Sensors and Actuators A 115 (2004) 60–66

Fabrication of SOI-based nano-gratings for Moirémeasurement using focused ion beam

Dong Yan∗, Jing Cheng, Alyssa ApselSchool of Electrical and Computer Engineering, Cornell University, 406 Phillips Hall, Ithaca, NY 14850, USA

Received 25 November 2003; received in revised form 10 March 2004; accepted 11 March 2004

Available online 5 May 2004

Abstract

High-frequency nano-gratings were fabricated using focused ion beam (FIB) and the Moiré fringes were then demonstrated usingscanning electron microscope (SEM). A customized silicon-on-insulator (SOI) process was developed to fabricate the micro-cantileverused for the nano-grating patterned by FIB. The nano-gratings were precisely fabricated on SOI-based cantilever by controlling the millingcurrent and the milling time of FIB under certain magnification and contrast. A theoretical analysis of the Moiré fringe is introduced andthen demonstrated by the experimental results using FIB and SEM. The measurement is discussed by changing the pitch of the nano-grating,the raster scanning lines, magnification, contrast and the rotation angles of SEM. The currently patterned high frequency nano-gratingsusing focused ion beam can be adjusted to serve as a calibration tool of NEMS and MEMS fabrication which can also be extended to broadcategories of engineering applications.© 2004 Elsevier B.V. All rights reserved.

Keywords: Moiré fringe; Nano-grating; Focused ion beam; Scanning electron microscope; NEMS; MEMS; Silicon-on-insulator

1. Introduction

The Moiré fringe method has become a standard techniquefor the measurement of the behavior of materials and struc-tures after it was developed in the late 1970s. Moiré methodhas many potential applications in NEMS and MEMS[1].One of the most studied areas is the local strain/displacementmeasurement of micro-structures[2]. Recently, a great dealof attention has been paid to metrology technology related tothe calibration of micro-fabrication. Usually SEM, FIB andatomic force microscope (AFM) can be used as the monitor-ing tools of Moiré fringe for micro-structures. Compared toSEM and FIB, the AFM method is more complex becausereplication of a specimen grating is needed[3]. The capabil-ities of FIB include nano-scale milling, imaging, platinumdeposition, and device modifications of NEMS and MEMSsystems. The ion column uses an ultra-rigid mechanical as-sembly ensuring stable beam position and high resolutionimages at spot diameters as low as 30 mm[4]. However, ac-curate critical placement of the beam requires high currentsettings which severely damages the nano-gratings due to

∗ Corresponding author. Tel.:+1-607-255-0603; fax:+1-607-254-3508.E-mail address: dy48@cornell.edu (D. Yan).

the scanning with high current density of gallium ions. Thisdamage restrains the imaging of Moiré fringes using FIB.

The focus of this work is on the investigation ofnano-gratings patterned by FIB and the experimental mea-surement of the Moiré fringe using SEM.

As known, the performance of the NEMS and MEMS-based sensors and actuators depends on the accuracy, sta-bility and reliability of the nano-scale fabrication, assemblyand packaging. IC technology requires even higher accuracymicro-fabrication. Therefore, the calibration of every man-ufacturing step is vital to assure the quality of the product.The high frequency nano-gratings patterned by FIB can beused to monitor and measure the local strain/displacementof the NEMS and MEMS sensors and actuators theoretically[5]. As a result, the various stages of the fabrication processcan be accurately and successfully calibrated.

A customized SOI technology-based fabrication processwas developed to make the SOI-based cantilever on whichthe nano-gratings are patterned. The SOI-based process hasonly one mask, and can be extended to a compatible pro-cess in any SOI technology. The SOI-based process can alsobe directly used to fabricate NEMS and MEMS-based sen-sors and actuators. In this paper, we demonstrate specificMoiré fringes of multiplication, division, and rotation underdifferent conditions such as magnification, resolution, and

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D. Yan et al. / Sensors and Actuators A 115 (2004) 60–66 61

scanning raster. Theoretical analysis on Moiré fringe wasverified experimentally results using SEM imaging.

2. Operational principle

When a specimen surface carrying a high-frequency linegrating is examined under SEM, Moiré fringes will be ob-served at certain magnifications, contrast, scanning angleand resolution. The fringes are characterized by their spa-tial frequency, orientation, and contrast. These features ofthe Moiré fringes depend on the spatial frequency mismatchbetween the specimen grating and the raster scan lines, thediameter of the electron beam, and the detailed topographyof the lines on the specimen. The nano-scale bending andtwisting of the micro-structures will then be reflected fromthe change of Moiré fringes related to that of the topogra-phy of specimen.Fig. 1shows the schematic raster scanninglines of SEM, nano-gratings patterned by FIB and Moiréfringes monitored by SEM.

The scattering function of the nano-gratings and the scan-ning raster lines can be described by a Fourier series[6].As reported, the local intensity of the image is proportionalto the product of the local scattering power of the specimennano-gratings and the local intensity of the scanning rasterline. Theoretically, the scattering function of the specimennano-gratings and the scanning raster line can be describedby formulae (1) and (2), respectively:

G(y′) = 12g0 +

∞∑

n=1

gn cos(2πnf′gy′) (1)

B(y) = 12b0 +

∞∑

m=1

bm cos(2πmfby) (2)

whereG(y′) is the scattering function for the specimen grat-ing. Thegn is the Fourier coefficients andf ′

g is the spatialfrequency of the grating lines. The subscript g stands forgrating line of the specimen. Similarly,B(y) is the scatteringfunction for the scattering raster. Thebm are the Fourier co-efficients andfb is the spatial frequency of the scanning rasterlines, subscript b stands for beam of the scanning raster. Ifthere is no misalignment between the specimen grating andthe scanning raster lines, they′- andy-axes coincide.

Fig. 1. Schematic of the generation of Moire fringes.

The spatial frequency of the electron beam raster is usuallyscanning along they-direction. Under certain conditions,Moiré fringes of multiplication, division and rotation areobserved. The Moiré pattern intensityI(y) can be describedas the product of the raster function and the grating functionas shown in the following:

I(y) = G(y)B(y) (3)

the term in the double series expansion that produces theimage observed and identified as the Moiré fringe pattern isas follows:

M(y) =∞∑

m=1

∞∑

n=1

12gnbm cos2π(nfg − mfb)y (4)

Therefore,Eq. (4)can be simplified as follows:

I(y) = c(y) +∞∑

m=1

∞∑

n=1

12gnbm cos 2π(nfg − mfb)y (5)

where c(y) is the intensity of the background with a fre-quency too high to be observed. The coefficients ofgn in thespecimen grating functionG(y′) fall off rapidly with n be-cause of the topography of the grating. The coefficientsbm

of the scanning beam raster functionB(y) do not decay asrapidly with increasing orderm of the expansion. The con-trast of the Moiré fringe depends on the termg1b1/2. There-fore, the Moiré fringe depends upon the spatial frequenciesof the scanning raster lines and the nano-gratings accordingto Eq. (5). Different Moiré fringes can be generated by ad-justing the magnification, contrast, and resolution of eitherthe FIB or SEM, such as the multiplication, division, androtation Moiré fringes that can then be used to measure thedisplacement of MEMS and NEMS sensors and actuators.

3. Process development

Silicon-on-insulator (SOI) technology has increased inpopularity due to its desirable electrical characteristics. Re-cently, more attention has been paid to reducing the costof SOI manufacturing process. Currently there are commer-cially available SOI wafers fabricated by direct oxygen im-plantation or bonded layer transfer technologies. UsuallySOI with the structural layer less than 1�m is used for VLSIapplications, SOI with a thicker structural layer is widelyused for power devices, NEMS and MEMS[7]. A wide va-riety of SOI-based sensors and actuators have already beensuccessfully developed[8–10].

Here, a customized SOI process was developed to fab-ricate the micro-cantilever on which the nano-gratings arepatterned by FIB. It is a one-mask process as schematicallyshown inFig. 2.

The SOI process begins with photoresist spinning andphotolithography patterning as shown in steps 1 and 2 inFig. 2. Shipley 1813 is used for the photolithography. Dur-ing step 1, the only mask for this SOI-based process is used

62 D. Yan et al. / Sensors and Actuators A 115 (2004) 60–66

Fig. 2. Fabrication process flow of the SOI-based micro-cantilever.

to pattern the micro-cantilever on the photoresist. The pho-toresist is exposed using EVG 640 contact aligner. The pat-terned photoresist is developed in AZ 300 MIF developer,and the DIRE process (Bosch process) is used to pattern themicro-cantilever. The wafer is etched for 2 minutes usingArgon, SF6 and C4F8. Every etch cycle is around 15 s. Thefirst 6 s are used for polymer deposition, the following 2 sare used for etch, and the last 7 s are used for strong etch.All etches are performed at a pressure of 23 mT. Under thiscondition, the etch rate is approximately 2�m/min. The pat-terned micro-cantilever is schematically shown at step 3 inFig. 2, which is ready to be patterned with nano-gratingsusing FIB.

Two experiments were performed to pattern thenano-gratings using FIB. In the first experiment, we pat-terned the nano-gratings using FIB right after the photoresistwas removed by oxygen plasma etch using DryTek Asher.In this case, the buried silicon oxide (BOX) layer remainsbetween the cantilever and the substrate. In the second ex-periment, we made nano-grating after the micro-cantileverwas fully released in buffered oxide etch (BOE) process.In this case, the BOX layer was removed except the an-chor areas.Fig. 3 shows the nano-grating patterning withBOE release. With the existence of the BOX layer, a thinmetal layer has to be deposited to resolve the charge issueduring FIB patterning. Therefore, an Au/Pd alloy layer wassputtered on the whole die using Deton VacuumTM desk IIsputter coating tool. The FEI FIB 200 system was used forpatterning the nano-gratings as shown at step 1 inFig. 3. Abasic set of milling patterns with nano-gratings that specifythe area for the ion beam milling was generated, this canbe programmed using FIB according to the designed pitch,length, and depth of the nano-gratings. The patterns were

Fig. 3. Nano-gratings patterning using FIB.

repeated across the entire length of the micro-cantilever.The alignment accuracy between any adjacent patterningsets is the one of the most difficult and critical techniquesusing FIB. Finally the micro-cantilever was released inBOE solution as shown at step 2 inFig. 3. The BOE hasa concentration of 6:1. As is well known, the etch rate ofthe BOE process depends on factors including the qualityof the BOX layer of the SOI wafer, the concentration ofBOE solution, the surface open areas of the wafer, andthe etching temperature. The etch rate is within a range of0.2–0.6�m/min at room temperature for the release of themicro-cantilever. Since the BOX layer of the SOI wafer isonly 1�m, there is a stiction problem generated in both thelateral and vertical directions. To resolve this issue, the crit-ical point CO2 process was performed using a critical pointdryer (CPD) at cornell nano-fabrication facility (CNF). Af-ter the CPD drying, the yield rate of the micro-cantilever isalmost 100% within a range from 3 to 4�m in width andfrom 80 to 300�m in length.Fig. 4shows the cross-sectionof the fully released micro-cantilever after the CPD process.

By comparison with the previously reported nano-gratingpatterning results, a higher magnification and lower milling

Fig. 4. SEM image of the fully released micro-cantilever.

D. Yan et al. / Sensors and Actuators A 115 (2004) 60–66 63

Fig. 5. Testing structure of nano-gratings with a pitch of 30 and 40 nm,respectively, patterned by FIB.

current are used to obtain more accurate nano-gratings as acompromise of the accuracy of patterning and milling time[2,5].

As a test of patterning, the ruling of the nano-gratingswith a pitch of only 60 nm (ruling width 30 nm) has beensuccessfully fabricated using 1 pA milling current under 65 kmagnification using FEI FIB 200. Since the resolution ofthe FEI FIB 200 system is about 30 nm, the nano-gratingswith a pitch of 60 nm already reach its patterning limitationwith a reasonable patterning quality. The dimension of asingle ruling can be patterned to be 20 nm under highermagnification, but it is right at the resolution that the FEI FIB200 system can get. Therefore, it is impossible to organizeevery single ruling to be integrated with a uniform spacing.Meanwhile, when the magnification is larger than 65 k, thedamage due to gallium ion imaging is very severe even usingthe lowest milling current of 1 pA at lowest resolution.Fig. 5shows the patterned nano-gratings with rulings of 30 and40 nm, respectively, patterned using 1 pA with a spot sizeof 8 nm at a magnification of 65 k. As a comparison,Fig. 6shows the patterned nano-gratings with rulings of 40 and100 nm, respectively, using 4 pA with a spot size of 12 nmat a magnification of 50 k.

Although rulings of 30 and 40 nm were successfully fab-ricated, their performances were greatly influenced by issuesrelated to the milling by FIB. Two most commonly occur-ing problems are the overlapping and shifting of the rul-ings. These two issues may result from the unstable millingcurrent or the environment vibration noise. Meanwhile, theruling alignment of the two separate millings becomes verytricky for patterning smaller nano-gratings. It is very difficultto calibrate the accuracy of the alignment of the nano-gratingwith a dimension less than 40 nm.Fig. 7shows the overlap-

Fig. 6. Testing structure of nano-gratings with a pitch of 40 and 100 nm,respectively, patterned by FIB.

ping and shifting of the nano-gratings with a pitch of 40 nmduring milling by FIB. Development of this calibration isstill under investigation.

Since the surface roughness of the micro-cantilever willhave great influence on the scattering function of the spec-imen gratings, the structural layer of the SOI wafer waspre-polished.Fig. 8 shows the surface roughness analysisresults within an area of 10�m × 10�m by AFM. As canbe seen, the RMS value of the surface roughness is lessthan 2 nm which assures good optical performance of thenano-gratings. By comparison with the previously reported

Fig. 7. The overlapping and shifting in the middle of milling ofnano-grating with a pitch of 40 nm.

64 D. Yan et al. / Sensors and Actuators A 115 (2004) 60–66

Fig. 8. Roughness analysis of the structural layer of the SOI wafer usingAFM.

poly-silicon process using surface micro-machining tech-niques[2], the surface roughness has been improved greatly,which ensures higher quality patterning and better imagingof Moiré fringes.

4. Experimental discussion

To prove the concept of Moiré fringe of multiplication,division and rotation, the fabrication of the nano-gratingswith ruling of 100 nm and the measurement of their Moiréfringes will be discussed in details.

Experimental results show that the nano-gratings with apitch of 200 nm (ruling width 100 nm) can be patternedwithin a magnification range of×35–65 k times at a milling

Fig. 9. Nano-gratings with a pitch of 200 nm by FIB.

Fig. 10. Enlarged image of the nano-gratings with a pitch of 200 nm bySEM.

current range of 1–70 pA. By optimizing the milling currentand magnification of the FIB, nano-gratings with a pitch of200 nm have been successfully patterned on the SOI-basedcantilevers at×50 k and 4 pA as a compromise of the millingcurrent, magnification and the milling time.Fig. 9 shows amicro-cantilever with a length of 100�m was fully patternedof the nano-gratings with a pitch of 200 nm using FEI FIB200.Fig. 10shows the enlarged image of the nano-gratingswith a pitch of 200 nm using SEM.

As discussed previously, by changing the magnification,contrast, resolution, scanning rotation angle of the SEM,the Moiré fringes of multiplication, division and rotationcan be generated and monitored. The experimental resultsunder certain testing conditions using SEM are shown inFigs. 11–14.

Fig. 11. Moire fringe of multiplication observed by SEM.

D. Yan et al. / Sensors and Actuators A 115 (2004) 60–66 65

Fig. 12. Moire fringe of division observed by SEM.

Fig. 13. Moire fringe of rotation with the scanning beam rotated 3◦.

Fig. 11shows the multiplication Moiré fringes when thescanning lines of the electron beam are set as 384 at themagnitude of× 4.16 k. Fig. 12 shows the division Moiréfringes when the scanning lines of the electron beam are

Fig. 14. Moire fringe of rotation with the scanning beam rotated 15◦.

set at 768 with a magnification of 4.16 k times. Both themultiplication and the division Moiré fringes can be usedto measure strain/displacement of the micro-structures bycalculating the change of Moiré fringes resulted from spatialfrequency change of the micro-structures.

Figs. 13 and 14show the rotation Moiré fringes withthe scanning beam rotated at different degrees. As shownin Fig. 13, the scanning beam is rotated 3◦ clockwise, andthe scanning line is 384 at a magnification of 11.98 k. Therotation Moiré fringes can be clearly generated and imaged.As can be seen fromFig. 14, the density of Moiré fringeswill increase and the images will become more blurry as therotation angle of the scanning beam increased. Similarly, therotation Moiré fringe will be generated if the lateral bendingor the twisting of the micro-structures happens instead ofrotating the scanning electron beam. Therefore, the rotationMoiré fringe can be used to measure the in-plane bendingor the out of plane twisting of the micro-structures.

5. Conclusion

The fabrication of SOI-based high frequency nano-gratingshas been studied by using nano-scale milling of FIB. ASOI-based process with only one mask was developed tofabricate the micro-cantilever, on which the nano-gratingswere patterned using FIB. Using the CPD process, the stic-tion problem was successfully resolved, and the yield rateof the micro-cantilever is almost 100% within a range of3–4�m in width and 80–300�m in length. Nano-gratingswith a ruling range of 30–100 nm were successfully fabri-cated. By utilizing high resolution SEM, high quality Moiréfringes were demonstrated. The demonstrated Moiré fringesof multiplication, division and rotation are consistent withthe theoretical analysis. The fabricated high frequencynano-gratings are designed to be used as calibration tools formicro-fabrication which is expected to have broad applica-tions in nano-technology engineering and material science.

Acknowledgements

The author Dong Yan is grateful for the Research Foun-dation of State University of New York at Albany throughSchool of NanoSciences and NanoEngineering to providethis research opportunity, and should like to thank all theteam members of microsystem group at the school for theirvaluable discussion and involvement of this research.

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Biographies

Dr. Dong Yan is currently working as a Research Associate in Depart-ment of Electrical and Computer Engineering at Cornell University.

He started his research in MEMS from 1995 as he worked in Depart-ment of Precision Instruments at Tsinghua University, PR China. FromFebruary 1998 to June 2000, he continued his work in Institute ofMaterials Research and Engineering in Singapore. In 2001, he workedas a Research Fellow in Department of Electronics and Electrical En-gineering at Strathclyde University in Scotland for 1 year. Before hejoined Cornell University, Dr. Yan worked as a Research Associate inthe School of NanoSciences and NanoEngineering in State Universityof New York at Albany for 2 years. His research interests are in theareas of the development of high speed interconnects with integratedphotonics and VLSI chips, optical MEMS devices, microfluidic devices,BioMEMS sensors and devices, inertial MEMS sensors, NEMS/MEMSprocessing technologies and NEMS/MEMS metrology and testingtechnologies.

Jing Cheng, graduated from the Engineering school of Rensselaer Poly-technic Institute in December 2003, and her research interests are in theareas of NEMS/MEMS products development and their integration withVLSI chips.

Dr. Alyssa Apsel’s primary area of interest is in optoelectronic VLSI sys-tems in CMOS which is toward the development of low power integratedCMOS systems which utilize the speed and computational benefits ofoptical processing and communication to complement the computationalpower and ubiquity of standard CMOS systems. A large portion of herwork is devoted to the design of low power arrays of optical intercon-nects for short distance and chip-to-chip communication. She is also in-vestigating intra-chip optical interconnects and their potential impact oncomputer architectures.