This work was funded by NSF grants DMR-0605716 and DMR-1101189. Also, the Cornell NanoScale Science and Technology Center was used to grow and pattern the oxides. We thank them for their assistance. One of us FMG is grateful for the support of the Moti Lal Rustgi Professorship.

1. Before Bonding, Wafer Preparation
This step except for 1.8 is done in the Cornell Nanoscale Facility cleanroom.
<ol>
	<li>Grow the oxides in a standard thermal oxidation furnace using a wet oxide process for thick oxides and, to achieve better thickness control, a dry oxide process for very thin oxides. Check the thickness for uniformity over the full wafer with ellipsometry.</li>
	<li>Create a mask for the geometry you wish to etch.</li>
	<li>Spin photoresist on the wafers being etched.</li...

<ol>
	<li>Gasparini, F. M., Kimball, M. O., Mooney, K. P., Diaz-Avila, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Finite-size+scaling+of+He-4+at+the+superfluid+transition.">Finite-size scaling of He-4 at the superfluid transition.</a> Rev. Mod. Phys. 80, 1009-1059 (2008).</li><li>Mehta, S., Kimball, M. O., Gasparini, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superfluid+transition+of+He-4+for+two-dimensional+crossover,+heat+capacity,+and+finite-size+scaling.">Superfluid transition of He-4 for two-dimensional crossover, heat capacity, and finite-size scaling.</a> J. Low Temp. Phys. 114, 467-521 (1999).</li><li>Reppy, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superfluid-Helium+in+Porous-Media.">Superfluid-Helium in Porous-Media.</a> J. Low Temp. Phys. 87, 205-245 (1992).</li><li>Mehta, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silicon+wafers+at+sub-mu+m+separation+for+confined+He-4+experiments.">Silicon wafers at sub-mu m separation for confined He-4 experiments.</a> Czech. J. Phys. 46, 133-134 (1996).</li><li>Tong, Q. Y., Cha, G. H., Gafiteanu, R., Gosele, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Low-Temperature Wafer Direct Bonding. J. Microelectromech. S. 3, 29-35 (1994).</li><li>Tong, Q. Y., Gosele, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Semiconductor+Wafer+Bonding+-+Recent+Developments.">Semiconductor Wafer Bonding - Recent Developments.</a> Mater. Chem. Phys. 37, 101-127 (1994).</li><li>Gosele, U., Tong, Q. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Semiconductor+wafer+bonding.">Semiconductor wafer bonding.</a> Annu. Rev. Mater. Sci. 28, 215-241 (1998).</li><li>Rhee, I., Petrou, A., Bishop, D. J., Gasparini, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bonding+Si-Wafers+at+Uniform+Separation.">Bonding Si-Wafers at Uniform Separation.</a> Physica B. 165, 123-124 (1990).</li><li>Rhee, I., Gasparini, F. M., Petrou, A., Bishop, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Si+Wafers+Uniformly+Spaced+-+Bonding+and+Diagnostics.">Si Wafers Uniformly Spaced - Bonding and Diagnostics.</a> Rev. Sci. Instrum. 61, 1528-1536 (1990).</li><li>Perron, J. K., Kimball, M. O., Mooney, K. P., Gasparini, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+behavior+of+coupled+4He+regions+near+the+superfluid+transition.">Critical behavior of coupled 4He regions near the superfluid transition.</a> Phys. Rev. B. 87, (2013).</li><li>Perron, J., Gasparini, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Specific+Heat+and+Superfluid+Density+of+4He+near+T+&#955;+of+a+33.6+nm+Film+Formed+Between+Si.">Specific Heat and Superfluid Density of 4He near T &#955; of a 33.6 nm Film Formed Between Si.</a> , 1-10 (2012).</li><li>Perron, J. K., Gasparini, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+Point+Coupling+and+Proximity+Effects+in+He-4+at+the+Superfluid+Transition.+.">Critical Point Coupling and Proximity Effects in He-4 at the Superfluid Transition. .</a> Phys. Rev. Lett.. 109, (2012).</li><li>Gasparini, F. M., Kimball, M. O., Mehta, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adiabatic+fountain+resonance+for+He-4+and+He-3-He-4+mixtures.">Adiabatic fountain resonance for He-4 and He-3-He-4 mixtures.</a> J. Low Temp. Phys. 125, 215-238 (2001).</li><li>Corruccini, R. J., Gniewek, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermal+expansion+of+technical+solids+at+low+temperatures;+a+compilation+from+the+literature.">Thermal expansion of technical solids at low temperatures; a compilation from the literature.</a> U.S. Dept. of Commerce, National Bureau of Standards. , (1961).</li><li>Kahn, H., Deeb, C., Chasiotis, I., Heuer, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anodic+oxidation+during+MEMS+processing+of+silicon+and+polysilicon:+Native+oxides+can+be+thicker+than+you+think.">Anodic oxidation during MEMS processing of silicon and polysilicon: Native oxides can be thicker than you think.</a> J. Microelectromech. S. 14, 914-923 (2005).</li><li>Tong, Q. Y., Gosele, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thickness+Considerations+in+Direct+Silicon-Wafer+Bonding.">Thickness Considerations in Direct Silicon-Wafer Bonding.</a> J. Electrochem. Soc. 142, 3975-3979 (1995).</li><li>Corbino, O. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Azioni+Elettromagnetiche+Doyute+Agli+Ioni+dei+Metalli+Deviati+Dalla+Traiettoria+Normale+per+Effetto+di+un+Campo.">Azioni Elettromagnetiche Doyute Agli Ioni dei Metalli Deviati Dalla Traiettoria Normale per Effetto di un Campo.</a> Nuovo Cim. 1, 397-420 (1911).</li><li>Diaz-Avila, M., Kimball, M. O., Gasparini, F. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Behavior+of+He-4+near+T-lambda+in+films+of+infinite+and+finite+lateral+extent.">Behavior of He-4 near T-lambda in films of infinite and finite lateral extent.</a> J. Low Temp. Phys. 134, 613-618 (2004).</li><li>Dimov, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anodically+bonded+submicron+microfluidic+chambers.">Anodically bonded submicron microfluidic chambers.</a> Rev. Sci. Instrum. 81, (2010).</li><li>Duh, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic+and+Nanofluidic+Cavities+for+Quantum+Fluids+Experiments.">Microfluidic and Nanofluidic Cavities for Quantum Fluids Experiments.</a> J. Low Temp. Phys. 168, 31-39 (2012).</li></ol>

The development of suitable silicon lithography in combination with direct wafer bonding has allowed us to make vacuum tight enclosures with highly uniform small dimensions over all the full area of a 5 cm diameter silicon wafer. These enclosures have allowed us to study the behavior of liquid 4He in the neighborhood of its phase transitions from a normal liquid to a superfluid. These studies have verified predictions of finite-size scaling, as well as pointed out failures which remain to be explored. The work...

When clean silicon wafers are brought into intimate contact at RT, they are attracted to each other via van der Waals forces and form weak local bonds. This bonding can be made much stronger by annealing at higher temperatures5,6. Bonding can be done successfully with surfaces of either SiO2 to Si or SiO2 to SiO2. Bonding of Si wafers are most commonly used for silicon on insulator devices, silicon-based sensors and actuators, and optical devices7. The work described here takes wafer direct bonding in a different direction by using it to achieve well-defined uniformly-spaced enclosures over the entire wafer area8,9. Having a well-defined geometry where fluid can be introduced allows measurements to be performed in order to determine the effect of the confinement on the properties of the fluid. Hydrodynamic flows can be studied where the small dimension can be controlled from tens of nanometers to several micrometers.
SiO2 can be grown on Si wafers using a wet or dry thermal oxide process in a furnace. The SiO2 can then be patterned and etched as desired using lithographic techniques. Patterns which have been used in our work include a pattern of widely spaced support posts which results upon bonding in a planar or film geometry (see Figure 1). We have also patterned channels for one-dimensional characteristics, and arrays of boxes, either of (1 &#181;m)3 or (2 &#181;m)3 dimension1 (see Figure 2). When designing a confinement with boxes, typically 10-60 million on a wafer, there needs to be a way to fill all of the individual boxes. A separate patterning of the top wafer with a design that stands off the two wafers by 30 nm or more allows for this. Or, equivalently, shallow channels can be designed on the top wafer so that all the boxes are linked. The thickness of the oxide grown on the top wafer is different from that on the bottom wafer. This adds another degree of flexibility and complexity to the design. Being able to pattern both wafers allows for a larger variety of confinement geometries to be realized.
The size of the geometric features in these bonded wafers, or cells, can vary. Cells with planar films as small as 30 nm have been made successfully10,11. At thicknesses below this, overbonding can take place whereby the wafers bend around the support posts thus &#34;sealing&#34; the cell. Recently, a series of measurements on liquid 4He have been performed with an array of (2 &#181;m)3 boxes with varying separation distance between them10,12. Features much larger in depth than 2 &#181;m are not very practical due to the increasing length of time required to grow the oxide. However, measurements have been made with an oxide as thick as 3.9 &#181;m9. The limits on the smallness of the lateral dimension arise from the limits of the lithography capabilities. The limit for the largeness of the lateral dimension is determined by the size of the wafer. We have successfully created planar cells where the lateral dimension spanned almost the entire wafer diameter, but one could just as easily imagine patterning several smaller structures on the order of tens of nanometers in width. However such structures would require e-beam lithography. We have not done this at this time.
In all of our work the bonded wafers formed a vacuum tight enclosure. This is achieved by retaining in the patterned oxide a solid ring of SiO2 of 3-4 mm in width at the perimeter of the wafer, see Figure 1. This, upon bonding, forms a tight seal. This design could be easily modified if one were interested in hydrodynamic studies which require an input and an output.
The bursting pressure of the bonded cells has also been tested. We found that with 375 &#181;m thick wafers, pressure up to approximately nine atmospheres could be applied. However, we have not studied how this could be improved by bonding over larger oxide areas or, perhaps, for thicker wafers.
The procedure for interfacing the silicon cells to a filling line and the techniques for measuring the properties of the confined helium at low temperature is given in &#160;Mehta et al.2 and Gasparini et al.13&#160;We note that changes in linear dimension for silicon are only 0.02% upon cooling the cells14. This is negligible for the patterns formed at RT.

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			<td height="20" style="height:20px;width:215px;">SmartCut</td>
			<td style="width:192px;">North American Tool</td>
			<td style="width:104px;">FL 130</td>
			<td style="width:600px;">Not much is needed per cell. Smaller sizes are available.</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Silicon Wafers</td>
			<td>Semiconductor Processing Co</td>
			<td></td>
			<td>There are many suppliers. Pay attention to thickness and thickness variation when ordering.</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Deionized Water</td>
			<td></td>
			<td></td>
			<td>General Availability</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Peroxide</td>
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			<td></td>
			<td>General Availability</td>
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		<tr height="20">
			<td height="20" style="height:20px;">Ammonium Hydroxide</td>
			<td></td>
			<td></td>
			<td>General Availability</td>
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			<td height="20" style="height:20px;">Nitrogen Gas</td>
			<td></td>
			<td></td>
			<td>General Availability</td>
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		<tr height="20">
			<td height="20" style="height:20px;">Helium Gas</td>
			<td></td>
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			<td>General Availability</td>
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			<td height="20" style="height:20px;">Diamond Paste</td>
			<td>Beuler Metadi II</td>
			<td>e.g. 406533032</td>
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			<td height="20" style="height:20px;">Diamond Drills</td>
			<td>Starlite</td>
			<td>e.g. 115010</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Pyrex Dishes</td>
			<td></td>
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			<td>General Availability</td>
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			<td height="20" style="height:20px;">Filter Paper</td>
			<td>Whatman</td>
			<td>1001-110</td>
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			<td height="20" style="height:20px;">Acetone</td>
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			<td>General Availability</td>
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			<td height="20" style="height:20px;">Methanol</td>
			<td></td>
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			<td>General Availability</td>
		</tr>
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			<td height="20" style="height:20px;">Quartz tubes for flushing furnace</td>
			<td></td>
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			<td>General Availability</td>
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			<td height="20" style="height:20px;">Rubber vacuum hose</td>
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</table>

fabrication of uniform nanoscale cavities via silicon direct wafer bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

Properly bonded wafers will have no unbonded regions. Attempting to split the wafers after annealing will cause the cell to break into pieces due to the strength of the bond. Infrared images of properly bonded wafer are shown in Figures 5 and 6. Often annealing improves the uniformity of the cell, especially if local unbonded regions are due to lack of flatness in the wafers. In Figure&#160;5 the light spots and border are bonded areas. The center bright spot is the hole...

Watch this Scientific Journal Video about Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding at JoVE.com

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

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6.2K visualizações.  The State University of New York at Buffalo. Um método para unir permanentemente dois wafers de silício para perceber que um gabinete uniforme é descrito. Isso inclui a preparação de wafer, limpeza, ligação RT e processos de ressarcimento. Os wafers ligados resultantes (células) têm uniformidade de gabinete ~1%1,2. A geometria resultante permite medições de líquidos confinados e gases.