1. Field of the Invention
The present invention relates to a microresonator.
2. Discussion of the Related Art
When an A.C. voltage is applied between resonant element 2 and pads 3 and 4, resonant element 2 tends to deform by expanding or retracting. When resonant element 2 starts resonating, the resonant element expands and retracts at the resonance frequency. Pads 3 and 4 and resonant element 2 are then equivalent to a capacitor having its capacitance varying at the resonance frequency.
The previously-described microresonator can be used in various fashions. An example of use as a filter is described hereafter. As shown in
A method for manufacturing the microresonator shown in
In a first phase, illustrated in
In a second phase, illustrated in
In a third phase, illustrated in
In a fourth phase, illustrated in
Apart from its complexity, the previously-described method has the disadvantage of forming a resonant element having a polycrystalline structure. Indeed, resonant element 12 is obtained by silicon deposition on a silicon oxide layer, which results in forming polysilicon. The polycrystalline structure of the resonant element is a disadvantage since this causes mechanical weaknesses. Further, the resonance frequency of a polysilicon resonant element may vary from one resonant element to another.
An object of the present invention is to provide a simple method for manufacturing a microresonator.
Another object of the present invention is to provide a microresonator structure, the resonant element of which has a good mechanical hold.
Another object of the present invention is to provide such a microresonator, the resonant element of which has a resonance frequency substantially constant from one resonator to another.
Another object of the present invention is to provide such a microresonator, the lateral motions of which are accurately detected.
To achieve these and other objects, the present invention provides a microresonator comprising a resonant element and at least one activation electrode placed close to the resonant element, in which the resonant element is placed in an opening of a semiconductor layer covering a substrate, the activation electrode being formed in the semiconductor layer and being level at the opening, wherein the resonant element is made of single-crystal silicon.
According to an embodiment of the present invention, the resonant element has the shape of a mushroom having its foot attached to the substrate.
According to an embodiment of the present invention, the microresonator further comprises a vertical detection transistor comprising a stacking of three doped semiconductor areas formed in the semiconductor layer and which is level at the opening, the lower and upper doped semiconductor areas forming source and drain areas of a first doping type, the intermediary semiconductor area forming a “substrate” area of a second doping type, the resonant element forming the transistor gate.
The present invention also provides a method for forming a microresonator comprising the steps of forming a sacrificial portion above a substrate; forming a first semiconductor layer above the previously-obtained structure; forming in the semiconductor layer at least one electrode area placed against or above a peripheral portion of the sacrificial portion; forming an opening in the first semiconductor layer above the first sacrificial portion, whereby said at least one electrode areas is level at the opening; forming a sacrificial layer covering the bottom, the wall, and the edges of the opening; forming a hole in the sacrificial layer at the bottom of the opening; forming a second semiconductor layer on the previously-obtained structure; etching the second semiconductor layer to keep a portion forming a resonant element placed in the openings and slightly extending over the edges of the opening; and removing the sacrificial portion and layer, and in which the sacrificial portion and layer are formed of a material such as silicon-germanium which is selectively etchable with respect to the substrate, to the first semiconductor layer and to the resonant element, and which is such that it enables forming a second single-crystal semiconductor layer at least in said opening.
According to an embodiment of the present invention, the method further comprises, after the forming of the sacrificial silicon-germanium portion, the forming of an insulating portion above the sacrificial portion, the insulating portion being used as an etch stop layer on forming of the opening by etching of the first semiconductor layer.
According to an embodiment of the present invention, the first and second semiconductor layers are silicon layers obtained by epitaxial growth, the resonant element being made of single-crystal silicon.
According to an embodiment of the present invention, the sacrificial layer has a thickness of a few tens of nanometers.
According to an embodiment of the present invention, the method is intended to form a microresonator comprising a vertical detection transistor, and comprises prior to the forming of the opening a step of performing three successive implantations from a same lithography mask shifted to form three stepped doped areas, the lower doped area being placed against or above the sacrificial portion and the upper doped region being placed at the surface of the first semiconductor layer, the doped areas being possibly partially etched in the forming of the opening in the first semiconductor layer, whereby the three doped areas are level with the opening.
According to an embodiment of the present invention, the insulating portion does not cover the entire sacrificial portion, the three doped areas being formed above an exposed area of the sacrificial portion.
The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale.
The method of the present invention is described hereafter in relation with
In an initial step, illustrated in
A silicon-germanium portion 103 is then grown by selective epitaxy above substrate portion 102. This epitaxial growth is conventionally performed according to a vapor phase deposition performed from a mixture of dichlorosilane and germane. The deposition method is selective to avoid growing silicon-germanium above insulating area 101.
An insulating layer portion 104 is then formed on silicon-germanium portion 103. Insulating portion 104 is smaller than silicon-germanium portion 103 to expose one or several peripheral areas of silicon-germanium portion 103. In this example, two exposed areas Z1 and Z2 are located respectively to the left and to the right of silicon-germanium portion 103. Insulating portion 104 can be obtained in several ways. A deposition of an insulating layer may be performed on the previously-obtained structure, after which this insulating layer may be etched by keeping a portion above a part of silicon-germanium portion 103. A thermal oxidation of silicon-germanium portion 103 may also be performed, after which the silicon oxide layer thus formed may be etched to expose certain parts of silicon-germanium portion 103.
At the next step, illustrated in
At the next step, illustrated in
At the next step, illustrated in
In this example, opening 130 is formed between electrode areas 120 and 121 so that they are level with opening 130. The used etch method is preferably anisotropic so that the opening wall is vertical. Insulating portion 104 is then eliminated, and thus does not appear in
It should be noted that in the previously-described method, insulating portion 104 is used as a stop layer in the etching of silicon layer 110. The step of forming an insulating portion on silicon-germanium portion 103 prior to the forming of silicon layer 110 may be avoided since there exist etch methods enabling etching silicon selectively with respect to silicon-germanium. In this case, silicon-germanium portion 103 would be used as a stop layer in the etching of silicon layer 110 to form opening 130. However, the use of an insulating portion as a stop layer enables better controlling the depth of opening 130, which enables more accurately defining the thickness of the resonant element of the microresonator, as will appear hereafter.
At the next step, illustrated in
At the next step, illustrated in
At the next step, illustrated in
At the next step, illustrated in
In the example illustrated in
In the example shown in
In the previously-described method, silicon-germanium portion 103 and layer 140 are sacrificial layers. A material other than silicon-germanium may be used. The selected material must be selective etchable with respect to substrate 100, to silicon layer 110, and to resonant element 160, and must enable depositing or growing by epitaxy a semiconductor layer exhibiting a single-crystal structure.
An advantage of the method of the present invention is that it enables forming a semiconductor resonant element exhibiting a single-crystal structure.
Further, to form the sacrificial portion and layer, a material enabling obtaining a very thin layer will preferably be used to finally have a small interval between resonant element 160 and electrode areas 120 and 121. It is possible to obtain an interval of a few tens of nanometers with a silicon-germanium layer. This feature is important since the motions of this type of resonant element are of a few angstroms. The detection of such small motions requires having a very small distance between the resonant element and the electrode areas to induce a significant capacitance variation.
The resonance frequency of such a microresonator may reach several GHz, the frequency depending on the biasing conditions of the resonant element.
A microresonator according to the present invention comprises a resonant element placed in an opening of a semiconductor layer covering a substrate. In the case where the semiconductor layer and the “support” substrate are formed of a same material, it can be considered that the layer and the substrate form a single substrate, the resonant element being then placed in a cavity of this substrate. Further, the resonant element is attached to the substrate by one or several “feet”. The microresonator further comprises one or several electrode areas placed against the walls of the opening in which the resonant element is placed. The portion of the resonant element placed in the opening has a shape substantially identical thereto so that the distance between the resonant element and the opening wall is relatively small in front of the electrode areas.
According to an aspect of the present invention, the resonant element is formed of a semiconductor material exhibiting a single-crystal structure.
In the case where the previously-described microresonator is an element of an integrated circuit, transistor-type components may be formed at the surface of silicon layer 110. A “bubble” may be placed above the microresonator, after which one or several metal connection levels may be formed above semiconductor layer 110 and the bubble. A metal contact may be placed above each electrode area 120 and 121 to connect them to other components of the circuit via a metal line. Further, to have access to resonant element 160, a doped layer buried in substrate 100 may be formed. This buried doped layer, for example, of type N if the substrate is of type P, then connects the foot of the resonant element to an area of silicon layer 110 by running under insulating area 101. This buried doped layer may be connected to a metal connection line via a doped area formed in silicon layer 110.
A microresonator according to the present invention may be used in various circuits, for example, as a filter. It may be used similarly to what has been described for the microresonator illustrated in
According to an alternative embodiment of a microresonator according to the present invention, the detection of the motions of the resonant element is performed by means of a vertical transistor having the resonant element as a gate. The source, drain, and channel areas of this transistor are placed in the substrate against the wall of the opening in which the resonant element is placed. The forming of such a microresonator comprises of adding to the previously-described method a vertical transistor forming step.
As illustrated in
A resin layer is deposited above silicon layer 110 and this resin is insolated to keep, after development, resin over the entire surface of the silicon layer except in an area located above exposed area Z2. In the case where the used resin is a positive resin, lithography mask M used to insolate the resin is shown at position “a” above the structure shown in
A second implantation is then performed. For this purpose, lithography mask M is shifted, leftwards in this example, so that the opening of mask M is shifted towards insulating portion 104, the mask being at position “b”. A new resin layer is deposited and insolated according to mask M, then developed. Dopant elements of a second type, for example, type N, are then implanted, to form a doped area 123 substantially above doped area 122 but shifted leftwards above the right-hand portion of insulating portion 104. The resin layer is then eliminated.
Then, a third implantation is performed by depositing a new resin layer. Lithography mask M is shifted leftwards again to position “c”, after which the resin is insolated and developed. Dopant elements of the first type, for example, type P in this example, are then implanted to obtain a doped area 124 placed substantially above doped area 123 in the upper portion of silicon layer 110. Three doped areas 122, 123, and 124 placed stepwise between silicon-germanium layer 103 and the surface of silicon layer 110 are thus obtained.
As a guide, doped areas 122 and 124 have a 40-nanometer thickness and doped area 123 has a 300-nanometer thickness. The sum of the thickness of areas 122, 123, and 124, that is, 380 nanometers, is equal to the thickness of silicon layer 110. The mask shift between each of the implantations approximately ranges from 200 to 250 nanometers.
One or several electrode areas are then formed. In this example, a single electrode 120 is formed above exposed area Z1. Electrode 121 has been replaced with doped areas 122, 123, and 124.
As illustrated in
As illustrated in
When resonant element 160 comes closer or moves away from area 123 of the substrate, the gate capacitance of the transistor varies. When the gate and the source/drain areas of the transistor are biased to turn on the transistor, a variation in the gate capacitance translates as a variation in the current flowing through the transistor. The transistor thus enables detecting the motions of the resonant element, especially when it starts resonating.
Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, it may be provided to form several electrode areas around the resonant element. All the electrode areas may be used to activate the resonant element, but it may be provided to use some of these areas to detect the motions of the resonant element.
Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.
Number | Date | Country | Kind |
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05 50276 | Jan 2005 | FR | national |
This application is a divisional of prior application Ser. No. 12/266,471, filed Nov. 6, 2008, entitled “Microresonator”, which is a continuation of prior application Ser. No. 11/888,245, filed Jul. 27, 2007 entitled “Microresonator” which application is a U.S. National Stage filing of International application Serial No. PCT/FR2006/050078, filed on Jan. 31, 2006, entitled “Microresonator” which application claims the priority benefit of French patent application number 05/50276, filed on Jan. 31, 2005, which applications are hereby incorporated by reference to the maximum extent allowable by law.
Number | Name | Date | Kind |
---|---|---|---|
6448622 | Franke et al. | Sep 2002 | B1 |
6856217 | Clark et al. | Feb 2005 | B1 |
6965128 | Holm et al. | Nov 2005 | B2 |
7205867 | Lutz et al. | Apr 2007 | B2 |
7625772 | Casset et al. | Dec 2009 | B2 |
7667200 | Watts et al. | Feb 2010 | B1 |
7791432 | Piazza et al. | Sep 2010 | B2 |
7800279 | Yang | Sep 2010 | B2 |
20020189350 | Tu | Dec 2002 | A1 |
20030173611 | Bertz et al. | Sep 2003 | A1 |
20030190776 | Wong et al. | Oct 2003 | A1 |
20040209435 | Partridge et al. | Oct 2004 | A1 |
20060071578 | Drabe et al. | Apr 2006 | A1 |
20070046398 | Nguyen et al. | Mar 2007 | A1 |
20080150647 | Yang et al. | Jun 2008 | A1 |
20090115283 | Tripard et al. | May 2009 | A1 |
20090152998 | Abele et al. | Jun 2009 | A1 |
Number | Date | Country |
---|---|---|
04-364380 | Dec 1992 | JP |
WO 0217482 | Feb 2002 | WO |
WO 2004027796 | Apr 2004 | WO |
Number | Date | Country | |
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20100295416 A1 | Nov 2010 | US |
Number | Date | Country | |
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Parent | 12266471 | Nov 2008 | US |
Child | 12850126 | US |
Number | Date | Country | |
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Parent | 11883245 | US | |
Child | 12266471 | US |