Note: Descriptions are shown in the official language in which they were submitted.
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EH 219 ~A
Thin-Film Absolute Pressure Sensors
and Methods of Manufacturing the Same -..
FIELD OF THE INVENTION
The present invention relates to thin-film absolute
pressure sensors with a base element and a diaphragm
which bound a hermetically sealed cavity, and to the
manufacture of such sensors.
BACKGROUND OF THE INVENTION
Such absolute pressure sensors have not become known un-
til now. The literature describes only those pressure
sensors which are manufactured by semiconductor pro-
cesses.
For instance, in the journal "J. Phys. E: Sci.
Instrum.", Vol. 20 (1987), pages 1469 to 1471, a pres-
sure sensor is described in which a polycrystalline
silicon diaphragm is provided on a single-crystal sili-
con substrate, so that the diaphragm is not electri-
Ke/Lo
20.06.94
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cally insulated from the substrate. The diaphragm is
first deposited on an Sio2 sacrificial layer, which is
then removed underneath the diaphragm.
Thus, a space-charge region is formed at the boundary
between polycrystalline silicon diaphragm and silicon
substrate, so that the capacitance of this pressure
sensor is strain- and temperature-dependent. Further-
more, the capacitance can be measured only by high-fre-
quency sensing, not by low-frequency sensing.
In the journal "VDI Berichte", No. 939, 1992, pages 185
to 190, a high-pressure sensor is described in which a
single-crystal silicon substrate supports a
polycrystalline silicon diaphragm insulated from the
substrate by means of a silicon-nitride film.
A similar pressure sensor with a polysilicon diaphragm
insulated from the silicon substrate and a method of
manufacturing such a sensor are described in
DE-A-40 04 179.
As is stated in the journal "Sensors and Actuators",
Vol. 28 (1991), pages 133 to 146, polysilicon films
deposited on silicon exhibit built-in compressive
strain. This results in a hysteresis of the pressure-
capacitance characteristic and deteriorates the response
of the pressure sensor to temperature changes.
In the case of a polysilicon diaphragm, the compressive
strain in the as-deposited film can only be prevented by
modifying the manufacturing process so that a defined
tensile strain is produced in the diaphragm.
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In the case of these prior art pressure sensors, and
also in the sensors described in the journal "Sensors
and Actuators", Vol. A21-A23 ( 1990 ) , pages 1053 to 1059,
the cavity is formed by first depositing an Si02 sacri-
ficial layer, then depositing the polysilicon diaphragm,
and subsequently removing the sacrificial layer by
etching in hydrofluoric acid.
This, however, has further disadvantages. After the
etching in hydrofluoric acid, which is followed by
rinsing in deionized water and drying, the thin poly-
silicon diaphragms generally stick to the silicon-sub-
strate surface. This can only be prevented by taking
complicated and costly countermeasures, which are ex-
plained in detail in the prior art mentioned above.
In addition, electrostatic fields, which are present at
the surfaces of the silicon substrate and the polysili-
con diaphragm as a result of the etching and thereafter,
lead to undesired deflection of the diaphragm. To elimi-
nate this deflection, a bias voltage is necessary.
DE-A-37 23 561 describes a further semiconductor process
for manufacturing a capacitive pressure sensor in which
the sacrificial layer defining the cavity to be formed
is etched away through openings in a first insulating
layer which lie within the diaphragm area. Then,
however, the material of the top electrode can penetrate
into the cavity. This degrades the electrical pro-
perties of the pressure sensor.
To prevent this,.a second insulating layer is provided
~13~J~J
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which has a plurality of openings displaced in relation
to the openings of the first insulating layer and pre-
vents material of the top electrode from penetrating
into the cavity. However, this second insulating layer
complicates the manufacture considerably.
By contrast with the prior art, the problem underlying
the invention is to provide resistive and capacitive
absolute pressure sensors and processes for manufactur-
ing same by surface-micromachining and thin-film
techniques, with
- the electrodes of the at least one capacitor in
a capacitive thin-film absolute pressure sensor
having a high insulation resistance relative to
each other,
- the respective diaphragms in resistive and capa-
citive thin-film absolute pressure sensors ex-
hibiting only little tensile strain in the
finished condition,
- no sublimation step being necessary to prevent
the diaphragm from sticking to the substrate,
- the diaphragm providing a measurement signal over
a wide pressure range even if it rests against
the substrate in the event of an overload,
- the measurement signal being virtually indepen-
dent of temperature, and
- only few chemical-vapor-deposition and photo-
lithographic steps being necessary.
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SUMMARY OF THE INVENTION
In accordance with the present invention there is
provided a thin-film process for manufacturing a capacitive
absolute pressure sensor with a base element and a diaphragm
which bound a hermetically sealed cavity, comprising the
following steps in the order given: a) depositing a first
metal layer over the entire surface of a glass substrate
serving as the base element, said first metal layer containing
a substrate electrode to be formed; b) depositing over the
entire surface a patternable material layer which defines the
height of the cavity and contains a sacrificial layer to be
formed; c) patterning the patternable material layer and the
first metal layer in a single, first photoresist step by
etching for simultaneously forming the substrate electrode,
first interconnection tracks connected therewith) and the
sacrificial layer, which is practically congruent with the
substrate electrode and the first interconnection tracks,
thereby partially exposing the glass substrate; d) depositing
a first insulating layer containing the diaphragm over the
entire surface, so that said first insulating layer firmly
adheres to the areas of the glass substrate exposed in step
c), even in an edge region next to the sacrificial layer;
e) forming, in a photoresist layer deposited over the entire
surface, a photoresist mask whose opening is congruent with a
top electrode to be formed, which will extend onto the edge
region of the first insulating layer next to the sacrificial
layer, and with second interconnection tracks connected with
.~.-.~,
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the top electrode; f) depositing a second metal layer
containing the top electrode over the entire surface of the
photoresist mask; g) removing the photoresist mask with the
overlying portions of the second metal layer by a lift-off
step; h) etching away the portions of the first insulating
layer not covered by the top electrode and by the second
interconnection tracks; i) removing the sacrificial layer by
lateral etching, starting from its portions lying on the first
interconnection tracks, and k) hermetically sealing the cavity
by depositing a second insulating layer over the entire
surface in a vacuum.
In accordance with the present invention there is
further provided a thin-film process for manufacturing a
capacitive absolute pressure sensor with a base element and a
diaphragm which bound a hermetically sealed cavity, comprising
the following steps in the order given: a) depositing a first
metal layer over the entire surface of a glass substrate
serving as the base element, said first metal layer containing
a substrate electrode to be formed; b) depositing over the
entire surface a patternable material layer which defines the
height of the cavity and contains a sacrificial layer to be
formed; c') patterning the patternable material layer and the
first metal layer in a single, first photoresist step by
etching for simultaneously forming the substrate electrode,
corner pads connected therewith, and the sacrificial layer,
which is practically congruent with the substrate electrode
and the corner pads, thereby partially exposing the glass
~" y,31
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substrate; d') depositing a first insulating layer containing
the diaphragm over the entire surface, so that said insulating
layer firmly adheres to the areas of the glass substrate
exposed in step c'), even in four lateral regions next to the
sacrificial layer, and etching openings into the first
insulating layer for the subsequent supply of an et chant to
the sacrificial layer, said openings lying over at least a
part of the four corner pads of one absolute pressure sensor;
e') forming, in a photoresist layer deposited over the entire
surface, a photoresist mask whose opening is congruent with a
top electrode to be formed and with interconnection tracks
connected therewith and is centered with the substrate
electrode without the corner pads; f) depositing a second
metal layer containing the top electrode over the entire
surface of the photoresist mask; g) removing the photoresist
mask with the overlying portions of the second metal layer by
a lift-off step; i') removing the sacrificial layer by
vertical etching and lateral etching through the openings, and
k') hermetically sealing the openings, and thus the cavity, by
depositing a second insulating layer in a vacuum.
In accordance with the present invention there is
further provided a thin-film process for manufacturing a
capacitive absolute pressure sensor with a base element and a
diaphragm which bound a hermetically sealed cavity, comprising
the following steps in the order given: a) depositing a first
metal layer over the entire surface of a glass substrate
serving as the base element, said first metal layer containing
28917-11
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_8_
a substrate electrode to be formed; b") depositing over the
entire surface a first patternable material layer containing a
first partial layer of a sacrificial layer to be formed, said
first partial layer defining a first part of the height of the
cavity; c") patterning the patternable material layer and the
first metal layer in a single, first photoresist step by
etching for simultaneously forming the substrate electrode,
first interconnection tracks connected therewith, and the
first partial layer, which is practically congruent with the
substrate electrode and the first interconnection tracks; c"')
depositing over the entire surface a second patternable
material layer which defines the remainder of the height of
the cavity and contains a second partial layer of the
sacrificial layer to be formed and two diametrically opposed
corner extensions, and patterning said second patternable
material layer in a second photoresist step so that it
completely covers the first partial layer; d") depositing a
first insulating layer containing the diaphragm over the
entire surface, so that said first insulating layer firmly
adheres to the areas of the glass substrate still exposed
after step c"'), and etching openings into the first
insulating layer above the corner extensions for the
subsequent supply of an etchant to the sacrificial layer; a")
forming, in a photoresist layer deposited over the entire
surface, a photoresist mask whose opening is congruent with a
top electrode to be formed and with second interconnection
tracks connected therewith and is centered with the substrate
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electrode; f) depositing a second metal layer containing the
top electrode over the entire surface of the photoresist mask;
g) removing the photoresist mask with the overlying portions
of the second metal layer by a lift-off step; i') removing the
sacrificial layer by vertical etching and lateral etching
through the openings, and k') hermetically sealing the
openings, and thus the cavity, by depositing a second
insulating layer in a vacuum.
In accordance with the present invention there is
further provided a thin-film process for manufacturing a
capacitive absolute pressure sensor with a base element and a
diaphragm which bound a hermetically sealed cavity, comprising
the following steps in the order given: a') depositing a
first metal layer over the entire surface of a glass substrate
serving as the base element, said first metal layer containing
a substrate electrode to be formed, and patterning said first
metal layer in a first photoresist step for forming the
subst rate elect rode and f first interconnect ion t racks connected
therewith; b" ' ) deposit ing over the ent ire surface a f first
patternable material layer which defines a first part of the
height of the cavity and contains a first partial layer of a
sacrificial layer to be formed, and patterning said first
patternable material layer in a second photoresist step so
that it completely covers the subst rate elect rode; c" " )
depositing over the entire surface a second patternable
material layer which defines the remainder of the height of
the cavity and contains a second partial layer of the
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sacrificial layer to be formed and two diametrically opposed
corner extensions, and patterning said second patternable
material layer in a third photoresist step so that it
completely covers the first partial layer; d") depositing over
the entire surface a first insulating layer containing the
diaphragm, so that said first insulating layer firmly adheres
to the areas of the glass substrate still exposed after step
c""), and etching openings into the first insulating layer
above the corner extensions for the subsequent supply of an
etchant to the sacrificial layer; a") forming, in a
photoresist layer deposited over the entire surface, a
photoresist mask whose opening is congruent with a top
electrode to be formed and with second interconnection tracks
connected therewith and is centered with the substrate
electrode; f) depositing a second metal layer containing the
top electrode over the entire surface of the photoresist mask;
g) removing the photoresist mask with the overlying portions
of the second metal layer by a lift-off step; i') removing the
sacrificial layer by vertical etching and lateral etching
through the openings, and k') hermetically sealing the
openings, and thus the cavity, by depositing a second
insulating layer in a vacuum.
The material for the first metal layer and/or second
metal layer is preferably chromium. For the sacrificial
layers, aluminum is preferably used. The insulating layers
are preferably of 5102, which is applied by plasma chemical
vapor deposition.
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A sixth solution to the above problem consists in the
provision of a thin-film process for manufacturing a
capacitive absolute pressure sensor with a base element
and a diaphragm which bound a hermetically sealed
cavity, comprising the following steps in the order
given:
a') depositing a first metal layer over the en-
tire surface of a glass substrate serving as
the base element, said first metal layer con-
taining a substrate electrode to be formed, and
subsequently patterning said first metal layer
in a first photoresist step for forming the
substrate electrode and first interconnection
tracks connected therewith;
b"') depositing over the entire surface a first
patternable material layer which defines a first
part of the height of the cavity and contains a
first partial layer of a sacrificial layer to be
formed, and patterning said first patternable
material layer in a second photoresist step so
that it completely covers the substrate electrode;
c"~~) depositing over the entire surface a second
patternable material layer which defines the
remainder of the height of the cavity and con-
tains a second partial layer of the sacrificial
layer to be formed and two diametrically opposed
corner extensions, and patterning said second
patternable material layer in a third photo-
resist step so that it completely covers the
first partial layer;
d«) depositing over the entire surface a first in-
sulating layer containing the diaphragm, so
that said first insulating layer firmly adheres
to the areas of the glass substrate still ex-
posed after step c ~~ ~~ ) , and etching openings in-
to the first insulating layer above the corner
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extensions for the subsequent supply of an
etchant to the sacrificial layer;
a") forming, in a photoresist layer deposited over
the entire surface, a photoresist mask whose
opening is congruent with a top electrode to
be formed and with second interconnection
tracks connected therewith and is centered
with the substrate electrode;
f) depositing a second metal layer containing the
top electrode over the entire surface of the
photoresist mask;
g) removing the photoresist mask with the over-
lying portions of the second metal layer by
a lift-off step;
i') removing the sacrificial layer by vertical
etching and lateral etching through the open-
ings, and
k') hermetically sealing the openings, and thus
the cavity, by depositing a second insulating
layer in a vacuum.
In these quotations of the independent claims, identical
letters indicate the identity of the respective features
in several claims, while apostrophes used with identical
letters indicate that these features are equal in
principle but differ in detail.
The material for the first metal layer and/or second
metal layer is preferably chromium. For the sacrificial
layers, aluminum is preferably used. The insulating
layers are preferably of SiOz, which is applied by
plasma chemical vapor deposition.
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In the first process variant of the invention, the top
electrode may serve as a mask for patterning the first
insulating layer.
One of the advantages of the invention is that, in con-
trast to the above-explained prior art semiconductor
processes for manufacturing pressure sensors, no
diffusion steps are necessary in the invention.
Unlike the above-mentioned semiconductor pressure
sensors with a polysilicon diaphragm, the absolute
pressure sensors according to the invention have a dia-
phragm of an insulating material, preferably an SiOz
diaphragm.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more apparent from the follow-
ing description of embodiments when taken in conjunction
with the accompanying drawings, in which:
Fig. 1 is a top view of four capacitive absolute
pressure sensors according to the first
variant of the invention which are manu-
factured according to the first process
variant;
Fig. 2 shows sections taken along line A-A or
B-B of Fig. 1;
_ 14 _ 2130505
Fig. 3 is a top view of four capacitive absolute
pressure sensors according to the first
variant of the invention which are manu-
factured according to the second process
variant;
Fig. 4 shows sections taken along line A-A of
Fig. 3;
Fig. 5 is a top view of four capacitive absolute
pressure sensors according to the first
variant of the invention which are manu-
factured according to the third process
variant;
Fig. 6 shows sections taken along line A-A of
Fig. 5;
Fig. 7 is a top view of four capacitive absolute
pressure sensors according to the first
variant of the invention which are manu-
factured according to the fourth process
variant;
Fig. 8 shows sections taken along line A-A of
Fig. 7, and
Fig. 9 is a top view of a resistive absolute
pressure sensor according to the second
variant of the invention.
15 _ 213~~05
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 is a top view of an array of four circular
absolute pressure sensors according to the first variant
of the invention which are manufactured according to the
first process variant, as will be explained below. Of
these four absolute pressure sensors, only the one at
the upper left is shown in its standard form, while in
the case of each of the other three, a characteristic
portion, which will be explained below, is set off by
hatching.
Figs. 2a to 2h are sections taken along line A-A or B-B
of Fig. 1 which illustrate the results of sequential
process steps.
The manufacture starts with a glass substrate 11 serving
as a base element. A first metal layer 12, preferably of
chromium, containing, inter alia, a substrate electrode
12' to be formed, is deposited over the entire area of
the substrate, which may be done by vapor evaporation,
for example. The thickness of a chromium metal layer is
100 nm, for example.
It should be pointed out that the term "deposition over
the entire surface" as used herein means a process in
which a layer is deposited over the entire surface of a
structure already present at the beginning of a given
process step, thus completely covering that surface.
A patternable material layer 13, preferably of aluminum,
which defines the height of the cavity of the finished
absolute pressure sensor and contains a sacrificial
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layer 13' to be formed, is deposited over the entire
surface of the first metal layer 12. If made of
aluminum, the material layer has a thickness of, e.g.,
500 nm.
The result of these process steps is shown in Fig. 2a,
which is a section taken along line A-A of Fig. 1.
Next the material layer 13 and the first metal layer 12
are patterned in a single, first photoresist step, in
which a suitable photoresist mask serves as an etch
mask, as is well known, such that, on the one hand, the
individual substrate electrodes 12' are separated from
each other and that, on the other hand, interconnection
tracks 14 are left behind, which extend from the sub-
strate electrode 12'. By this process step, the
sacrificial layer 13' is formed on the substrate
electrode and the interconnection tracks, see Fig. 2b,
which is a section taken along line B-B of Fig. 1 (the
interconnection tracks are not visible in Fig. 2b).
This common patterning of the first metal layer 12 and
the material layer 13 is performed through the same
photoresist mask in two etch steps, namely in a first
etch step for etching the (aluminum) material layer 13
and in a subsequent, second etch step for etching the
(chromium) metal layer 12.
As can be seen in Fig. 1, if a plurality of absolute
pressure sensors are fabricated simultaneously and are
to be electrically connected in parallel, horizontal
interconnection tracks 14 are provided which intercon-
nect the substrate electrodes 12' of these absolute
pressure sensors and lead to a substrate electrode
contact pad 140.
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If absolute pressure sensors are to be used singly, one
of the two interconnection tracks 14 shown in Fig. 1 for
each absolute pressure sensor can, of course, be
omitted.
Over the structure formed so far, i.e., over the sur-
faces of the sacrificial layer 13' and the glass sub-
strate 11, a first insulating layer 15 is deposited, a
part of which will later become a diaphragm 15'. This
insulating layer 15 is preferably of Sio2, which is
applied by plasma chemical vapor deposition, and it has
a thickness of, e.g., 3,um, cf. Fig. 2c, which is a sec-
tion taken along line B-B of Fig. 1.
The insulating layer 15 makes a firm bond with the glass
substrate 11, even in an edge region 110 next to the
substrate electrode 12'. This is illustrated in Fig. 1
at the lower right by the horizontally hatched area 110.
There is no hatching above the interconnection tracks 14
and the overlying portions of the sacrificial layer 13' ,
since the insulating layer will be etched away there.
The entire surface of the structure formed so far is
then covered with a photoresist layer. Therein a
photoresist mask is formed whose opening is congruent
with a top electrode 16' to be formed, which extends
onto the edge portion 110 of the first insulating layer
15 next to the sacrificial layer 13', and with second
interconnection tracks 17 to be formed, which are
connected to the top electrode 16'. A second metal
layer, containing the top electrode 16', is then
deposited over the entire surface of the photoresist
mask. Next the photoresist mask with the overlying
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portions of the second metal layer is removed by a lift-
off step. This is done, for example, in an acetone bath
under the action of ultrasound. Of the second metal
layer, the top electrode 16' and the interconnection
tracks 17 are thus left, cf. Fig. 2d, which is a section
taken along line B-B of Fig. 1.
Then, the parts of the first insulating layer 15 not
covered by the top electrode 16' and the second inter-
connection tracks 17 are etched away, exposing, inter
alia, those parts of the sacrificial layer 13' which lie
over the interconnection tracks 14, cf. Fig. 2e, which
is a section taken along line B-B of Fig. 1. If the
first insulating layer 15 is of Sio2, the etching will
preferably be carried out in a CF4-O2 plasma.
The sacrificial layer 13' is then removed by lateral
etching. The etching begins in those places 130 which
lie over the interconnection tracks 14 at the edge of
the top electrode 16', from which places the etchant,
after having etched away the portions of the sacrificial
layer 13' overlying the first interconnection tracks 14,
penetrates under the diaphragm 15' and continues to etch
inwards.
Thus a cavity 18 is created, namely the aforementioned
cavity by which the substrate electrode 12' and the
diaphragm 15' are separated, cf. Fig. 2f, which is a
section taken along line A-A of Fig. 1.
The result of this lateral etching with respect to the
first insulating layer 15, which contains the diaphragm
15', is also illustrated in Fig. 1 at the upper right
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by the vertically hatched area 150, which marks the un-
covered portions of the insulating layer 15. It can be
seen that the diaphragm 15' is also exposed above a re-
spective part of the interconnection tracks 14.
Finally the cavity 18 is hermetically sealed, in the
laterally open places 130, by depositing a second in-
sulating layer 19 over the entire surface of the struc-
ture in a vacuum. Thus, after the sealing, the cavity is
evacuated, which is essential for absolute pressure sen-
sors, as is well known.
The second insulating layer 19 also serves as a protec-
tive layer of the absolute pressure sensor, cf. Fig. 2g,
which is a section taken along line B-B of Fig. 1, and
Fig. 2h, a section taken along line A-A of Fig. 1.
The second insulating layer 19, too, is preferably of
Si02, which is applied by plasma chemical vapor de-
position and has a thickness of, e.g., 3 Vim.
As mentioned, during the patterning of the second metal
layer, like during the patterning of the first metal
layer 12, interconnection tracks 17 are left which are
extensions of the top electrodes 16'. As shown in Fig.
1, if a plurality of absolute pressure sensors are
fabricated simultaneously and are to be electrically
connected in parallel, two interconnection tracks 17 are
provided per sensor which are arranged diametrically in
a vertical direction, interconnect the top electrodes
16' of the individual absolute pressure sensors, and
lead to a top electrode contact pad 170. If absolute
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pressure sensors are to be used singly, one of the two
interconnection tracks 17 can, of course, be omitted.
Fig. 3, a representation comparable to that of Fig. 1,
is a top view of an array of four virtually square
absolute pressure sensors 20 according to the first
variant of the invention which are manufactured
according to the second process variant, as will be ex-
plained below. In Fig. 3a, different layers and parts of
the absolute pressure sensors are set off by different
hatchings, while Fig. 3b shows the finished absolute
pressure sensors in a top view.
Figs. 4a to 4h are sections taken along line A-A of Fig.
3 which show the results of sequential process steps.
The second process variant of the invention, like the
first, starts with a glass substrate 21, of which only
one surface can be seen in Figs. 4a to 4h as the lower
edge of the respective section to simplify the illu-
stration.
A first metal layer 22, preferably of chromium, which
contains, inter alia, a substrate electrode 22~ to be
formed, is deposited on the glass substrate 21, a . g. , by
evaporation. The thickness of the chromium sub-
strate electrode is again 100 nm, for example.
The first metal layer 22 is covered with a patternable
material layer 23, preferably an aluminum layer, which
defines the height of the cavity 28 of the finished
absolute pressure sensor. If this layer is of
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aluminum, its thickness is 500 nm, for example. The re-
sult of these process steps is shown in Fig. 4a.
The patternable material layer 23 and the first metal
layer 22 are then patterned in a single, first
photoresist step by etching to simultaneously form the
substrate electrode 22, corner pads 24 connected
therewith, and the sacrificial layer 23', which is
virtually congruent with the substrate electrode and the
corner pads, exposing parts of the surface of the glass
substrate 21, cf. Fig. 4c.
This common patterning of the metal layer 22 and the
material layer 23 is again performed through the same
photoresist mask in two etch steps, namely in a first
etch step for etching the (aluminum) material layer 23
and in a subsequent, second etch step for etching the
(chromium) metal layer 22. The results of these two etch
steps are shown in Figs. 4b and 4c, respectively.
In the second process of the invention, like in Fig. 1,
a contact pad may be provided for the substrate elec-
trode; this pad is not shown in Fig. 3 for convenience
of illustration.
In the next process step, a first insulating layer 25,
which contains the diaphragm 25', is deposited over the
entire surface, so that it firmly adheres to the exposed
areas of the glass substrate 21, even in four lateral
regions 210 next to the substrate electrode 22'.
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This insulating layer 25 is preferably of Si02, which is
deposited by plasma chemical vapor deposition, and it
has a thickness of, e.g., 3 ,um, cf. Fig. 4d. In the
aforementioned lateral regions 210, the insulating layer
25 forms a firm bond with the glass substrate 21.
Thereafter, openings 230 for the subsequent supply of an
etchant to the sacrificial layer 23' are etched into the
first insulating layer 25 over at least a part of the
four corner pads 24 belonging to a respective one of the
absolute pressure sensors. Preferably, the openings 230
are respectively formed over two diametrically opposed
corner pads, cf. Fig. 4e and Fig. 3a, in which such an
opening 230 is marked at the upper left with a checkered
pattern, and from which it is apparent that the openings
230 are disposed at the periphery of the absolute
pressure sensor outside the diaphragm 25' to be formed.
If the insulating layer 25 is of Sio2, the etching will
preferably be carried out in a CF4-OZ plasma. During this
etching, the sacrificial layer 23' prevents the glass
substrate from being etched at the surface; in the first
variant, this superficial etching is tolerated.
Next a photoresist mask is formed in a photoresist layer
deposited over the entire surface. The opening of the
mask is congruent, on the one hand, with a top electrode
26' to be formed and, on the other hand, with horizon-
tal and vertical interconnection tracks 27 connected
with the top electrode 26', and is centered with the
substrate electrode 22' - without regard to the corner
pads 24 -, cf. the obliquely hatched areas in Figs. 3a
and 3b, and Fig. 4f.
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The entire surface of the photoresist mask is then
coated with a second metal layer containing the top
electrode 26'. Thereafter, as in the above-mentioned
first variant, the photoresist mask with the overlying
portions of the second metal layer is removed by a lift-
off step. Thus, of the second metal layer, the top
electrodes 26' and the interconnection tracks 27
connected therewith are left, cf. Fig. 4f.
In the next process step, the cavity 28 is formed by re-
moving the sacrificial layer 23' by vertical etching in
the openings 230 and by lateral etching under the dia-
phragm 25'. The substrate electrode 22' and the
diaphragm 25' are now separated by the cavity 28, cf.
Fig. 4g.
Finally, the openings 230 and, thus, the cavity 28 are
hermetically sealed by vacuum-depositing a second in-
sulating layer 29, cf. Fig. 4h. The second insulating
layer 29, too, is preferably of Sio2, which is applied
by plasma chemical vapor deposition, and it has a thick-
ness of 3 Vim, for example.
Fig. 5, a more schematical top view than the views of
Figs. 1 and 3, shows an array of four virtually square
absolute pressure sensors 30 according to the first
variant of the invention which are made according to the
third process variant, as will be explained below.
Figs. 6a to 6h are sections taken along line A-A of Fig.
which illustrate the results of sequential process
steps.
213o~os
- 24 -
In the third process variant of the invention, instead
of a sacrificial layer deposited in a single step, a
sacrificial layer 33 deposited in two steps and thus
consisting of a first partial layer 33' and a second
partial layer 33~~ is used.
Therefore, a first metal layer 32, which contains a sub-
strate electrode 32' to be formed, is deposited on a
glass substate 31, cf. Fig. 6a. In Figs. 6a to 6h, like
in Figs. 4a to 4h, only one surface of the glass
substrate 31 can be seen as the lower edge of the
respective sectional view.
After this process step, the entire surface is coated
with a first patternable material layer which defines a
first part of the height of the (yet-to-be-formed)
cavity 38 and contains the first partial layer 33' of
the sacrificial layer 33 to be formed.
The first material layer and the first metal layer 32
are then patterned in a single, first photoresist step
by etching to simultaneously form the substrate
electrode 32', first interconnection tracks 34 connected
therewith, and the first partial layer 33', which is
virtually congruent with the substrate electrode and the
interconnection tracks. If the material used for the
substrate electrode is chromium and the partial-layer
material is aluminum, this is done sequentially like in
the first process variant by applying the appropriate
etchant, but through the same photoresist mask.
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Thereafter, a second patternable material layer, which
defines the remainder of the height of the cavity 38 and
contains the second partial layer 33" of the sacrificial
layer 33 to be formed and two diametrically opposed
corner extensions 310, is deposited over the entire sur-
face and is so patterned by a second photoresist step as
to completely cover the first partial layer 33', and
thus the underlying substrate electrode 32', cf. Fig.
6c. The second partial layer 33" thus has, around the
first partial layer 33' and the substrate electrode 32' ,
a covering region 331 where it firmly adheres to the
glass substrate 31.
At this point it should be noted that the terms "cover-
ing" and "cover" as used herein mean that a layer
deposited on a structure that has already been formed
extends beyond the entire periphery of this structure
onto the layer supporting this structure and firmly ad-
heres to this layer.
The second partial layer 33" is preferably thinner than
the first. The entire sacrificial layer 33 is therefore
stepped in its height, as can be seen in Fig. 6c.
A first insulating layer 35, which contains the
diaphragm 35' , is now deposited over the entire surface,
cf. Figs. 5 and 6d, so that this layer firmly adheres to
the areas of the glass substrate 31 still exposed after
the preceding step. In the first insulating layer 35,
openings 330 for the subsequent supply of an etchant to
the sacrificial layer 33 are etched above the corner ex-
tensions 310 in the covering region 331 of the second
partial layer 33", cf. Figs. 5 and 6e.
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If the insulating layer 35 is of Si02, the etching is
again preferably carried out in a CF4-OZ plasma. During
this etching process, the second partial layer 33" pre-
vents the glass substrate 31 from being etched at the
surface.
Next a photoresist mask is formed in a photoresist layer
deposited over the entire surface. Its opening is con-
gruent with a top electrode 36' to be formed and with
second interconnection tracks 37 connected therewith,
and is centered with the substrate electrode 32'. The
entire photoresist mask is then coated with a second
metal layer containing the top electrode 36' and the
second interconnection tracks 37. The photoresist mask
with the overlying portions of the second metal layer 36
is then removed by a lift-off step. This gives the
structure shown in Fig. 6f; top electrode 36' and the
second interconnection tracks 37 are left.
The sacrificial layer 33 is then removed by vertical
etching and lateral etching through the openings 330, so
that a cavity 38 forms under the diaphragm 35' in the
place of the sacrificial layer 33. This is done in two
stages, the first of which removes only a portion of the
second partial layer 33~~ near the opening 330. An etch
channel is thus formed for the subsequent etching of the
remaining second partial layer 33~~ and the first partial
layer 33' , i. e. , of the entire sacrificial layer 33 . The
result of this process step can be seen in Fig. 6g.
Finally the openings 330, and hence the cavity 38, are
hermetically sealed by depositing a second insulating
layer 39 in a vacuum, cf. Fig. 6h.
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Fig. 7, also a more schematic top view than Figs. 1 and
3, shows an array of four virtually square absolute
pressure sensors 40 according to the first variant of
the invention which are manufactured according to the
fourth process variant, as will be explained below.
Figs. 8a to 8h are sections taken along line A-A of Fig.
7, which illustrate the results of sequential pro-cess
steps.
In the fourth process variant of the invention, too, in-
stead of a sacrificial layer deposited in a single pro-
cess step, a sacrificial layer 43' deposited in two
steps and thus consisting of a first parial layer 43'
and a second partial layer 43" is used.
Therefore, a first metal layer 42 containing a sub-
strate electrode 42' to be formed is deposited on a
glass substrate 41 and subsequently patterned by a first
photoresist step to form the substrate electrode 42' and
first interconnection tracks 44 connected therewith, cf.
Fig. 8a. Of the glass substrate 41, only one surface can
be seen in Figs. 8a to 8h as the lower edge of the
respective section.
After this step, a first patternable material layer,
which defines a first part of the height of the cavity
48 (to be formed) and contains the first partial layer
43' of the sacrificial layer 43 to be formed, is
deposited over the entire surface and is so patterned by
a second photoresist step as to completely cover the
substrate electrode 42', cf. Figs. 7 and 8b.
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A second patternable matrial layer, which defines the
remainder of the height of the cavity 48 (to be formed)
and contains the second partial layer 43" of the sacri-
ficial layer 43 to be formed, is then deposited over the
entire surface and is so patterned by a third photo-
resist step as to completely cover the first partial
layer 43' and to have two diametrically opposed corner
extensions 410, cf. Figs. 7 and 8c. The second partial
layer 43" thus has, around the first partial layer 43',
a covering region 431 where it firmly adheres to the
glass substrate 41.
Preferably, the second partial layer 43" is thinner than
the first. The entire sacrificial layer 43 is therefore
stepped in its height.
A first insulating layer 45 containing the diaphragm 45'
is then deposited over the entire. surface, cf. Figs. 7
and 8d, so that it firmly adheres to the areas of the
glass substrate 41 still exposed after the preceding
step. Into the first insulating layer 45, openings 430
for the subsequent supply of an etchant to the sacri-
ficial layer 43 are etched above the corner extensions
410 in the covering region 430 of the second partial
layer 43", cf. Figs. 7 and 8e.
If the insulating layer 45 is of Sio2, the etching will
preferably be carried out in a CFA-OZ plasma. During this
etching, the second partial layer 43" prevents the glass
substrate 41 from being etched at the surface.
2~.305n~
- 29 -
In a photoresist layer subsequently deposited over the
entire surface, a photoresist mask is formed whose
opening is congruent with a top electrode 46' to be
formed and with second interconnection tracks 47
connected therewith and is centered with the substrate
electrode 42'.
The entire surface of the photoresist mask is then
coated with a second metal layer containing the top
electrode 46' and the second interconnection tracks 47.
The photoresist mask with the overlying portions of the
second metal layer is subsequently removed by a lift-off
step. This gives the structure shown in Fig. 8f; the top
electrode 46' and the second interconnection tracks 47
are left.
The sacrificial layer 43 is then removed by vertical
etching and lateral etching through the openings 430, so
that the cavity 48 forms under the diaphragm 45' in the
place of the sacrificial layer 43. This is done in two
stages, the first of which removes only a portion of the
second partial layer 43" near the opening 430. This
creates in an etch channel for the subsequent etching of
the remaining second partial layer 43~~ and the first
partial layer 43', i.e., of the entire sacrificial layer
43. The result of this process step can be seen in Fig.
8g.
Finally the openings 430, and thus the cavity 48, are
hermetically sealed by depositing a second insulating
layer 49 in a vacuum, cf. Fig. 8h.
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The division, in the third and fourth process variants,
of the sacrificial layers 33 and 43 into the two partial
layers 33', 33" and 43', 43", respectively, has the
advantage that at the transition from the glass
substrate 31, 41 to the sacrificial layer 33, 43, the
insulating layer 35, 45 need not cover the height of the
sacrificial layer in a single step as in the first and
second process variants, but that this height is divided
into two smaller steps. At these transitions, therefore,
the thickness of the respective insulating layer 35, 45
is more uniform. Furthermore, the openings 330, 430 can
be better hermetically sealed than the openings 230 in
the second process variant, shown in Figs. 3 and 4.
The first process variant requires two photoresist masks
or photoresist steps, the second process variant three
photoresist masks or photoresist steps, the third pro-
cess variant four photoresist masks or photoresist
steps, and the fourth process variant five photoresist
masks or photoresist steps.
Fig. 9 is a schematic top view of an embodiment of a
resistive absolute pressure sensor 60 according to the
second variant of the invention.
The resistive thin-film absolute pressure sensor 60 has
a glass substrate and a diaphragm 65 which bound a
hermetically sealed cavity. The diaphragm 65 is made of
the material of a first insulating layer which firmly
adheres to the substrate around the edge of the cavity.
The diaphragm supports, on the side remote from the
cavity, a piezoresistive half bridge or a piezoresistive
full bridge and a second insulating layer which com-
21.3Q505
- 31 -
pletely covers this bridge and the diaphragm 65 and
hermetically seals the cavity below the diaphragm.
In Fig. 9, a full bridge 61 consisting of four piezo-
resistors 611, 612, 613, 614 is shown. The opposite
piezoresistors are identical in construction, the
piezoresistors 611, 613 lying in areas of the diaphragm
65 which will be compressed when pressure is applied,
and the piezoresistors 612, 614 lying in areas of the
diaphragm 65 which will expand when pressure is applied.
At the edge of the diaphragm 65, the four piezoresistors
are interconnected by interconnection tracks 62 to form
a full bridge, the interconnection tracks extending onto
the substrate outside the diaphragm 65.
To manufacture resistive absolute pressure sensors
according to the second variant of the invention, the
above-explained process variants can be used, with the
following modifications: Since no substrate electrode is
necessary, the process steps for depositing and pattern-
ing the substrate electrode are omitted. Instead of the
steps for depositing and patterning the top electrode,
the piezoresistors of the half or full bridge and their
connections are formed.
Throughout the above explanation of the invention, for
linguistic and definitional convenience, the term "dia-
phragm" only means the patterned first insulating layer
even though further layers, such as the top electrode
and the second insulating layer, are deposited thereon.
The capacitance- or resistance-pressure characteristic
of the absolute pressure sensor is, of course, deter-
mined by the flexural properties of the whole diaphragm
structure with all its layers.
