Research

Research interest: Nanofabrication, NEMS/MEMS, Biosensors, Nanotechnology, MEMS Sensor, Nanosensor etc.

My research focuses on micro-nanofabrication process technology, though we also work on various device applications. We use the facilities at the three cleanrooms in campus: G2N, Quantum nanoFAB and CIRFE, Watlab, particularly its SEM/EBL, HIM and FIB system.

AFM images of a hexagonal silicon pillar array with the center-to-center distance between adjacent pillars of 400 nm and a pillar height of 440 nm.

Overview: Nanoscale science and technology is one of the fastest growing research areas in recent years. Nanofabrication techniques are the cornerstone for this rapid growth; without this capability, nanotechnology would be a mere speculation. In fact, while the birth of nano-science/technology was occurred over fifty years ago, from Richard Feynman’s famous 1959 lecture “There’s Plenty of Room at the Bottom”, growth in this field was rather slow in its first 30 years – mainly due to the lack of nanofabrication capability. It is only since the mid-1990s that nano-research has experienced explosive grown, coinciding with the development and more availability of a broad range of nanofabrication techniques.

There are two approaches to enabling fabrication at the nanoscale: “bottom-up”, which involves chemical synthesis and self-assembly, and “top down”, where nanostructures are generated or duplicated by lithography. For the “top-down” method, electron beam lithography (EBL), focused ion beam (FIB) etching/lithography and nanoimprint lithography (NIL) are generally used to create or duplicate nano-structures.

I. 1 Nanofabrication – Electron beam lithography

Electron beam lithography (EBL) is the most popular nano-lithography method for R&D, device prototyping and photo-mask manufacturing. It uses an focused electron beam to expose the resist, to make it more soluble (positive tone) or less soluble (negative tone) in the developer. It has demonstrated sub-5 nm resolution.

We systematically studied negative resist polystyrene, and achieved both ultra-high resolution (15 nm pitch) when using low molecular weight, and ultra-high sensitivity (1 µC/cm2) when using high molecular weight. We have also shown that low cost broad disperse polystyrene performs as well as the high cost narrow disperse polystyrene. Moreoever, demostreated that polystyrene can be evaporated, which, unlike spin-coating, can be applied to any irregular surface such as an AFM tip or optical fiber. Lastly, by adding Cr to the evaporated polystyrene resist, the etching selectivity to silicon is greatly incrased, making it possible to etch very high aspect ratio silicon structures using the polystyrene-Cr nanocomposite resist.

Moreover, we also demonstrated that due to the lack of feedback, conventional electron beam lithography (EBL) is a ‘blind’ open-loop process where the exposed pattern is examined only after ex-situ resist development, which is too late for any improvement. Here, we report that self-developing nitrocellulose resist, for which the pattern shows up right after exposure with out ex-situ development, can be used as in-situ feedback on the e-beam distortion and enlargement. We first exposed identical test pattern in nitrocellulose at different locations within the writing field; then, we examined in-situ at high magnification the exposed patterns and adjusted the beam (notably working distance) accordingly. The process was repeated until we achieved a relatively uniform shape/size distribution of the exposed pattern across the entire writing field. Once the beam was optimized using nitrocellulose resist, under the same optimal condition, we exposed the common resistPMMA. We achieved approximately 80-nm resolution across the entire writing field of 1 mm × 1 mm, as compared to210 nm without the beam optimization process.

SEM and AFM images of structures in nitrocellulose.

I. 2 Nanofabrication – Oxidation Sharpening

Thermal oxidation sharpening of silicon tips is probably the most widely employed tip sharpening method, and it is at present routinely used for commercial AFM tip fabrication. This method has a self-limiting characteristic when it is performed below950C ,i.e., when the remaining silicon becomes very thin with the radius of curvature of just a few nanometers, oxidation will be practically stopped with rates<0.2 nm/h, rather than continuing to rapidly erode the thin silicon wire in the center till its disappearance.9As a result, longer oxidation will not turn the already sharpened silicon tip into a blunt one, and thus, this method has a relatively broad process window and high yield as needed for commercial applications.

Sharp tips are essential for high-resolution atomic force microscopy (AFM) imaging and high-performance electron emitters in vacuum microelectronic devices. Thermal oxidation at high temperature followed by oxide removal is widely used in the nanofabrication of sharp silicon AFM/emitter tips. This method relies on the fact that oxide grows slower on areas with a smaller radius of curvature.Thermal oxidation is commonly carried out in a dedicated oxidation furnace that is costly, and the tips or wafer of tips must be cleaned thoroughly using Radio Corporation of America (RCA) cleaning.Here, the authors report that oxidation sharpening can also be attained using a very low-cost generic box furnace in the atmospheric environment that does not require the tips to go through an RCA clean-ing process. As is apparent, such cleaning is not convenient for millimeter-scale AFM probes. The minimum tip apex radius of 2.5 nm was obtained by oxidation at 950C in the atmospheric environment. The obvious application of this approach is the regeneration of sharp tips out of worn out and thus blunt AFM probes at very low cost.Published by the AVS..

SEM images of cone structures fabricated on the Si wafer. (a) Before oxidation in the atmospheric environment; (b) right after oxidation;(c) after HF etching of oxide. The apex diameters are 75, 95, and 14 nm, respectively.
SEM images of truncated cone structures fabricated on Si. (a) Before oxidation; (b) after first oxidation and HF etching of oxide;(c) after second oxidation and HF etching; [(d) and (e)] SEM images of random pyramid structures fabricated on Si using maskless KOH etching. (d) Before first oxidation; (e) after third oxidation and HF etching of oxide, showing a pointed apex.

II. Nanostructured ICP Plasma Etching

In nanofabrication, use of thin resist is required to achieve very high resolution features. But thin resist makes pattern transferring by dry etching difficult because typical resist has poor resistance to plasma etching. One widely employed strategy is to use an intermediate hard mask layer, with the pattern first transferred into this layer, then into the substrate or sublayer. Cr is one of the most popular hard etching mask materials because of its high resistance to plasma etching. Cr etching is carried out in O2and Cl2or CCl4 environment to form the volatile etching product CrO2Cl2, but addition of O2gas leads to fast resist etching. In this work, the authors show that Cr2O3can be etched readily in a Cl2/O2gas mixture with less oxygen than needed for Cr etching, because Cr2O3contains oxygen by itself. Thus it is easier to transfer the resist pattern into Cr2O3than into Cr. For the subsequent pattern transferring into the substrate here silicon using non-switching pseudo-Bosch inductively coupled plasma-reactive ion etching with SF6/C4F8gas and Cr or Cr2O3as mask, it was found that the two materials have the same etching resistance and selectivity of 100:1 over silicon.Therefore, Cr2O3 is a more suitable hard mask material than Cr for pattern transferring using dry plasma etching.

Etching rates of Cr, Cr2O3, and PS for four different etching recipes. The pressure, RF power, ICP power, and temperature are fixed at 12 mTorr, 10 W, 1200 W, and 50C, respectively.

III. Additional Research

Additional research thrusts include electrochemical biosensors consisting of nano-electrode arrays for improved sensitivity, contact guidance of cell growth for tissue repair, guided growth of carbon nanotubes, fabrication of contrast agent for medical ultrasonic imaging.