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The authors have fabricated a microfluidic system for the control of DNA motion consisting of 400 or 900nm wide Si channels covered by a bonded glass layer containing integrated dual electrodes. By applying an ac potential to an electrode pair on both sides of the channels, precise lateral positioning of DNA has been demonstrated for the first time. By modifying the amplitude or frequency of the ac potential applied to the electrode pairs, they control DNA motion within the channel, causing it to increase speed, decrease speed, reverse directions, or halt. The precise control of DNA average position within channels is expected to be an important step in the development of microfluidic single molecule analysis systems.

The authors demonstrated the fabrication of patterned arrays of polymer light emitting diodes (PLEDs) with a polyfluorene based emissive layer using reversal imprint lithography. A stack of patterned metal and polymer films is transferred to a glass substrate in a single reversal imprint step. Two different techniques for coating the patterned Si mold are shown, one involving the spin coating of polymer layers directly onto the mold and one involving inking of the polymers onto the mold from a spin-coated PDMS inkpad. Using these techniques, PLED devices with a minimum feature sizes as small as 1μm and luminescence starting at 3.5V have been demonstrated.

A soft inkpad imprinting technique to produce stacked micrometer and submicrometer polymer patterns on substrates is presented. A thin soft inkpad is used to coat a polymer film onto the protrusions of a surface treated hard mold. The polymer film on the protrusions of the hard mold is then transferred to a substrate. Simultaneously, a negative pattern of the hard mold is formed on the soft inkpad that may also be transferred to a substrate. Numerical simulations are used to study the mechanisms of pattern transfer by soft inkpad imprinting. With the use of polymer blends, both positive and negative polymeric gratings with 700 nm period were produced. The soft inkpad allows multiple transfers of polymers with similar solubilities to the hard mold since no chemical solution is used for coating. High aspect ratio polymer stacks can be formed without alignment. This capability is an important advantage when forming submicrometer and nanometer multiple-layered polymer structures because current nanoimprint sy...

The electric field distribution for an electron cyclotron resonance (ECR) cavity was measured using microcoaxial probes. Typically, two to three resonant modes are found for each plasma condition and longer cavity length provides a more stable plasma. A 30° periodicity is seen in the angular dependence of the electric field, and the peak right across the microwave input probe becomes more dominant at shorter cavity length. The longitudinal dependence of the electric field has a peak close to the middle of the cavity, and the measured field patterns agree well with the theoretical prediction. The resonant modes were identified by fitting the experimental measurements to linear combinations of electromagnetic resonant modes with characteristic length close to the cavity height. Using this method, all the modes in this ECR cavity were found to be multimode, and the resonant mode with the longest cavity length was found to be a mixing of TE314 and TE611 modes. Significant changes in the electric field pattern...

Si has been etched with an electron cyclotron resonance source in a Cl2 plasma to fabricate microsensors with high‐aspect‐ratio structures. Resonating elements in Si with an aspect ratio of 20 were generated. For these high‐aspect‐ratio sensing elements, the etch rate and profile strongly depend on feature size and etch depth. When the spacing between elements was increased from 2 to 50 μm, the Si etch depth varied from 21.5 to 24.6 μm for a 156 min etch. Typical etch conditions were 100 W microwave power, 100 W rf power, at 3 mTorr and 8 cm source distance. The Si etch rate decreased linearly from 156 to 119 nm/min as the trench aspect ratio varied from 2 to 34. The etch rate also decreased from 151 to 124 nm/min in 2‐μm‐wide trenches and from 163 to 141 nm/min in 10‐μm‐wide trenches as the pressure was increased from 3 to 30 mTorr. Etch profile was vertical at 3 mTorr and it became undercut at pressures ≥10 mTorr. Larger undercut widths were found for trenches with narrower spacings. As microwave power ...

Etching of high‐aspect‐ratio Si structures for microsensors was carried out in a Cl2 plasma generated by a multipolar electron cyclotron resonance (ECR) source. Si etch rate is found to increase with microwave and rf power, and it is the highest at 1 mTorr over the range of 0.5–50 mTorr. Optical emission spectroscopy (OES) and mass spectrometry (MS) were used to monitor the etch process to provide precise control for the sensor fabrication. The Si emission intensity at 288.1 nm and the mass spectrometric signal for 63SiCl+ follow the Si etch rate directly. The selectivity of etch masks decreases at higher rf power and Ni has higher selectivity than SiO2. Si etch rate is reduced for structures with smaller linewidth, but this microloading effect decreases at lower pressure. For the etch conditions used, the Si etch rate remains almost constant when flow rate is changed, but the optical emission and mass spectrometric signals of the etch products are found to decrease with increasing flow. Endpoint detectio...

Optical emission spectroscopy (OES) and mass spectrometry (MS) have been used to monitor the etching of InP and GaAs in a Cl2/Ar plasma. The plasma was generated by an electron cyclotron resonance source. Emission from atomic In and Ga has been detected with the strongest signal to background measured at 410.2 nm for In and 417.2 nm for Ga. The optical emission signals have been studied for varying etch conditions to determine their effectiveness for real‐time control applications. It was found that, by increasing the total gas flow rate, the etch rate remained unchanged but the detected etch product signals decreased due to reduced residue in the plasma system. The residence time of the etch products causes the optical emission signals to be dependent upon previous etch conditions. The In and Ga emission signals have been correlated to the etch rate for various etch conditions. A downstream mass spectrometer was able to detect P and As etch products. The strongest signal for InP etching was 101PCl+2 and the strongest signal for GaAs etching was 145AsCl+2. The ability of the mass spectrometer to detect the group V etch products coupled with the ability of OES to detect the group III elements allows for noninvasive study of a variety of III–V material systems. Smooth, vertical etching of InP via holes at 2.7 μm/min for GaInAs/AlInAs‐based power devices has been monitored. Optical emission measurements showed that the surface oxide removal time for the via hole etching can be decreased from 95 s to an immediate removal by increasing the self‐induced dc bias (‖Vdc‖) from 50 to 300 V, in agreement with MS results. End point detection for via hole etching of GaInAs/AlInAs power devices to Ti/Au contacts on InP substrate was studied using 145AsCl+2 and 155TiCl+3 signals. The thickness of Ti removed can be limited to 18 nm. The optical emission from Ga and Al (at 396.1 nm) have been used to monitor etching of GaInAs/AlInAs heterojunction bipolar transistors. Less than 10 nm of the GaInAs base material is removed at the end of the emitter etch.

A multipolar electron cyclotron resonance (ECR) plasma source was characterized by Langmuir probe measurements and the charged particle energy and density are related to photoresist etch rate. Ion density was found to increase with microwave power but decrease with source distance, and is independent of rf power and flow rate. Among the different gases investigated, Ar plasma was found to have higher ion density compared to O2, N2, and Cl2 discharge. Ion density peaked at 5 mTorr for Ar and 2 mTorr for O2 plasma. For a N2 plasma, the maximum ion density was found to occur at 10 mTorr at 3 cm and 2 mTorr at 23 cm below the ECR source. Electron temperature is ∼3 eV for pressure ranging from 1 to 5 mTorr but decreases to 2 eV at 20 mTorr. Photoresist etch rate follows the same trend as ion density. It increases with microwave power and decreases with source distance. On the other hand, photoresist etch rate increases with rf power and flow rate, even though the ion density remains constant. This suggests tha...

A Cl2 plasma generated by a multipolar electron cyclotron resonance (ECR) source was used to etch Si. Etch rate of Si was found to increase with microwave power and rf power, but decrease with source distance and Ar addition. Etch rate increases with pressure up to 1 mTorr, then decreases at higher pressure. Temperature ranging from − 130 to 100 °C has no significant influence on the etch rate, profile, and morphology. Vertical profile and smooth morphology in Si can be obtained by using low microwave power and low pressure. With a Cl2 plasma at 1 mTorr, 50 W microwave power, 70 W rf power, and 15 cm below the ECR source, 0.15‐μm‐wide features in Si were etched with 7:1 aspect ratio. Schottky diodes were fabricated on the etched surface to evaluate etch‐induced damage. Breakdown voltage was found to increase with microwave power but decrease with rf power and source distance. Ideality factor and barrier height degrade at high rf power and long source distance. One of the most important factors to maintain...

High resolution patterning on nonplanar substrates with large height variation using electron beam lithography is reported. Using an automatic, high precision, noncontact laser probe microscope, a three-dimensional map of the nonplanar substrate to be patterned is obtained first. This data are converted to a format for the electron beam lithography system, which performs the write by adjusting the plane of electron beam focus based on the mapping data. As a proof of concept of this patterning scheme, three different kinds of nonplanar substrates were used including a tilted Si wafer for a uniform and unidirectional tilt, a planoconvex lens for a multidirectional tilt, and deep Si trenches with a step height variation. The patterning scheme was tested for resolution, field stitching accuracy, and field placement accuracy. The results of these tests are in very close agreement with typical results that are obtained for flat substrates with similar patterning conditions. For wafers with 10 mm tilt, 50 nm gra...

Reversal imprint is used to pattern three-dimensional (3D) metal structures over substrates with complex topography. Metal films are deposited onto SU-8 polymer layers on glass molds, lithographically patterned, and transferred to Si or SU-8 substrates with nanosctructured surfaces by reversal imprint at 5MPa, 80°C, and 1s of UV exposure. The transfer of metal patterns of 50∕20nm thick Au∕Ti and 0.5μm thick Al as well as 3D Al structures have been demonstrated.

The profile shape and the flow behavior of polymer nanoscale gratings made by a thermal nanoimprint process are precisely examined using visible light angular scatterometry. Nanoimprinted poly(methyl methacrylate) (PMMA) lines with 60–800nm width, 100–200nm height, and varied residual thicknesses of 70–400nm have been investigated using this optical approach, and insightful observations are made regarding residual stress buildup during thermal nanoimprint. In addition, a nonlinear profile model has been developed for scatterometry to monitor the “melting” behavior of PMMA gratings under annealing around its glass transition temperature. The polymer nanostructures were found to relax primarily at high stress regions.

Low temperature Si to glass bonding using polymethylmethacrylate (PMMA) as an adhesive layer is developed to integrate electrodes with Si channels. The integrated microsystem contains channels dry etched in Si with widths ranging from 3to100μm and depths ranging from 100nmto30μm. The channels are bonded to a 100μm thick glass consisting of 600nm thick patterned PMMA and 20∕50nm thick Cr∕Au electrodes, with PMMA as an adhesive layer. The typical bond strength is 3MPa, obtained by bonding at 110°C with 600nm thick PMMA. Fluidic flow studies are carried out in channels that are 50 and 100μm wide with a depth of 100nm. De-ionized water flows through the sealed Si channels due to capillary pressure with an initial velocity of 0.65mm∕s for 50μm wide and 100nm deep channels. Electric fields are used to induce DNA motion with velocities from 2.4to14.5μm∕s in 100μm wide and 20μm deep channels. The forces generated by the fields and the fluid flow are also used to stretch the tethered DNA molecules up to 15μm long ...

Three-dimensional (3D) nanostructures are needed in many applications including sensors, actuators, and microanalysis systems. In this work, a novel technology combining UV lithography, hybrid mold, and reversal imprint for fabricating 3D structures is developed. A hybrid mold made from quartz is used. The mold has structures patterned by lithography and dry etching. The quartz mold also has selected patterns formed by 50nm thick Cr. A layer of UV definable SU-8 polymer is spin coated onto the hybrid mold and patterned by optical lithography. The mold with the patterned SU-8 layer and no residue is then transferred to a substrate with topography by reversal imprint with temperatures as low as 50°C and pressures of nominally 2MPa. Depending on the dimensions of patterns on the mold compared to the ones on the substrate and the imprint pressure, patterns can be selectively transferred to substrates through reversal imprint. This technology greatly simplifies the fabrication process and provides more flexibi...

In this work, three-dimensional (3D) SU-8 micro- and nanostructures were fabricated using a reversal UV imprint process at low temperature and low pressure. The SU-8 polymer was coated on a patterned glass mold and then transferred onto various substrates by reversal UV imprint at a typical temperature of 50°C, pressure of 1MPa, and UV exposure of 1s. The lower temperature and pressure used compared to conventional thermal imprint shorten the imprint time and alleviate pattern distortion. A combination of silanes was used to generate a medium surface energy on the imprint molds to enable polymer spin coating and mold release after imprint. In addition, an O2 plasma was used for glass mold treatment to improve uniformity of silane coating and to increase substrate surface energy for better polymer adhesion. Using this technology, 100nm–1μm wide SU-8 gratings were fabricated on flat or patterned substrates with good fidelity. By repeating this process, multiple-level nanochannels, cavities, or air-bridging ...

Nanochannels are widely used in biomedical applications such as DNA analysis and biomolecule detection. We report a study using sacrificial polymer and oxide to form three-dimensional (3D) nanochannels. Polymer nanostructures were patterned on Si substrates using optical lithography or nanoimprint lithography, followed by oxide deposition to form the sealed channels. A high-speed dry etching technique for removing the sacrificial polymer was developed using an O2 plasma at high power, high pressure, and elevated temperature. This dry etching technique provides a fast lateral etch rate of 3.91μm∕min for the polymer inside nanochannels, which is an order of magnitude higher than conventional reactive ion etching. High selectivity of 1200 was obtained between the lateral etch rate of polymer inside the nanochannels and the vertical etch rate of oxide. Etch rate dependence on pressure, temperature, and channel width were studied. It was found that the etch rate increases with pressure and temperature. To form...

There is a wide range of applications for three-dimensional (3D) nanochannels in biomedical systems and fluidic control. In this article, a simple and versatile technique to create 3D nanochannels with width from 200nm to 2μm is demonstrated using sequentially stacked reversal UV nanoimprint of SU-8. Its advantages include controllable channel profile, low pressure and temperature for imprints, and flexibility in designing 3D channels by stacking. In a typical reversal UV imprint, SU-8 is spin coated on a glass mold and then transferred onto Si substrates by an UV imprint process at low temperature of 55°C, low pressure of 2MPa, and UV exposure of 1–4s. While reversal UV imprinting top SU-8 layer onto bottom SU-8 layer, the UV exposure and imprint sequence of the top SU-8 layer and its effect on channel profile control are investigated. It has been found that initially UV-cured top SU-8 layer is preferred for good channel profile control because UV-cured SU-8 layer is prevented from flowing down into bott...

A technique to create sealed three-dimensional nanochannel networks is developed using sequentially stacked thermal nanoimprint lithography on planarized self-supporting dielectric sealing material over polymer sacrificial layers. Void-free plasma enhanced chemical vapor deposited SiO2 encloses and seals nanochannels that are formed upon the removal of the sacrificial polymer. Planarization of the SiO2 surface allows the utilization of the vertical dimension to sequentially apply nanoimprint lithography for the formation of multiple-level nanochannel networks. Removal of the sacrificial polymer is performed with a high-power and high-pressure O2 plasma. Wet chemical processes using common solvents are found to be ineffective in removing the sacrificial polymer. Two level nanochannels with cross-sectional dimensions of 300nm×200nm and lengths of 65μm that are aligned offset from one another and aligned on top of one another are demonstrated.

We have developed a method to pattern polymeric materials, including nonthermoplastic polymers and biomaterials, at low temperature and low pressure. In this method, plasticizers are added to increase the chain mobility of the polymers, resulting in lower imprinting temperature and/or pressure. Three established imprinting and transfer techniques were chosen to demonstrate this method: conventional nanoimprint lithography (NIL), microcontact printing (μCP), and soft ink-pad (SIP). These three techniques were used to pattern poly(3,4-ethylenedioxythiophene) (PEDOT) and chitosan. PEDOT and chitosan were chosen because both of them are nonthermoplastic polymer and therefore cannot be easily patterned using conventional NIL. Imprinting of PEDOT and chitosan films from the poly(dimethylsiloxane) mold was achieved at a low pressure of 10kPa and 25°C by controlled addition of glycerol as a plasticizer using conventional NIL; well-defined arrays of 2μm wide, 185nm high PEDOT dots have also been demonstrated by μC...

Poly(3,4-ethylenedioxythiophene) (PEDOT) and chitosan structures on Si, patterned by plasticizer-assisted imprint lithography (PAIL), are examined under a variety of imprinting conditions. The stabilities of pattern dimension and chemical functionality of these polymers are presented. Thermal annealing for 5min at 80°C is found to be an effective method to stabilize imprinted PEDOT patterns. Biofunctionality in chitosan as a function of imprint temperature and pressure is examined through fluorescence spectroscopy. The accessibility of the amine group of chitosan is observed to decrease for imprint temperatures above 80°C, whereas the chemical functionality is not affected by pressure up to 1MPa. Fluorescence spectra of the chitosan are observed to be strong functions of exposure time to O2 plasma.

We present a new method of imprinting three dimensional (3D) polymeric micro- and nanostructures using a duo-mold process. In this method, two patterned Si molds are surface treated to have different surface energies. A polymer solution is spin-coated onto one of the molds, forming a planarized thin film on the mold. The two molds are pressed together at an appropriate temperature and pressure and then released. The patterned thin film adhering to one of the molds is then pressed onto a substrate at an appropriate temperature and pressure to form supported 3D structures. Alternatively, the patterned thin film on the mold may be released to form freestanding 3D structures. A key success factor in this process is the silane-based surface treatments to enable the patterning of the thin polymer film and final release from the mold to form 3D structures. We demonstrate various polymethyl methacrylate 3D structures with combinations of grating, circular-, and square-patterned molds. The duo-mold process is a po...

A polymer inking technique was developed to form micro- and nanopatterns on a substrate. In this process, a polymer thin film is spin coated on a patterned mold. After contacting the substrate at a suitable temperature and pressure, the polymer on the protruded surfaces of the mold is transferred to the substrate and a positive image of the mold is obtained. A selective surface treatment method has been developed to improve the edge smoothness of the inked pattern. During selective surface treatment, the protruded surfaces of the mold are first treated with a flat poly(dimethylsiloxane) stamp impregnated with a silane that has medium surface energy. The mold is then immersed into the solution of another silane with very low surface energy to treat the trenches of the mold. Because the surface energy of the sidewalls is lower than that on the protrusions, polymer dewetting from the sidewalls is promoted, which makes the polymer film discontinuous along the edges of patterns. Therefore, inked polymer patter...

Thick thermally isolated microheaters in Si are fabricated using high aspect ratio etching technology. These thick microheaters a with large surface area provide the large adsorbent capacity needed for high sensitivity microfabricated preconcentrators in a microgas chromatography system. In addition, air gaps around the microheaters provide good thermal isolation to lower the power consumption. A 520 μm thick Si microheater with good thermal isolation has been made and its back side is anodically bonded to a pyrex glass substrate. To provide good thermal isolation, a 500 μm wide air gap around the microheater, thin poly-Si interconnects on top of dielectric membrane, and air gap isolation from the bottom glass substrate are used. Microheaters with a 500 μm air gap can be heated up 50% faster to a temperature of 270 °C compared to those with a 100 μm air gap, while the power consumption is 25% less and there is a larger temperature difference between the microheater and the bonding area. Operating the microheater in a vacuum results in lower power consumption. At 250 °C, 35% less power is needed at 1.2 Torr compared to atmospheric pressure. The power consumption is further reduced by minimizing the contact area with the heater support substrate. Up to 34% power reduction has been demonstrated by placing heaters on a thin membrane or etching trenches in the supporting substrate. These thick thermally isolated Si microheaters can provide good power efficiency, large adsorbent capacity, and high mechanical strength for microfabricated preconcentrators.

Freestanding, high aspect ratio microstructures in Si were micromachined as thick microheaters. These microheaters, combined with adsorbents, can be used as preconcentrators in a micromachined gas chromatography system. Dry etching of Si using etch and passivation cycles has been developed to produce 240 μm thick Si microheaters with 3 μm wide wires, achieving a high aspect ratio of 80:1. This optimized dry etching technology results in high etch rates with vertical profiles for thick Si microheaters up to 535 μm. Microheaters with 40 μm wide wires, 110 μm gaps, 400 μm thick, and an area of 9 mm2 have been fabricated. With the heater on a 140 μm thick Si membrane, it takes 1320 mW to increase the temperature by 290 °C. The power consumption is reduced to 447 mW for the same temperature raise with a freestanding Si microheater. Heating response for freestanding Si microheaters with different thicknesses is also studied. These Si microheaters have fast response times and reach 75% of the final temperature i...

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. “Embossing” occurs at temperatures well above the glass transition temperature (Tg) of a polymer; “inking” occurs at temperatures around Tg with nonplanarized polymer coating surface on the mold; and “whole-layer transfer” occurs at temperatures around Tg but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the s...

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patte...

Microstructures with submicrometer gaps in Si were fabricated by etching with Cl2 and F-based gases in inductively coupled plasma systems. Released cantilevered beams with a 0.1 μm gaps and 4 μm thick were fabricated using Cl2 etching. Etching in Cl2 provides vertical profile and smooth morphology for submicrometer trenches. It has been applied to form a clamped–clamped beam resonator with 0.2 μm gaps between the resonating beam and electrodes. For trenches with 0.1 μm openings, the F-based etching resulted in tapered etch profile and the top of the trenches was eventually closed due to polymer deposition. Based on faster lateral etch rate for n++ Si in Cl2 etching, a one step dry etching process was used to pattern and release microstructures from the substrate. Residues on the surface after F-based etching can be removed by overetching and the charging effect can be avoided by etching in Cl2.

A fabrication technique for single electron transistors is presented. The charge island for the single electron transistor is confined in the z direction by two epitaxial layers serving as tunnel barriers and the lateral confinement in the x-y plane results from the dry etching of a source-drain pillar using an inductively coupled plasma source. The gate is deposited using a self-aligned process with the source contact serving as a shadow mask and separated from the conducting channel by a small gap for closed coupling of gate voltage, reduced leakage current, and high breakdown voltage (approximately −60 V). The designed and measured values of the tunnel resistance of the epitaxial layers were in good agreement.

The differences between Cl2 and F-based dry etching are compared in this article. Inductively coupled plasma sources have been used to generate plasmas using both Cl2 and SF6/C4F8 chemistries. Trenches etched using Cl2 suffered less aspect ratio dependent etching effects because the trenches can be etched at a much lower pressure than with F-based gases. A 1.4 μm wide, 65 μm deep trench can be obtained with an aspect ratio of 46 in 12 h. The average Si etch rate was 90 nm/min and the selectivity to electroplated Ni was 23. The sidewall was vertical and smooth and the trench openings were nearly the same width before and after etching. Adjacent trenches with 0.14 μm mask opening and 2 μm line width were etched using these two etching technologies. With Cl2 etching, a wider 0.25 μm trench opening, due to the mask erosion effect, with a depth of 5.6 μm was obtained in 50 min. However, the 0.33 μm undercut increased the trench opening to 0.8 μm for 10.7 μm deep trenches after the F-based etching for 55 min. T...

Optical interferometry has been applied to determine the displacement of p++ Si beams. Clamped-clamped Si beams and cantilevered beams were fabricated with short and long B diffusion processes and characterized. Measurements of beam bending for released Si structures with length varying from 50 to 1000 μm, width varying from 5 to 15 μm, and thickness varying from 6 to 37 μm were obtained. By taking advantage of an etch-diffusion process, thicker beams can be fabricated which have less bending due to stress gradients. A 6.0-μm-thick cantilevered beam had a deflection of 11.2 μm due to stress gradients, while a 36.7-μm-thick beam had a deflection of only 0.3 μm. Beams fabricated using a dissolved wafer process with a 12 h B diffusion were found to bend the same amount as those fabricated with a 4 h diffusion. This indicates that bending in doped Si beams not only depends on the gradients in the B concentrations, it could also be related to the distribution of dislocations. Using the deep-etch shallow-diffus...

Low energy Cl species generated in an inductively coupled plasma source have been used to passivate etch induced damage in GaAs and InGaAs. Improved electrical and optical characteristics were measured after Cl2 plasma passivation. The ideality factor and barrier height of etched GaAs Schottky diodes were improved back to the values of an unetched sample with a 10 min passivation. No etching occurred during passivation due to the presence of a surface oxide layer. The growth conditions of the oxide layer were found to have a large effect on the ability of the Cl2 plasma to passivate the surface. It was found that native oxides allow more effective passivation by Cl species as compared to plasma grown oxides. The passivation techniques were used to passivate damage along an etched sidewall for improved electrical conductivity of GaAs wires and increased photoluminescence signal from etched gratings containing an InGaAs quantum well.

The effects of etch induced damage on the electrical and optical properties of AlGaAs/InGaAs quantum wells (QWs) were studied. From the variations in the photoluminescence (PL) intensity and the conductivity of etched gratings, the optical cutoff width was found to be 33 nm whereas the electrical cutoff width was 136 nm. The PL intensity of the gratings indicated that increased stage power during etching causes more damage. Comparisons were also made between the sheet resistivity (ρs) of transmission lines and the conductivity of wires after etching of AlGaAs/InGaAs and AlInAs/InGaAs QWs grown on GaAs and InP substrates, respectively. The AlGaAs/InGaAs QW transmission lines showed reduced ρs after etching with higher stage power, although the ρs was still higher than that of the unetched control sample. The AlInAs/InGaAs QW transmission lines had a higher ρs as the stage power was increased. The two material systems also showed different etch time and sidewall damage characteristics. The AlInAs/InGaAs QW ...

An inductively coupled plasma (ICP) source has been used to generate a Cl2 plasma for the etching of Si field emitters and resonators. An optimized etch condition was used to etch Si field emission devices with controllable sidewall angles as well as good uniformity for large arrays of emitters. A mask erosion technique was used to control the Si emitter profile. The sidewall angle of Si emitters increased from 55° to 75° as plasma pressure was increased from 0.1 to 5.0 mTorr. Sharp tips with good uniformity and a high packing density of 6.25×106 tips/cm2 were fabricated using 200 W ICP source power, 100 W stage power at 1 mTorr with 20 sccm of Cl2 flowing and an ICP source to sample distance of 8 cm. Released Si mechanical resonators were also fabricated with a vertical etch profile and smooth surfaces. An optimized etch condition of 250 W ICP source power and 70 W stage power at 5 mTorr with 20 sccm of Cl2 flow and an ICP source to sample distance of 6 cm provided an etch rate of 219 nm/min and a select...

Nanostructures in Al were generated by printing with hard SiC molds. This nano-printing technology replaces the lithography and the etching or deposition processes to produce patterns directly in metal. Dots, short lines, and long lines were formed in the SiC molds by electron beam lithography and reactive ion etching. High aspect ratio features as small as 40 nm with depth up to 840 nm were patterned in the SiC molds. By pressing the SiC mold onto the Al substrate at room temperature, nanostructures in the SiC mold were reproduced accurately and uniformly in Al. Large arrays of nanostructures down to 40 nm were printed in Al with similar results for dots, short lines, and long lines. Using atomic force microscopy to analyze the cross sections of the SiC molds and printed Al nanostructures, depth dependence on feature size was observed. This nano-printing technology simplifies the processes for generating nanostructures with high throughput and high uniformity.

Arrays of uniform gated Si field emitters with small gate-tip spacing and sharp tips were fabricated using a newly developed self-aligned process. Emitter tips with 80 nm gate-tip spacing were fabricated and low temperature plasma oxidation was used to sharpen the emitter tips. The enhancement factor of the field emitters increased by a factor of 2.6 after the emitter tips radius was reduced from 67 to 8 nm by plasma oxidation. Effects of surface passivation on the emission current of gated Si field emitters were investigated by exposing emitter tips to Cl2 and H2 plasmas prior to testing. Considerable improvements in the emission characteristics were observed after plasma passivation. The emission current was enhanced and the turn-on voltage was reduced after Cl2 and H2 plasma passivation, which corresponds to a work function reduction of 0.5 and 0.9 eV, respectively.

A self-aligned process was developed to control the gate–tip spacing, gate diameter, tip sharpness, and tip protrusion of Si field emission devices (FEDs). Such control is needed for the fabrication of FEDs with low turn-on voltage and high emission current. Using the mask erosion technique, 2-μm-tall Si emitter tips were initially etched in a Cl2 plasma generated by an electron cyclotron resonance source. Plasma oxides with different thicknesses were grown on the Si emitters as a sacrificial layer which creates close spacing between gate and tips, as well as sharpens the tip when removed in wet etchant. Sharp emitters with 8 nm tip radius and 80 nm gate–tip spacing were formed using plasma oxidation. Polyimide was used as the insulator and Mo was sputtered to form the self-aligned gate. By changing the polyimide thickness, gate diameter ranging from 220 to 880 nm and tip apex position ranging from 320 nm below the gate (nonprotruding) to 930 nm above the gate (protruding) were demonstrated. Emission curr...

The effects of etch time on the damage induced during dry etching have been studied. GaAs- and InP-based materials were etched for different times in an electron cyclotron resonance source and the electrical and optical characteristics were measured. Variations in Schottky diode ideality factor (n) and barrier height (φb) were used to measure dry-etch-induced electrical damage at the surface of GaAs after different etch times. The contact resistance of In0.53Ga0.47As, extracted from transmission line measurements, was also investigated for different etch times and compared to the etch time dependence of GaAs. Capacitance–voltage (C–V) measurements from Schottky diodes and photoluminescence (PL) intensity from GaAs/Al0.30Ga0.70As multiple quantum well (MQW) structures were used to investigate the time dependence of the etch damage farther below the surface. The damage in GaAs as shown by n and φb shows marked deterioration after 10 s of etching but improves after longer etching. The contact resistance of I...

Etching of InP using an electron cyclotron resonance source in a Cl2/Ar plasma has been investigated for the fabrication of microcavity laser structures. Fast, smooth, and reproducible etching was achieved with etch rates of 2.14 μm/min. It was found that the etch profile depended on the Cl2 concentration in the Cl2/Ar plasma. Vertical etch profile was obtained for Cl2 concentration ⩽15%. Hard-baked photoresist and Ni have been used as etch masks. Typical etch rate of hard-baked photoresist was 0.10 μm/min and etch selectivity between InP and photoresist was 21. Typical etch rate of Ni was 13 nm/min with a selectivity of 165. The etched InP surface was smoother for samples with photoresist mask. For samples with Ni mask, the etched surface became rougher when narrower or deeper trenches were etched. With optimized etch conditions, high aspect ratio microstructures in InP that were 0.75 μm wide and 10.7 μm tall were fabricated with nearly vertical profile and smooth etched surface using photoresist mask. S...

This article discusses etching and boron diffusion in Si for the purpose of fabricating micromachined devices with high aspect ratio features. The etch rate of Si under various doping conditions in a Cl2 plasma generated by an electron cyclotron resonance (ECR) source was measured. It was found that lightly boron and phosphorus doped Si were etched at rates of 0.17 μm/min, whereas heavily boron doped Si had a similar etch rate of 0.16 μm/min. Si doped with high phosphorus concentrations had a faster etch rate of 0.31 μm/min. These etch rates were measured for Si etched at 100 W microwave power, 100 W rf power, 3 mTorr, with 20 sccm of Cl2 flow, and an ECR source to sample distance of 8 cm. The difference between the p++ and n++Si etch rates was more significant when etched at higher microwave power, higher rf power, or higher temperature. The depth of a heavily doped boron diffusion layer was measured for different feature sizes, trench openings, and aspect ratios. The diffusion layer thickness was found ...

Distributed Bragg reflector mirrors were etched in InP and GaAs. The mirrors were patterned using electron‐beam lithography and etched with an electron cyclotron resonance source. This fabrication technique has the advantage that no regrowth step is required and the length of the passive mirror region is confined to a few micrometers. The structure requires etching vertical profiles with smooth surface morphology and high selectivity to the masking material. The effects of rf power, Cl2 percentage in Ar, chamber pressure, and stage temperature were studied. Increasing the rf power from 100 to 250 W caused the InP selectivity to Ni to decrease from 147 to 55. A similar effect was observed for etching GaAs. For both InP and GaAs, a vertical profile can be obtained by reducing the Cl2 percentage in Ar and by using low chamber pressure of 1 mTorr. Increasing the Cl2 percentage, however, improves selectivity to the Ni mask. With 20% Cl2 in Ar, vertical profiles and high selectivities are obtained for both InP ...

Dry etching and low temperature plasma oxidation were used to sharpen Si field emitter tips. Si emitters with tip radii of 67 nm were formed by dry etching in a Cl2 plasma at 5 mTorr. An electron cyclotron resonance (ECR) source was used to generate the plasma and erosion of a SiO2 mask was employed to control the Si emitter geometry. The Si emitters were sharpened by dry etching in the Cl2 plasma generated at 0.7 and 5 mTorr after the SiO2 mask was completely eroded. After etching the emitters for 7 min at 0.7 mTorr, the tip radius decreased from 67 to 22 nm. The sharpening effect could be attributed to enhanced sputtering yield at sloped surfaces. At higher pressure of 5 mTorr, the sharpening effect was reduced and the tip radius only decreased from 67 to 62 nm. Optical emission spectroscopy and mass spectrometry were used to monitor the etching and sharpening of Si emitter tips. Sharp increases in the Si emission signal at 288.1 nm and mass spectrometric signal for 63SiCl+ were observed once the SiO2 m...

A Cl2 plasma generated by an electron cyclotron resonance source was used to etch field emitter arrays in Si. Compared to wet etching, emitter arrays with sharper emitter tips and higher packing density can be formed by dry etching. Mask erosion was used to control the etch profile in Si. SiO2 masks with different profiles were patterned by wet and dry etching, and the effects of the initial SiO2 masks slope and etch conditions on the resultant Si profile were investigated. The lateral etch rate of SiO2 decreased from 363 to 150 nm/min as the SiO2 slope increased from 17° to 45°. As a result, the slope of the Si etch profile increased from 52° to 78° after dry etching to a depth of 1.8 μm. The ion flux and energy, controlled through coupled microwave and rf power, were used to obtain the desired etch rate and basewidth of the emitters. By increasing the pressure during etching, the lateral etch rate of SiO2 was reduced and more vertical Si profiles were developed. As pressure was increased from 0.5 to 10 ...

Various plasma passivation techniques for removal of etch‐induced damage on GaAs have been studied. It was found that a Cl2 plasma generated with an electron cyclotron resonance source can efficiently remove dry etch‐induced damage with minimal etching of the GaAs. Complete recovery of the electrical characteristics of both the Schottky diodes and unalloyed transmission lines was found with a 30 s Cl2 plasma passivation at 25 °C. The chlorine reactive species used for passivation were generated with 50 W microwave power at 2 mTorr without any rf power applied at the stage. The Cl2 passivated surface was thermally stable up to 450 °C. Similar recovery was also observed for diodes passivated with a N2 plasma. Compared to Cl2, however, N2 plasma passivation requires a higher temperature (350 °C) and higher microwave power (500 W). Capacitance–voltage measurements show that the presence of H2 in the plasma during passivation results in dopant depletion near the surface, but the dopants can be reactivated afte...

Quadrupole mass spectrometry (QMS) has been used as an in situ diagnostic for GaAs etching with a Cl2/Ar plasma generated by an electron cyclotron resonance source. Partial pressure from reactive species and volatile etch products down to 10−10 Torr can be detected. AsCl+x (x=1–3) products have been detected from 6×6 mm2 GaAs samples. The 145AsCl+2 signal is generally the strongest. For an increase of microwave power from 50 to 400 W, the GaAs etch rate increased from 151 to 263 nm/min and the 145AsCl+2 partial pressure increased from 0.6×10−8 to 2.8×10−8 Torr. The effects of changing source distance, rf power, and pressure were also studied and the changes in etch rate can be correlated to the mass spectrometric intensities of AsCl+x. QMS has been used to monitor the etching of surface oxide on GaAs for different dc biases on the sample. It is found that the oxide etch time is a function of ion energy. For a self‐induced dc bias of −100 V, the 145AsCl+2 signal remained at 1.4×10−9 Torr for the first 15 s...

Surface damage induced by dry etching on GaAs with an electron cyclotron resonance source has been studied using both electrical measurements and surface analysis techniques. It is found that the unalloyed contact resistance extracted from the transmission lines is very sensitive to the etch‐induced damage, and it increases significantly with ion energy and ion flux but decreases with etch temperature. Conducting wires that are 40 to 1000 nm wide have been etched down to 1.3 μm deep with vertical profile and smooth surface morphology. The extracted sidewall damage depth of these wires ranges from 2.7 to 20.4 nm and it increases with ion energy and ion flux but decreases with etch temperature. At 200 W rf power, the sidewall damage depth decreases from 13.1 to 4.0 nm after removing 10 nm of the etched surface using low‐energy reactive chlorine species. Results from cross‐sectional transmission electron microscopy show a higher defect density and a shallower defect depth for samples etched with higher rf po...

GaInAs was etched in a Cl2 plasma generated with an electron cyclotron resonance source. The effects of changing etch parameters on etch rate, morphology, and surface damage were analyzed. Increased microwave power, rf power, and Cl2 percentage in Ar caused the etch rate to increase monotonically. The etch rate decreased with increasing distance and reached a maximum for a pressure of 1.0 mTorr. The etch conditions were chosen to maintain smooth morphology, which requires a balance between ion energy, ion flux, concentration of reactive species, and pressure. Ohmic contacts for transmission lines and Schottky contacts for diodes were deposited directly on the etched GaInAs surface for the evaluation of etch induced damage. It was found that the transmission line measurements were more sensitive to surface damage than the diode characteristics and the specific contact resistivity (ρc) was a more sensitive measure of the damage than sheet resistivity. Defects generated by dry etching typically caused a redu...

High aspect ratio etching of polyimide in an O2 plasma generated by an electron cyclotron resonance (ECR) source was investigated. The dependence of etch rate, profile, and selectivity on microwave power, rf power, pressure, etch time, and gas composition were characterized. Etch rate was found to increase with microwave and rf power. Etch anisotropy increases with rf power but decreases with pressure and etch time. With 750 W microwave power, 300 W rf power, and 0.5 mTorr pressure, the polyimide etch rate was 0.91 μm/min and the anisotropy was 0.92. Etch selectivity between polyimide and the Ti mask was 3150:1 at 50 W and 536:1 at 300 W rf power. Compared to reactive ion etching, etching using an ECR source provides etch rate that is typically 10× faster and etch selectivity that is 4–6× higher. High aspect ratio (≳15:1) structures in polyimide that were 50 μm deep and 3 μm wide have been obtained using an O2 plasma generated by an ECR source.

A systematic analysis of how CH4/H2 based reactive ion etching affects the optical and electro‐optical properties of GaInAs/InP multiple quantum well pin diode strip‐loaded waveguides is reported. The study includes measurements of waveguiding properties, optical losses and electro‐optical phase modulation as function of etch depth, radio‐frequency (rf) power, pressure, and the CH4 content in the etch process. It is found that the optical losses of the waveguides are most sensitive to the etching conditions, in particular the rf power. As the rf‐power density was varied from 0.27 to 1.09 W/cm2, the optical losses increased from 6.8 to 19.6 dB/cm. The waveguiding and electro‐optical properties were found to be much less sensitive to the etching parameters. For a 5‐μm‐wide waveguide, the full width half‐maximum of the optical mode is typically 5 μm and the average voltage needed to produce a π phase shift is 4 V corresponding to a modulation response of 15°/V mm.

Controllable dry etching of GaAs and InP using a multipolar electron cyclotron resonance (ECR) source and a radio frequency (rf)‐powered electrode was investigated. The etch characteristics were studied as a function of microwave power, rf power, distance from the ECR source, pressure, and temperature. Etch rate is found to increase with microwave power initially, then decrease at higher microwave power due to reduction in ion energy. Surface morphology becomes rougher and etch profile is more undercut at higher microwave power, but can be improved using higher rf power or by Ar addition. As the ECR source distance increases, the concentration of ions and neutral species decrease, but the ion energy increases. Therefore, when etching is limited by the arrival rate of reactive radicals, etch rate decreases with source distance. When the process is limited by the ion‐enhanced reaction or removal rates, etch rate increases with source distance. Etch rate and self‐induced dc bias voltage (‖Vdc‖) typically inc...

Etching of photoresist in an O2 plasma generated by an electron cyclotron resonance source (ECR) was investigated. The ECR source was a microwave multipolar reactor at 2.45 GHz, and the stage was connected to a rf power supply at 13.56 MHz. Effects of microwave power, rf power, ECR source to sample distance, and pressure on photoresist etch rate were characterized. It has been found that the photoresist etch rate increases with microwave and rf power, but decreases with source to sample distance. With microwave power at 1000 W and rf power at 300 W, smooth morphology and fast etch rate at 1.61 μm/min were obtained. Self‐induced dc bias voltage increases with rf power and source to sample distance, but decreases with microwave power. Etch rate uniformity better than 0.5% was obtained across 7.5 cm diam wafer even with a very close source to sample distance of 3 cm. Etch profile can be varied depending on the etch conditions. Vertical profile in polyimide has been obtained using a trilayer resist scheme.