U.S. patent application number 11/611336 was filed with the patent office on 2007-05-03 for capacitor with a dielectric including a self-organized monolayer of an organic compound.
This patent application is currently assigned to INFINEON TECHNOLOGIES AG. Invention is credited to Franz Effenberger, Marcus Halik, Hagen Klauk, Guenter Schmid, Ute Zschieschang.
Application Number | 20070099338 11/611336 |
Document ID | / |
Family ID | 34801432 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070099338 |
Kind Code |
A1 |
Klauk; Hagen ; et
al. |
May 3, 2007 |
Capacitor with a Dielectric Including a Self-Organized Monolayer of
an Organic Compound
Abstract
A capacitor is formed that includes a self-organized monolayer
of an organic compound between two electrodes.
Inventors: |
Klauk; Hagen; (Erlangen,
DE) ; Halik; Marcus; (Erlangen, DE) ;
Zschieschang; Ute; (Erlangen, DE) ; Schmid;
Guenter; (Hemhofen, DE) ; Effenberger; Franz;
(Stuttgart, DE) |
Correspondence
Address: |
EDELL, SHAPIRO & FINNAN, LLC
1901 RESEARCH BLVD.
SUITE 400
ROCKVILLE
MD
20850
US
|
Assignee: |
INFINEON TECHNOLOGIES AG
St.-Martin-Str. 53
Munich
DE
81669
|
Family ID: |
34801432 |
Appl. No.: |
11/611336 |
Filed: |
December 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11046905 |
Feb 1, 2005 |
|
|
|
11611336 |
Dec 15, 2006 |
|
|
|
Current U.S.
Class: |
438/99 ;
438/393 |
Current CPC
Class: |
H05K 1/162 20130101;
H01G 4/18 20130101 |
Class at
Publication: |
438/099 ;
438/393 |
International
Class: |
H01L 51/40 20060101
H01L051/40; H01L 21/20 20060101 H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2004 |
DE |
102004005082.1 |
Claims
1. A method for producing a capacitor comprising: depositing a
first electrode; bringing the first electrode into contact with an
organic compound to deposit a monolayer of the organic compound on
the first electrode so as to form a structure of the first
electrode and the organic compound; and depositing a second
electrode.
2. The method of claim 1, wherein the organic compound is brought
into contact with the first electrode in a solution, and the method
further comprises: rinsing the structure with solvent to remove
excess organic compound in solution from the structure prior to
depositing the second electrode; and evaporating the solvent prior
to depositing the second electrode.
3. The method of claim 2, wherein the concentration of the organic
compound in the solution is between 10.sup.-4% and 1% by
weight.
4. The method of claim 2, wherein the solvent comprises a dried,
low-polarity, aprotic solvent.
5. The method of claim 4, wherein the solvent comprises one of
toluene, tetrahydrofuran, and cyclohexane.
6. The method of claim 2, further comprising: after rinsing the
structure with solvent and prior to depositing the second
electrode, further rinsing the structure with volatile
solvents.
7. The method of claim 1, wherein the organic compound is brought
into contact with the first electrode in a gas phase to deposit the
monolayer of the organic compound on the first layer.
8. The method of claim 7, wherein the pressure during the gas phase
deposition of the organic compound is between 10.sup.-6 mbar and
400 mbar.
9. The method of claim 6, wherein the temperature during the gas
phase deposition of the organic compound is between 80.degree. C.
and 200.degree. C.
10. The method of claim 6, wherein the gas phase deposition of the
organic compound is carried out over a time period of between 3 min
and 24 hours.
11. The method of claim 1, wherein the second electrode is
vapor-deposited.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/046,905, filed on Feb. 1, 2005, and titled, "Capacitor
with a Dielectric Comprising a Self-Organized Monolayer of an
Organic Compound," which claims priority under 35 USC .sctn. 119 to
German Application No. DE 10 2004 005 082.1, filed on Feb. 2, 2004,
and titled "Capacitor with a Dielectric Comprising a Self-Organized
Monolayer of an Organic Compound." The entire contents of these
applications are hereby incorporated by reference in their
entireties.
FIELD OF THE INVENTION
[0002] The invention relates to capacitors for semiconductors in
which the dielectric includes an organic compound.
BACKGROUND
[0003] The semiconductor industry is developing chips which have
ever smaller structures, and consequently an ever increasing
density of components, to increase the computing speed of
processors, the storage volume of memory elements and the power of
capacitors, and also to lower the costs for the components.
[0004] The increasing miniaturization and integration of complex
electronic circuits requires not only the integration of
semiconducting components but also an integration of passive
components, such as, for example, capacitors, coils, resistors,
etc. The variety of application areas has had the effect that the
microchips have to meet much more stringent requirements. Since the
microchips are present in many everyday devices, such as, for
example, computers, cell phones, GPS receivers, CD/DVD drives,
cameras, pocket calculators, wristwatches, domestic appliances,
cars, etc., both the active semiconductor elements and the passive
semiconductor elements have to satisfy many different
requirements.
[0005] The capacitors at the present time are produced by using
inorganic dielectrics, for example, insulating metal oxides
arranged between two electrodes, and are initially completed as
individual pieces during the fabrication of printed circuit boards
and then placed individually on the PCB and soldered by means of
conventional construction and connecting techniques. However, these
techniques require the use of complex and cost-intensive
"pick-and-place" machines, with the effect of increasing production
costs.
[0006] Microchips that are presently used are generally based on
silicon as the semiconductor material. The production of the
passive components and the integration of these components are
still comparatively complex and expensive in spite of the advanced
methods of production. In the case of some areas of use, these
costs are not significant, since the memory units usually remain on
the item for a considerable time or are used for goods which
command a high price. There are, however, a whole series of
applications in which goods that are relatively inexpensive are
used and the attached microchips make up a significant proportion
of the costs, with the result that the remaining components that
would otherwise be used are ruled out of everyday use for reasons
of cost.
[0007] A considerable cost reduction and time saving could be
achieved, for example, by using RFID tags (Radio Frequency
Identification Tags) in the retail sector. In the case of these
applications, the price of an RFID tag for labeling products must
not exceed that of a conventional barcode tag. Therefore, in this
"low performance" area, the production costs must be fractions of a
cent.
[0008] Furthermore, the microchips must have properties such as
great robustness or low weight, to allow them to be processed
without any problems, or else have great flexibility, to allow them
to also be used on curved surfaces.
[0009] Attempts have been made to develop capacitors which can be
integrated in the various substrates without the use of
"pick-and-place" machines. For example, R. Ulrich and L. Shaper:
IEEE Spectrum, July 2003, have proposed creating a capacitor
including inorganic dielectrics and metal electrodes directly on or
in the printed circuit boards in a multistage production process.
This multilayer construction is intended to be used on various
substrates, such as, for example, fixed PCB substrates, for example
FR4 or flexible PCBs made of polyamide. The problem with this
technology is the relatively low thermal stability of the PCB
materials, since the depositing of high-quality inorganic
dielectrics generally requires temperatures above approximately
400.degree. C. The result is therefore a compromise in which a low
quality and reliability of the dielectrics is deliberately accepted
in order to permit integration in the PCBs.
[0010] A further possibility for the integration of capacitors on
flexible substrates specifically for cost-driven RFID applications
is described in D. Redinger et al.: Device Research Conference
Digest 2003, 187-188. This approach is based on printing techniques
in the case of which both the electrodes and the dielectric of the
capacitor are deposited by using low-cost printing processes.
Allowance is made for the low thermal budget of the flexible
polymeric substrates by using organic polymer dielectrics, such as
a polyamide with a maximum process temperature of 190.degree. C.
The problem with this solution is the large space requirement of
capacitors produced in this way, since the creation of polymer
layers with adequately good insulating properties generally
requires layer thicknesses of several 100 nm. The capacitor
proposed by Redinger et al. has a polyimide layer thickness of
approximately 1 .mu.m. However, the power of a capacitor depends on
the layer thickness and can be represented by the following
formula: C=.epsilon.A/t where C is the capacitance, .epsilon. is
the dielectric constant, A is the surface area and t is the layer
thickness of the dielectric. Consequently, a great layer thickness
inevitably leads to a small capacitance per unit area. When polymer
layers of approximately 1 .mu.m are used, as described by Redinger
et al., a large capacitance can be achieved only by increasing the
surface area. For example, RFID transponders which are intended for
the 13.56 MHz frequency band require a resonance capacitor of 0.4
nF, with the result that, if the layer thickness described by
Redinger et al. is used, this capacitor would take up a surface
area of approximately 25 mm.sup.2.
SUMMARY OF THE INVENTION
[0011] An object of the invention is therefore to provide
capacitors in which the dielectric has a layer thickness of just a
few nanometers, the capacitors can be produced by customary
semiconductor techniques, the capacitors can be easily incorporated
in printed circuit boards, and as a result do not require use of
"pick-and-place" machines.
[0012] Another object of the invention is to provide capacitors
that can be incorporated in various substrates and have good
electrical properties.
[0013] The aforesaid objects are achieved individually and/or in
combination, and it is not intended that the present invention be
construed as requiring two or more of the objects to be combined
unless expressly required by the claims attached hereto.
[0014] In accordance with the present invention, a capacitor
includes a first electrode, a second electrode, and a dielectric
layer arranged between the first electrode and the second
electrode. The dielectric layer is formed from a self-organized
monolayer.
[0015] Preferably, the organic compound includes an anchor group, a
linker chain and a head group, where the anchor group includes at
least one of R--SiCl.sub.3, R--SiCl.sub.2-alkyl,
R--SiCl(alkyl).sub.2, R--Si(OR).sub.3, R--Si(OR).sub.2-alkyl,
R--SiOR(alkyl).sub.2, R--PO(OH).sub.2, R--CHO, R--CH.dbd.CH.sub.2,
R--SH, R--S.sup.-, R--COO.sup.- and R--COSH, the linker chain
includes at least one of --(CH.sub.2).sub.n1-- and
--(O--CH.sub.2).sub.n2--, where n1 is from 2 to 20 and n2 is from 2
to 10, and the head group includes an aromatic group.
[0016] In accordance with another embodiment of the present
invention, a method for producing a capacitor includes the steps of
depositing a first electrode, bringing the first electrode into
contact with an organic compound to deposit a monolayer of the
organic compound on the first electrode so as to form a structure
of the first electrode and the organic compound, and depositing a
second electrode.
[0017] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed description of specific embodiments thereof,
particularly when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 depicts a schematic of a capacitor structure in
accordance with the invention.
[0019] FIG. 2 is a diagram of current density vs. voltage for a
capacitor structure formed in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The capacitor according to the invention includes a number
of significant advantages including, without limitation, the
capacitor being produced by a simple method and used on any desired
substrates while using organic self-organized monolayers as the
capacitor dielectric. The construction of these thin-film
capacitors takes place by conventional layer construction
(electrode-dielectric-electrode), it being possible for the three
layers to be created one after the other by using evaporation
processes, printing processes or immersion processes. In this case,
the evaporation temperatures of the organic compounds which form
the dielectric monolayer on the electrode surface are particularly
favorable for deposition on flexible substrates, since the
temperature at which the depositing takes place is generally less
than 200.degree. C. The layer thickness of the dielectric is merely
the thickness of a monolayer and corresponds approximately to the
molecule length, with the result that the dielectric thickness is
in the range of approximately 2 to 10 nm. On this basis outstanding
electrical properties of the capacitor according to the invention
can be achieved.
[0021] For example, for an RFID transponder which is intended for
the 13.56 MHz frequency band and requires a resonance capacitor of
0.4 nF, a capacitor according to the invention merely requires a
surface area of 0.04 mm.sup.2. That is 0.16% of the area
requirement of the polyimide capacitor described by Redinger et al.
Capacitors with greater or smaller capacitances can be created by
definition of the electrode surface areas.
[0022] An important factor for the outstanding dielectric
properties is the molecular design of the organic compound, which
includes an anchor group, a head group and a linker chain linking
the anchor group and the head group. Here, the reactive anchor
group attaches the molecule to the electrode surface, preferably by
a covalent bond, which results in particularly high thermal,
mechanical and chemical stability of the monolayer. The linker
chain, which is preferably formed by an n-alkyl chain or an ether
chain, brings about a virtually orthogonal alignment, and
consequently the densest possible packing of the molecules. The
head group, which preferably has a .pi.-system or some other
radical capable of intermolecular interactions, serves for
stabilizing the monolayer by using the intermolecular interactions
(.pi..pi.-interaction) to make molecules interact to a greater
extent with their respective neighbor and consequently be
additionally stabilized mechanically and electrically. As a
consequence, such layers are better insulators than comparable
monolayers without corresponding head groups.
[0023] One particular advantage of the materials according to the
invention is the variability with respect to the electrode material
obtained by choosing suitable reactive anchor groups. In principle,
all metals or alloys or semimetals which have a natural oxide film
and/or can be superficially oxidized in a simple way are suitable
as electrode material. Furthermore, other metals and their alloys
that are capable of forming covalent bonds or other strong
interactions with organic reactive groups, such as, for example,
gold, silver, copper and gallium arsenide in the case of the thiol
anchor groups, are also suitable as electrode material for use in
the invention.
[0024] Thus, the capacitor according to the invention permits
technically simple integration on any desired substrates, the
production of capacitors with a comparatively small area
requirement is possible (since the layer thickness of the
dielectric layer is in the nanometer range), and there also is
great variability in the choice of electrode materials in forming
the capacitor.
[0025] In one particular embodiment of the invention, the layer
thickness of the dielectric has the length of a single molecule and
is in the range of about 1 to about 10 nM. The length of the
molecule is intended to permit an orthogonal alignment, with the
result that it is very difficult for shorter molecules that are
less than 1 nm to form a monolayer. In the case of the molecules
that are longer than 10 nm, it is difficult to obtain the
orthogonal alignment because of many degrees of freedom. In a
preferred embodiment, the layer thickness of the dielectric is in
the range of about 2 nm to about 5 nm.
[0026] As already mentioned above, the head group can be any group
that permits an intermolecular interaction between two molecules.
According to the invention, .pi.-systems may serve as the head
group, since a .pi..pi.-interaction can come about as a result,
which contributes to the stabilization of the monolayer. The
.pi.-systems according to the invention may also be substituted by
heteroatoms.
[0027] All groups permitting an orthogonal alignment of the
molecule and keeping the spacing between the head group and the
anchor group stable are suitable as linker groups. In one
particular embodiment of the invention, the linker chains are
formed by n-alkyl chains or polyether chains. The .pi.-alkyl or
polyether chains have repeat units --(CH.sub.2).sub.n-- or
--(O--CH.sub.2--CH.sub.2).sub.n--, with n in the range from
approximately 2 to 20 for the n-alkyl chain and in the range from 2
to 10 for the polyether chain.
[0028] As already described above, the electrodes may consist of
all metals or metal alloys or semimetals, the only important factor
being that the electrode material preferably enters into a covalent
bond with the anchor group. However, some other interaction, such
as, for example ionic interaction, hydrogen bridging or
charge-transfer interaction, also comes into consideration.
[0029] Aluminum, titanium, gold, silver, copper, palladium,
platinum, nickel, silicon and gallium arsenide are preferred for
the electrode materials. If the electrode material consists of
aluminum or titanium, the surface can be oxidized in a simple way,
to allow it to be reacted with anchor groups. To this end,
R--SiCl.sub.3; R--SiCl.sub.2-alkyl, R--SiCl(alkyl).sub.2;
R--Si(OR).sub.3; R--Si(OR).sub.2-alkyl; R--SiOR(alkyl).sub.2 and/or
R--PO(OH).sub.2 are preferred as anchor groups.
[0030] If silicon with a native or specifically generated silicon
oxide layer, such as, for example, hydroxy-terminated silicon, is
used, the following are preferred as anchor groups: R--SiCl.sub.3;
R--SiCl.sub.2-alkyl; R--SiCl(alkyl).sub.2; R--Si(OR).sub.3;
R--Si(OR).sub.2-alkyl and R--SiOR(alkyl).sub.2.
[0031] If silicon with a hydrogen surface is used as the electrode
material, the preferred anchor groups are R--CHO(h.nu.) and
R--CH.dbd.CH.sub.2(h.nu.).
[0032] For the second electrode, which does not have to enter into
a covalent bond with the self-organized monolayer, all electrically
conductive materials are suitable, in particular metals and
conductive polymers.
[0033] Since the depositing of the organic molecules which form the
dielectric monolayer on the electrode surface is particularly
gentle, and very suitable for flexible substrates, in a preferred
embodiment the capacitor according to the invention is incorporated
in flexible substrates.
[0034] The schematic construction of a capacitor according to the
invention is depicted in FIG. 1. Between first electrode 2 and
second electrode 4 of the capacitor 1, there is a layer 6 of an
organic molecule that enters into a covalent bond (via its anchor
group) with the first electrode 2, has a virtually orthogonal
alignment (from its linker chain to its head group) between the two
electrodes and by which the .pi.-system for the second electrode 4
is stabilized. The first electrode 2 consists of natively oxidized
silicon and the second electrode 4 consists of gold.
[0035] The production of the capacitor according to the invention
takes place by depositing the first electrode, bringing the first
electrode into contact with the organic compound to obtain a
self-organized monolayer of the compound on the first electrode,
appropriate rinsing of the structure with the solvent in which the
compound was dissolved is then conducted to remove the excess
compound, followed by evaporating of the solvent and depositing the
second electrode.
[0036] The rinsing of the excess compound only takes place if the
compound is dissolved in a solvent. In a preferred embodiment such
as previously described, the compound is brought into contact with
the first electrode in the solution. However, other methods for
depositing the organic compound are possible.
[0037] The concentration of the organic compound of which the
solution is brought into contact with the first electrode is
preferably between about 10.sup.-4 and about 1% by weight. This
concentration in the range from about 10.sup.-4 to about 1% by
weight is suitable in particular for producing dense layers. It is
also possible, however, to use less concentrated or more (i.e.,
highly) concentrated solutions of the organic compounds. The
depositing can then take place by immersing the substrate with a
defined first electrode in the prepared solution, after which the
rinsing with the pure process solvent can take place. Optionally,
the structure obtained in this way may subsequently be rinsed with
a readily volatile solvent, such as acetone or dichloromethane for
example, and subsequently dried under inert gas. The preferred
solvents for dissolving the organic compounds are dried,
low-polarity, aprotic solvents. Examples of such solvents include
toluene, tetrahydrofuran and cyclohexane.
[0038] If the organic compound is brought into contact with the
first electrode from the gas phase, the pressure is preferably
between about 10.sup.-6 mbar and about 400 mbar, depending upon the
volatility of the organic compound. The process temperature is
preferably in the range from about 80.degree. C. to about
200.degree. C. and the depositing time lies between about 3 min and
about 24 hours.
[0039] When the molecular self-organized monolayer is obtained, the
second electrode can be deposited, for example, by
vapour-depositing.
[0040] The electrical properties of the capacitor according to the
invention are explained with reference to FIG. 2. In particular,
FIG. 2 depicts the current-voltage characteristic of a capacitor
including a lower electrode made of monocrystalline, natively
oxidized silicon, an organic self-organized monolayer as the
dielectric ((18-phenoxyoctadecyl)-trichlorosilane, 2.5 nm thick)
and an upper electrode made of thermally vapor-deposited gold (30
nm thick). The characteristic demonstrates the excellent insulating
properties of the self-organized monolayer, which can withstand an
electrical voltage of up to 4 V, which corresponds to an electric
field strength of 16 MV/cm. The leakage current (which, although
undesired, is unavoidable and flows over the dielectric on account
of the voltage present between the electrodes, leading to a slow
loss of charge) increases in the expected way as the voltage
increases, until a breakdown occurs at a voltage of 4 V (16 MV/cm).
In the case of the capacitor which can be used for an RFID
transponder, an extremely small leakage current of approximately 10
pA flows when there is a voltage of 2 V.
[0041] While the invention has been described in detail and with
reference to specific embodiments thereof, it is believed that
other modifications, variations and changes will be suggested to
those skilled in the art in view of the teachings set forth herein.
It is therefore to be understood that all such variations,
modifications and changes are believed to fall within the scope of
the present invention as defined by the appended claims and their
equivalents.
* * * * *