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Integrated SPM probes with NEMS technology
Integrated SPM probes with NEMS technology
Two kinds of integrated scanning probe microscope (SPM) probes are developed. The first kind is AFM probes realized with a novel maskedâ€œmaskless combined etching process. Both the nano-tips for scanning and the bending cantilevers are simultaneously formed with the maskedâ€œmaskless combined anisotropic etching technique. The simultaneous formation method effectively avoids damage to the previously formed tips when the cantilever shaping is processed. The testing results for the probes show the imaging quality comparable with commercial probes. The second kind of probes is an integrated probe with both a piezoresistive sensor and an electric-heated tip. This kind of probe is used for thermalâ€œmechanical data storage, with the pulse-heated tip for data writing and the piezoresistive sensor for data reading. Nano-sized bumps have been formed by probe scanning on PMMA thin film, resulting in a storage density beyond 30 GB/in.2 .
SARASWATHI GANESAN & UMA RAMASAMY
SUDHARSAN ENGINEERING COLLEGE
Along with the rapid development of nano-science and tech- nology, SPM has been becoming a powerful tool for character- ization and imaging of nano-structures. The key component of the SPM (scanning probe microscope) is probes. Silicon based NEMS (nano-electromechanical systems) technologies can be used for the formation of integrated SPM probes that are useful for investigation of surface, nano-manipulation and investiga- tion of nano-size-effects. As mentioned in published literatures, the tips for SPM probes can be either prepared using struc- ture silicon substrate as a mold or etched out of the silicon wafer directly . In the former method, probe-shaped holes are etched in a silicon substrate and, thereafter, filled with the desir- able probe/cantilever materials. Then the probes are released by etching away the silicon wafer, which serves only as a mould substrate . This method is usually used for fabricating sili- con nitride probes, which generally used in contact mode. In the latter method, the probes are etched directly by undercut- ting under the etching-mask with KOH solution  or reactive ion etching technique . This method is usually used for fab-
ricating silicon probes, which is commonly used in non-contact mode.
Following we will describe two novel SPM probes produced with the second method. The first probe is a novel AFM (atomic force microscope) probe realized with maskedâ€œmaskless two- step etching process. Low-cost and mass-production suitable fabrication techniques are used instead of the expensive and difficultly controlled processes, such as the combination of anisotropic and isotropic RIE (reactive ionic etching). The sec- ond probe presented here is a piezoresistive probe with both an electro-thermal nano-tip and a piezoresistive sensor integrated in a silicon cantilever, for NEMS high-density data storage. Detailed process flow of the probes is presented and the per- formances of the fabricated probes are tested as follows.
2.1. Fabrication of AFM probes with simultaneous formation of both probes and cantilevers
AFM has been proved being a powerful tool for highly resolved investigation of surface in air, liquid and ultrahigh vac- uum environment. For decreasing the micro-fabrication cost of AFM probes, new technique is needed for high-yield, high- throughput and quality-controllable fabrication of the AFM
Fig. 1. The process flow for AFM probes.
probes. Anisotropic etching with KOH is a good solution. How- ever, with previous methods , the tip formation has to be completed before the formation of the cantilever. During the etching process for the cantilever, the formed tip is easily damaged during operation unless thick photo-resist is spun on probes whose height ranges from several microns to 20 pm. Unfortunately, thick photo-resist would degrade the accuracy of transferring the mask for the probes to the wafers. Therefore, a maskedâ€œmaskless etching technique in this paper is devel- oped for the tip-cantilever simultaneous formation. Besides, the anisotropic etching plus post oxidization sharpening is used for low-cost, high-throughput fabrication.
Fig. 1 is the fabrication flow for the AFM probes. SOI (silicon on insulator) wafer is thermally oxidized and patterned. Then the step is anisotropic-etched with the step-height, h, slightly thicker than the designed thickness of cantilevers. A thin layer of SiO2 is patterned for the tip contour. Then masked anisotropic etching is performed at the tip region, while maskless etching is processed at the cantilever step. Along with the etching downward, the maskless etching of (3 1 1) plane makes the cantilever contour recessing laterally. The lateral recession of the edge is :
OM* = (2.3452r3 - 2.1213)d (1)
where d is the vertical etching depth of the (0 0 1) plane. The etching rate ratio between (3 1 1) and (1 0 0), denoted as r3 , is measured as 1.715 for 40% aqueous KOH. Mean- while, the convex-corner undercutting of (4 1 1) plane shapes the tip-shaped cone under the etching mask of SiO2 layer. By optimized design, when the top diameter of the tip is within 0.5 pm, the cantilever shaping is also completed with the buried oxide layer of SOI wafer exposed (see Fig. 2). After
Fig. 2. The cross-sectional geometric evolution of the probe during the maskedâ€œmaskless etching.
Fig. 3. SEM image of a fabricated AFM probe with the close-up view of the tip at the right side.
the maskedâ€œmaskless combined etching, low temperature oxi- dization at 950 C is performed to further sharpening the tip into nano-scale radius. Protected with the SiO2 layer, TMAH anisotropic etching from backside removes the substrate beneath the probes. The probes are finally freed by striping SiO2 with wet HF. Fig. 3 shows a fabricated probe with the close-up view of the nano-tip.
Higher than 80% fabrication yield for the AFM probes has been realized in 4 in. wafers. The formed tips are generally with a radius of smaller than 30 nm. The height of fabricated probes is around 6.5 pm. Their opening angle is between 25 and 35.
2.2. Fabrication of integrated probes with both a piezoresistive sensor and an electric-heated tip
The probe presented in this section is a silicon piezoresistive probe with integrated electro-thermal nano-tip and piezoresis- tive sensor used for NEMS high-density data storage. Previous work either uses two probes for independently piezoresistive reading and electro-thermal writing, respectively, or uses one heating-tip for both reading and writing [5,6]. The former can- not realize reading and writing with the same device. The latter does not facilitate independent optimization of the reading and writing properties. Integration of the piezoresistive sensor and
Fig. 5. Micrograph of the piezoresistive heating probes.
3. Testing results and applications
Fig. 4. The process flow of the piezoresistive heating probes: (a) oxidation and pattern the tip area, (b) KOH etch for tip, © light boron diffusion for heater, (d) dense boron diffusion for hole, (e) light boron diffusion for piezoresist, (f) pattern Al for wire, (g) etching silicon for cantilever and (h) deep trench etch through wafer to release component.
thermal tip together on a cantilever is considered to be a good solution. During the nano-indenting recorder, the tip is kept slid- ing on the surface of the memory medium (such as PMMA thin film). When pulsed voltage applied on the heating resistor, the tip can be heated beyond the glass transition temperature of the material. The nano-indentation bumps can be formed as a datum point. During the data reading, the piezoresistive sensor can detect the change of the cantilever deflection when the tip reaches a datum bumps.
Fig. 4 shows the process flow. (1 0 0) SOI wafer is used for fabrication. The tip with about 30 nm apex curvature-radius is
formed by KOH etching and oxidization-sharpening at 950 C.
Boron is doped two times for the piezoresistor and the heat- ing resistor, respectively. After aluminum interconnection, RIE is processed from the front side to shape the cantilever in the top layer of the SOI wafer. Then deep RIE is performed from backside to etch through the substrate. After the SiO2 layer stripped with buffered HF, The cantilevers are finally released.
The length, width and the thickness of the cantilever are 102,
38 and 2 pm, respectively. The height of the fabricated probes is around 6.5 pm. The formed probe (see Fig. 5) is characterized,
resulting in piezoresistive sensitivity of /XR/R0 = 3.558 Ãƒâ€” 10-4
and the first mode frequency of the cantilever probe as
The thermal characteristics of the piezoresistive probes can be electrically evaluated, by using the temperature-dependent resistivity of the heater as an on-chip thermometer. When a volt- age pulse is applied to the heater, the tip resistance will increase by heating. This increase in resistance can be measured by mon- itoring the current through the heater. After the electric pulse, the heater resistance would gradually decrease to its room tem- perature value as it cools. The cooling rate corresponds to the thermal time constant and the writing speed of the nano-data.
In the experiment, varied heating pulses with varied voltage and time period are applied across the heater. Shown in Fig. 6 is a measured tip-temperature versus time under a 4 V, 3 ps pulse, together with the ANSYS simulation results for comparison. The measurement indicates that the heater can be heated to 463.15 K in 3 ps heating period with 4 V pulse supply, which is sufficient for data writing on most polymer media films. Furthermore, the cooling rate is high enough to ensure the data writing speed. Therefore, the single-probe data writing speed of about 100 KHz can be realized by employing the 3 ps heating pulse.
Fig. 6. The tested and the simulated temperature of the heater vs. heating time under voltage of 4 V.
Fig. 7. The datum bumps recorded by the probe under 240 ps heating period and 9 V voltage.
When the thermal probe is in physical contact with the data recording PMMA film, however, the heating voltage and the data writing period has to be changed. Experimental results show that both the voltage and the period should be increased. In our experiment, the PMMA thin film is spinning coated on a sili- con wafer. The thermal loss to the PMMA film and the beneath silicon substrate make the difference. After optimization to the data recording conditions, 9 V voltage, 240 ps heating period is finally chosen. With 120 nN normal load for the probe, about
31.6 GB/in.2 data-writing density is experimentally obtained on
the PMMA polymer thin film by using a contact AFM mode. The recorded data bumps are in situ scanned with the image shown in Fig. 7.
Fig. 8. The image of a nano-grating scanned by the AFM probes. The grating period is 500 nm and the height is 500 nm.
For evaluating the performance of the AFM probes fabricated with the maskedâ€œmaskless etching technique, ten probes have been tested in an AJ-III AFM made by Shanghai AiJian Nano- science Development Co. Ltd. The scanning images for a nano- grating are obtained in taping mode, showing similar results as demonstrated in Fig. 8. This indicates the usefulness of the AFM probes.
In this study, we present two kinds of SPM probes. For the AFM probes, the cantilever and tip simultaneous forma- tion method effectively avoids damage to the previously formed tips when the cantilever shaping is processed. In addition, low- cost and mass-production suitable fabrication techniques are used instead of the expensive and difficult-controlled processes such as the combination of anisotropic and isotropic RIE, etc. The testing results for the probes show the imaging quality comparable with commercial probes. For data-storage probes, the heating-tip integrated piezoresistive cantilever is formed and the fundamental working principle of the probe has been illustrated. The microsecond instantaneous electro-thermal per- formance and the high density data writing are experimentally achieved.