Patent Application
20110133597
1. Field of Invention
The current claims relate to electromechanical systems and waveguides, devices that incorporate the electromechanical systems and waveguides, and methods of production.
2. Discussion of Related Art
Microelectromechanical systems (MEMS) devices comprise microstructures that are usually separated from other control elements, for example control electrodes, by narrow air gaps. The MEMS devices have movable structures that can move over the space provided by the gap. Usually, this movement is used to make a contact with an electrode, such as in a MEMS switch. Variation of the width of the gap can be used to change electrical characteristics of the device, such as a variable capacitor. In many applications, the MEMS structure is used as a sensitive element and the movement of the microstructure is used to detect an external effect such as pressure, acceleration, etc. In many applications, performance of the MEMS structure is limited by the size of the gap. Larger gaps require greater forces to enable the MEMS structure to move. Accordingly, the sensitivity of the MEMS structure is lower for larger gaps and higher power is needed to control it. For example, electrical voltages that are needed for the operation of MEMS devices is relatively high, i.e., of the order of a few tens of Volts. To improve performance of MEMS devices, significantly smaller gaps are needed. The smaller gaps can improve sensitivity and allow for lower power operation of the devices. Most traditional MEMS devices require a vacuum in the gap and, therefore, require special protection from ambient atmosphere, such as sealing and encapsulation.
MEMS structures are usually produced using a layer referred as a sacrificial layer because it is removed during fabrication. The sacrificial layer is prepared in the space which later becomes a gap which separates the movable component of the MEMS structure from other parts of the device, for example, control electrodes. The sacrificial layer is temporarily included in the device to enable preparation of the movable component of the MEMS structure on top of it. After that, the sacrificial layer is removed by etching to provide the gap that separates the movable component of the MEMS structure and the electrodes. The use of the sacrificial layer and etching process limits the minimum size of the gap because it is more difficult to etch thinner sacrificial layer materials underneath the top structure. To improve etching processes one can make holes in the movable component of the MEMS structure. However, the smallest gaps obtained by this approach are in the range from 1 to 1.5 micrometers. In addition, the etching process is incompatible with the production of many electronic circuits on the same substrate, such as CMOS integrated circuits. Therefore, there remains a need for improved electromechanical systems, methods of producing electromechanical systems and devices that incorporate the electromechanical systems.
A method of producing an electromechanical device according to an embodiment of the current invention includes forming a layer of density-changing material on a substructure, and forming a support layer on at least a portion of the layer of density-changing material. The density-changing material has a first density during the forming the layer and a second density subsequent to the forming the support layer, the second density being greater than the first density such that the layer of density-changing material shrinks in at least one dimension to provide a gap between the layer of density changing material and at least one of the support layer and the substructure.
A combined electronic and electromechanical device according to an embodiment of the current invention has a substrate, an electronic circuit formed on the substrate, and an electromechanical system formed on the substrate to provide a combined electronic and electromechanical device on a common substrate. The electromechanical system comprises a structure that is free to move within a gap defined by the electromechanical system.
An electromechanical system according to an embodiment of the current invention has a substructure, and a movable component attached to the substructure such that a gap is provided between the movable component and the substructure. The gap is less than about 500 nm.
An apparatus according to an embodiment of the current invention has an electromechanical system that has a substructure, and a movable component attached to the substructure such that a gap is provided between the movable component and the substructure. The gap is less than about 500 nm.
A method of producing a waveguide according to an embodiment of the current invention includes providing a substructure, forming a layer of density-changing material on the substructure, and forming an upper layer on at least a portion of the layer of density-changing material. The density-changing material has a first density during the forming the layer and a second density subsequent to the forming the upper layer, the second density being greater than the first density such that the layer of density-changing material shrinks in at least one dimension to provide a gap between the layer of density changing material and at least one of the upper layer and the substructure.
Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.
Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification are incorporated by reference as if each had been individually incorporated.
Some embodiments of the current invention are directed to Micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) and methods of producing the MEMS and NEMS. The MEMS and NEMS according to some embodiments of the current invention can be applied in RF applications, mobile technologies, photonics, energy, solar cells, telecommunications, transistors and diodes for example. Methods of fabricating MEMS and NEMS devices according to some embodiments of the current invention allow the fabrication of structures having nanogaps which are significantly smaller than 1 micrometer and can be as small as a few nanometers in size, or even less than 1 nm (e.g., 0.5 nm). (In the remainder of this description, we often refer to MEMS devices, but that should be interpreted as including NEMS devices.) There can be several nanogaps in one device and the device may have multigap or multi-nanogap structures, according to some embodiments of the current invention. Such structures can be used, for example, in transistors. In addition, methods of producing devices according to some embodiments of the current invention allow us to simplify fabrication of MEMS devices by avoiding encapsulation or other protection procedures such as sealing, etc. In addition, methods of production according to some embodiments of the current invention can also be used for encapsulation of devices. The fabrication methods according to some embodiments of the current invention can be used to make planar waveguides for light transmission or transmission lines for RF signals of high frequency in the GHz range. Long structures having nanogaps inside can be a new type of nanotubes. In addition, fabrication methods according to some embodiments of the current invention allow for the fabrication of vacuum gaps that do not need special sealing or protection against ambient atmosphere. For example, fabrication methods according to some embodiments of the current invention allow for fabrication of gaps inside the material and therefore, the gap automatically provides a vacuum relative to the surrounding environment.
The term vacuum as used here is intended to have a broad meaning to include partial vacuums as well as substantially complete vacuums, as long as the gas pressure within the gap is less than that of the surrounding environment. Nonetheless, in some cases, the vacuum can also be a high vacuum so that there is little easily discernable gas in the gap.
An embodiment of the current invention provides a method of fabricating ultrathin gaps that can have a size of few nanometers, for example. We call this gap a nanogap. According to some aspects of the current invention, the nanogap can be formed when two or more specially chosen materials chemically react with each other to form a final material with higher density than the initial materials. Accordingly, the higher density will result in smaller volume. If these materials are placed between two solid structures, then the chemical reaction will lead to shrinking the volume occupied by the inner material. As a result, the difference between the initial volume and the final volume will release an empty space that will form the gap. The greater the difference between the densities of initial components and the density of the final material, the larger the volume that can be released. In addition to chemical reaction, a diffusion process can take place. The fabrication of an example of a device that has a nanogap according to an embodiment of the current invention is illustrated schematically in
In
The effect can be estimated mathematically using the law of conservation of mass. For example, consider the case in which the initial density of the first layer is ρ1 and its thickness is h1, and the initial density of the second layer is ρ2 with a thickness of h2. After the reaction, the density of the first layer changes to ρ1x, and its thickness changes to h1x, and the density of the second layer changes to ρ2x and its thickness changes to h2x. The gap hg can be obtained from the following relation
h
1
+h
2
=h
1x
+h
2x
+h
g. (1)
Using the law of conservation of mass leads to the relation
ρ1h1+ρ2h2=ρ1xh1x+ρ2xh2x, (2)
where one can assume that thin layers change their volumes along thickness and don't change their surface areas. One can be seen from these equations that different conditions can result in providing a gap, as follows:
The nanogap can be formed above the layered structure during shrinking as shown in
According to another embodiment of the current invention, there can be combinations of three or more layers. The chemical reaction can occur with three or more participating layers. Accordingly, the final effect will be obtained from similar equations. For example, for three layers
h
1
+h
2
+h
3
=h
1x
+h
2x
+h
3x
+h
g, (3)
and from law of conservation of mass
ρ1h1+ρ2h2+ρ3h3=ρ1xh1x+ρ2xh2x+ρ3xh3x. (4)
Similarly, one can design more complex reactions when chemical reactions take place in separate stages, for example, material M1 can react first with material M2 to form material M12. Then material M12 reacts with material M3 resulting in material M123 etc. Physical diffusion, temperature, radiation, incident illumination and other physical processes can be used to control the reaction.
According to further embodiments of the current invention, more complex structures can be used for the fabrication of more than one nanogap in a device. One can prepare two or more gaps in one device by methods similar to those described above. Furthermore, a single layer of material can also be used to produce structures that have a gap. For example, the single layer can undergo a phase change such that the final phase has a greater density than the initial phase. In another embodiment, the single layer could undergo sublimation such that the layer increases in density. Furthermore, phase changes and/or sublimation processes could be combined with chemical reactions in more complex layer structures in some embodiments of the current invention.
In a more general sense, a method of producing an electromechanical device according to an embodiment of the current invention includes forming a layer of density-changing material on a substructure, and forming a support layer on at least a portion of the layer of density-changing material. The density-changing material has a first density during the forming the layer and a second density subsequent to the forming the support layer, the second density being greater than the first density such that the layer of density-changing material shrinks in at least one dimension to provide a gap between the layer of density changing material and at least one of the support layer and the substructure.
The substructure can be a substrate in some embodiments of the current invention, or can be a more complex structure. For example, the substructure in the example of
The density-changing material changes density due to a phase change in its structure in some embodiments of the current invention. In other embodiments, the density-changing material changes density due to sublimation. In further embodiments, the density changing material changes density due to one or more chemical reactions. In some embodiments, the density changing material changes density as the result of a combination of one of more of a phase change, sublimation, chemical reactions or diffusion.
In some embodiments of the current invention, the layer of density-changing material provides a membrane structure after it shrinks. In some embodiments, the membrane can be formed in a vacuum relative to a surrounding environment. In other embodiments, external components can form a membrane, such as the structure 5 in the example of
In some embodiments, energy is transferred to the layer of density-changing material to initiate a phase change, sublimation and/or a chemical reaction. The energy transfer can include at least one of heating, passing an electrical current through the layer of density-changing material, or directing radiation onto the layer of density-changing material The radiation can be electromagnetic radiation, for example.
The forming the layer of density-changing material can include forming a plurality of sub layers of respective pluralities of materials. The general concepts of the current invention are not limited to a particular number of sub-layers of materials, which can be two, three or more than three sub-layers, for example.
In some embodiments of the current invention, the gap is less than 1 μm such that said electromechanical device is a micro-electromechanical system or nano-electromechanical system. However, the general concepts of the method of producing electromechanical devices are not limited to producing only MEMS and NEMS devices. Devices with larger gaps can also be produced according to some embodiments of the current invention. In some embodiments, the gap is less than 0.5 μm such that the electromechanical device produced is a nano-electromechanical system. In further embodiments, the gap in the NEMS devices produced is less than 1 μm and greater than 0.5 nm, and in some embodiments the gap is less than 0.5 μm and greater than 0.5 nm.
Another embodiment of the current invention is directed to an electromechanical system that has a substructure, and a movable component attached to the substructure such that a gap is provided between the movable component and the substructure. The gap is less than about 500 nm. In some embodiments, the gap is less than about 200 nm. In further embodiments, the gap is less than about 100 nm and greater than about 0.5 nm.
The movable component can be a membrane in some embodiments or cantilever (see, for example, U.S. patent application Ser. No. 12/370,882 by A. Pavlov et al., “Resonant MEMS Device that Detects Photons, Particles and Small Forces”, filed Feb. 13, 2009, the entire content of which is incorporated herein by reference) in some embodiments or a bridge-like structure in some embodiments of the current invention. The electromechanical system can include enclosing structures in some embodiments of the current invention such that the gap is substantially a vacuum relative to a surrounding environment.
Further embodiments of the current invention include an apparatus that includes one or more electromechanical systems according to the current invention. Another embodiment of the current invention is directed to a method of producing a waveguide that includes providing a substructure; forming a layer of density-changing material on the substructure; and forming an upper layer on at least a portion of the layer of density-changing material. The density-changing material has a first density during the forming the layer and a second density subsequent to the forming the upper layer, the second density being greater than the first density such that the layer of density-changing material shrinks in at least one dimension to provide a gap between the layer of density changing material and at least one of the upper layer and the substructure.
An example of a device based on the fabrication method according to an embodiment of the current invention is shown in
a shows a device that can work as an accelerometer according to an embodiment of the current invention. The device comprises a substrate 40, isolation layer 41, another isolation layer 42, 43 forming step structure; electrodes 65, 66, 67, 68 and 69; density-changing layer 50, nanogap 49 and a top MEMS structure. In this device electrodes are separated from each other and can sense motion of the MEMS structure in different direction sensing forces F11,F12, F21,F22 and F3 shown in
The term “waveguide” is considered in a broad sense to mean that the device can transfer electro-magnetic waves, light, radiation or electrical charge depending on materials forming the waveguide. For transferring light or radiation it is an optical waveguide. For transferring electrical charge it is a nanotube. Accordingly, materials for the device will vary depending on the application. Prior art optical waveguides use photonic crystal fibers or hollow fibers. The prior art fibers cannot be integrated on a chip with other devices like laser diodes and photodetectors and they are limited by a few materials that can be used in the process. We disclose a waveguide device that can be made of different materials, the inner gap can be very small, of the nanometer size, and the size can be predicted with a very good accuracy. An example of a waveguide device according to an embodiment of the current invention is shown in
One can estimate the effect, for example, for aluminum (Al). It has a density of 2.7 g/cm3. When Al reacts with oxygen, the oxide Al2O3 will have a density of 3.96 g/cm3. If one deposits a thin layer of Al on a substrate, then oxidation will result in decreasing its thickness because the density of the oxide is greater than that of Al and the thickness of the oxide will be accordingly smaller. The relation can be obtained, taking into account that the total mass of the layer remains almost the same, i.e. if the initial thickness of the layer is h1, initial density ρ1, thickness after oxidation h1x and final density ρ1x, then h1x=h1 ρ1/ρ1x. The thickness of Al2O3 will be thinner by a ratio of 2.7/3.96=0.68, or 68% of the initial thickness. If, for example, the initial thickness is 10 nm, then the final thickness will be about 7 nm. When the initial layer is placed between two fixed plates, such shrinking of the layer will release a gap of about 3 nm. If the layer is prepared inside (surrounded by or enveloped by) the other material, the gap will be substantially a vacuum with respect to the surrounding environment formed inside the structure.
The formation of the nanogap according to some embodiments of the current invention depends on reaction time within the layered structure. There are three main cases:
Electronegativity of the materials can be chosen for particular applications. The buffer layer can also be used for the transfer of charge and electrons according to some embodiments of the current invention. The thickness of the buffer layer can change too in some embodiments of the current invention.
Below are some combinations of materials that can be used for some applications of some embodiments of the current invention. However, the general concepts of the current invention are not limited to only these examples.
Metals, oxides and modifications:
Some embodiments of the current invention can utilize the following regularities: formation of oxides of light metals such as sodium Na, potassium K, calcium Ca, lithium Li, magnesium Ma, beryllium Be, and others will result in an increase of the density; on the other hand, formation of oxides of heavy metals results in a decrease of the density.
Almost all inter-oxide compounds form with a decrease in density. However, increases of density can be obtained in phase transitions in metals. For example, ordinary white tin alpha-Sn has a density of 5.85 g/cm3. It transforms to gray-tin β-Sn at temperature minus 20° C. The β-Sn has a density of 7.29 g/cm3.
Increases of density can be obtained in phase transitions in oxides. For example, TiO2 anataz has density of 3.8 g/cm3, but TiO2 rutile has density of 4.2-4.3 g/cm3. One can add a small fraction of another material such as niobium Nb or tantalum Ta to control the electrical conductivity.
Other examples include, but are not limited to,
α-Ga2O3(5.88 g/cm3)→β-Ga2O3(6.48 g/cm3);
β-GeO2(4.70 g/cm3)→α-GeO2(6.24 g/cm3); and
Hexagonal SiO2(2.65 g/cm3)-Tetragonal SiO2(3.44 g/cm3).
Density can increase in reactions that lead to a decrease of Entropy. For example, density can increase as a result of reaction of oxides of a metal having different oxidation ratios. For example, oxide of Bismuth (II) BiO having density of 7.20 g/cm3 reacts with oxide of Bismuth (V) Bi2O5 having a density 5.10 g/cm3 to form the oxide of Bismuth (III):
4BiO+Bi2O5=3Bi2O3.
The resultant oxide Bi2O3 has a density of 8.9 g/cm3. Here 4 molecules of BiO and one molecule of Bi2O5 form 3 molecules Bi2O3. The reaction leads to a decrease of molecular volume, and accordingly a decrease of Entropy.
An increase of density can be obtained by sublimation of some easily evaporating volatile oxides: OsO4 (evaporation temperature 131° C.), Re2O7 (evaporating temperature 359° C.), for example.
An increase of density can be obtained during reduction of oxides of heavy metals with hydrogen:
CuO+H2=Cu+H2O.
Density can be significantly increased by reduction reactions of oxides of heavy metals releasing the metals. Some oxides, for example, oxides of Au, Ag, Pt and Ir metals decompose under heating to yield the metal and oxygen, such as:
Ag2O→2Ag+½O2(T>=200° C.)
IrO2→Ir+O2(T>1000° C.)
In the latter reaction, the density will increase from 3.15 g/cm3 for IrO2 to 22.4 g/cm3 for Ir.
Oxides of Noble metals can be reduced by hydrogen too.
The reaction can be enhanced by thermal conditions, electromagnetic fields, illumination of light or radiation. One can use DC electrostatic fields or AC variable electrical fields or variable electrical fields at resonant oscillation of the MEMS structure.
In an embodiment of the current invention, two or more layered structures can be used for fabrication of multigaps in one device.
Below are some examples of materials that can be used for the formation of vacuum cavities using reduction of Cu from CuO.
The Copper can be reduced from its oxide. There are the following reactions:
CuO—Ti
2CuO+Ti=TiO2+2Cu
ΔH:2*162+0−944=−620 kJ (1)
hCuO=(2*80)/6.45=24.8 nm
hTi=(47.8)/4.5=10.6 nm
hTiO2+Cu=(80)/4.2+(2*64)/8.96=19+14=33 nm
Δh=35.4−33=2.3 nm(6.5%).
CuO—Al
3CuO+2Al=Al2O3+3Cu (2)
ΔH:3·162+0−1676=−1190 kJ
hNiO=(3·80)/6.45=37.2 nm
hAl=(2·27)/2.7=20 nm
hAl2O3+Cu=(102)/3.96+(3·64)/8.96=26+21=47 nm
Δh=57−46=11 nm(19%).
CuO—Mg
CuO+Mg=MgO+Cu (3)
ΔH:162+0−601=−439 kJ
hCuO=(80)/6.45=12 nm
hMg=(24.3)/1.74=14 nm
hMgO+Cu=(40)/3.58+(64)/8.96=11+7=18 nm
Δh=26−18=8 nm(30%).
CuO—Ca
CuO+Ca=Cu+CaO (4)
ΔH:162+0−635=−473 kJ
hCuO=(80)/6.45=12.4 nm
hCa=(40)/1.54=26 nm
hCaO+Cu=(56)/3.4+(64)/8.96=16+7=23 nm
Δh=38−23=15 nm(40%).
CuO—Ba
CuO+Ba=Cu+BaO (5)
ΔH:162+0−548=−307 kJ
hCuO=(80)/6.45=12 nm
hBa=(137.3)/3.76=37 nm
hCu+BaO=(64)/8.96+153/5.72=7+27=34 nm
Δh=49−34=15 nm(30%).
CuO—Be
CuO+Be=Cu+BeO (6)
ΔH:162+0−609=−447 kJ
hCuO=(80)/6.45=12.4 nm
hBe=(9)/1.85=5 nm
hCu+BeO=(64)/8.96+25/3.01=7+8=15 nm
Δh=17−15=2 nm(12%).
CuO—Zr
2CuO+Zr=2Cu+ZrO2 (7)
ΔH:2·162+0−1100=−776 kJ
hCuO=(2.80)/6.45=25 nm
hZr=(91.2)/6.45=14 nm
hCu+ZrO2=(2.64)/8.84+107.2/5.86=14+16=30 nm
Δh=39−30=9 nm(24%).
CuO—V
3CuO+2V=3Cu+V2O3 (8)
ΔH:3·162+0−1219=−733 kJ
hCuO=(3·80)/6.45=37 nm
hV=(2·50)/5.96=17 nm
hCu+V2O3=(3·64)/8.96+100/4.87=21+21 nm=42 nm
Δh=57−42=15 nm(26%).
CuO—Si
2CuO+Si=2Cu+SiO2 (9)
ΔH:2·162+0−908=−584 kJ
The reaction is thermodynamically advantageous and is accompanied by heat emission.
In order for the substances to have reacted completely one needs to take 2 mole of CuO and 1 mole of Si, resulting in formation of 2 mole of Cu and 1 mole of SiO2.
For complete reaction one needs to use the following thicknesses of the films
hCuO=(2.80)/6.45=25 nm
hSi=(28)/2.33=12 nm
hCu+SiO2=(2.64)/8.96+60/2.65=14+22=36 nm
Δh=47−36=11 nm(24%).
The resulting shrinkage of the film is 24%.
CuO—Cu
CuO+Cu=Cu2O (10)
ΔH:162+0−173=−11 kJ
hCuO=(80)/6.45=12.5 nm
hCu=(64)/8.96=7.14 nm
hCu2O=(143)/6.0+60/2.65=28.83 nm
Δh=12.5+7.14−28.83=−9.19 nm(−46%).
CuO—H2
CuO+H2=Cu+H2O (11)
ΔH:162+0−285=−123 kJ
hCuO=(80)/6.45=12.5 nm
hCu=(64)/8.96=7.14 nm
Δh=12.5−7.14=5.36 nm(42.8%).
Ag2O—H2
Ag2O+H2=2Ag+H2O (12)
ΔH:31.1+0−285=−254 kJ
hAg2O=(231.6)/7.14=32.4 nm
hAg=(2·107.8)/10.5=20.53 nm
Δh=32.4−20.53=11.86 nm(36.6%).
Re2O7—H2
Re2O7+7H2=7H2O+2Re (13)
hRe2O7=452.4/8.2=59 nm
hRe=(186·2)/21=18 nm
Δh=59−18=41 nm(69.9%).
1) GeO2—Al
3GeO2+2Al=Al2O3+3Ge
2) SiO2—Ti
SiO2+Ti=TiO2+Si
3) SiO2—Al
3SiO2+2Al=Al2O3+3Si
4) TiS2
TiS2+3O2=TiO2+2SO2 The above-described methods are not limited to only horizontal gaps. Vertical gaps can also be fabricated. Also, combinations of horizontal and vertical gaps can be made as shown in
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.
This application claims priority to U.S. Provisional Application No. 61/267,027 filed Dec. 5, 2009, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61267027 | Dec 2009 | US |
