by second
layer 9 could be brought above a solder bath. For that, the contact
pads, connected to the conductive strips which were explained in
FIGS. 1 to 5, have not to be covered. For this operation, it is
possible to leave the photoresist used to form the second layer
which protects said pads, as well as the end of the conductive
strips.
[0110] In this fifth embodiment, passages 10 are arranged like a
grid on the top portion of second layer 9 in the same way as that
which could be achieved within the knowledge of those skilled in
the art with a porous polysilicon layer. By way of reference, one
can refer to the article of the 12th IEEE international conference
MEMS 99 of 17 to 21 January 1999, entitled, "Micro Electro
Mechanical Systems" at pages 470 to 475. This polysilicon layer is
used in certain microsystem encapsulations for the removal of a
sacrificial layer by a chemical etchant passing through said porous
polysilicon.
[0111] The encapsulation method which has just been described could
also be applied to the encapsulation of a single microsystem
mounted on a substrate. However, in order to reduce as much as
possible the manufacturing costs in this field of micrometric
devices, it is more economical to encapsulate several microsystems
on a common substrate at the same time.
[0112] Other variants or combinations of preceding embodiments for
an hermetic metal encapsulation of microsystems which were not
explained above, but are within the grasp of those skilled in the
art, may also be envisaged without departing from the scope of the
invention.
* * * * *