1、1,Electron beam lithography (EBL),Overview and resolution limit. Electron source (thermionic and field emission). Electron optics (electrostatic and magnetic lens). Aberrations (spherical, chromatic, diffraction, astigmation). EBL systems (raster/vector scan, round/shaped beam) Interaction of electr
2、ons with matter (scattering, x-ray, Auger). Proximity effect and how to reduce it. Resist contrast and sensitivity. Several popular resist materials. High resolution EBL, resolution limit. Grey-scale EBL for 3D structure fabrication. Anti-charging techniques. Electron projection and multi-beam litho
3、graphy,ECE 730: Fabrication in the nanoscale: principles, technology and applications Instructor: Bo Cui, ECE, University of Waterloo; http:/ece.uwaterloo.ca/bcui/ Textbook: Nanofabrication: principles, capabilities and limits, by Zheng Cui,2,Resolution enhancement process: ultrasonic development,Ul
4、trasonic helps to remove exposed resist (for positive tone) from inside narrow trenches.,3,Resolution enhancement process: low temperature development (ZEP),Comparison of edge roughness of ZEP-520 resist lines (40nm wide) developed at (top) room temperature; (bottom) -4oC.,Contrast curve for ZEP-520
5、 at various temperatures,Contrast as a function of development temperature.,Low T development increases contrast, but decreases sensitivity,4,Resolution enhancement process: low temperature development (PMMA),Figure 2-6: Schematic illustration of one possible explanation of resolution-enhancing mech
6、anism of cold development. When a feature is exposed in PMMA, the soluble resist in the exposed region is surrounded by a boundary region of resist that, due to the initial polydispersity of the PMMA and random nature of chain scission, contains both soluble and insoluble polymer chains. During deve
7、lopment, this region phase-separates, with the soluble chains diffusing toward the soluble region and the insoluble chains diffusing toward the insoluble region. The result is a region of soluble PMMA that is larger than the initial exposed feature, resulting in a degradation in resolution. Cold dev
8、elopment helps prevent this by limiting the diffusion that can occur in the boundary region, since diffusion is a thermally-dependent process.,The exact mechanism of cold development is still not clear. Another possible mechanism: colder developer is weaker solvent, so less attack to unexposed or pa
9、rtly exposed resists.,PhD thesis, Bryan M. Cord, MIT, June 2009,5,Figure 2-10: Measured contrast curves for PMMA developed in 3:1 IPA:MIBK at various temperatures. The initial resist film thickness was 160 nm and the development time was 60 seconds, except in the -40C and -50C cases (120 seconds) an
10、d the -60C case (600 seconds), where longer development times were needed to show any measurable dissolution at all.,Cold (to -60oC) development of PMMA,Best contrast (steepest slope) at -15oC. Because, at even lower temperature, need higher dose to expose, but PMMA becomes negative at doses 3000 C/
11、cm2 (i.e. the already exposed PMMA with short chain begins to cross-link upon further exposure).,PhD thesis, Bryan M. Cord, MIT, June 2009,IPA: iso-propanol; MIBK: methyl isobutyl ketone,6,Forward scattering,: forward scattering range, with unit nm,Figure 3-5: Forward scattering coefficients as a fu
12、nction of beam energy for various thicknesses of PMMA, calculated using CASINO, a free Monte Carlo modeling program. The forwarding scattering width decreases dramatically as the beam energy is increased, but using thicker resist results in more scattering.,PhD thesis, Bryan M. Cord, MIT, June 2009,
13、Why weaker forward scattering at higher energy,For simplicity, assume the electric field from the secondary electron acted on the primary electron is uniform covering a distance d.,Forward scattering: deflect primary electron, generate secondary electron to expose the resist,x,z,Higher energy, large
14、r Vz, so smaller Vx. V: velocity P: momentum A: acceleration),8,Resolution limit: forward scattering and lateral diffusion,Whether it is forward scattering or beam spot size that limits the minimum line-width, the effect of SE diffusion is to further broaden the line/reduce the resolution.,Backscatt
15、ered electron,Secondary electrons and its lateral diffusion,Figure 3-6: Schematic illustration of secondary electrons (SE), which typically travel/diffuse normal to the beam. (low energy SE is responsible for resist exposure),Backscattered electron,Forward-scattered electron,9,Figure 3-11: “Critical
16、 thickness” of resist as a function of beam energy. At resist thicknesses above critical thickness, forward scattering limits resolution, whereas below the critical thickness the only resolution limiter is the beam diameter, which is at least theoretically energy-independent.,Figure 3-19: Beam diame
17、ter as a function of beam energy as measured in the MIT Raith-150 system. While beam diameter is inexplicably large at voltages below 10kV, it seems to be reasonably close to its 3-4 nm specification at higher kV, suggesting that beam diameter is not the limiting factor in the resolution (see next s
18、lide).,PhD thesis, Bryan M. Cord, MIT, June 2009,Resolution limit: forward scattering and beam diameter,A third limiting factor is resist swelling by developer.,10,The highest resolution by far: 9nm pitch grating,Figure 3-20: 9-nm-pitch nested-L structure fabricated in HSQ on Si and imaged in the MI
19、T Raith-150 system (left) and on the Raith-150TWO prototype tool. The left image shows minimal to nonexistent modulation, while the discrete lines of the structure are clearly visible in the right image, possibly due to the much better vibration isolation.,Backscattering/proximity effect is not impo
20、rtant here, since pattern area only 50nm. For large area (1m) grating, such a small pitch may not be achieved due to proximity effect.,Here resist very thin, 25nm.,PhD thesis, Bryan M. Cord, MIT, June 2009,PMMA should have similar or even better resolution, but difficult to image (too soft, whereas
21、exposed HSQ is rigid SiO2 without deformation during imaging), PMMA doesnt stick to substrate well (may be lifted off during development), and lines may fall due to capillary force during drying after development.,11,Electron beam lithography (EBL),Overview and resolution limit. Electron source (the
22、rmionic and field emission). Electron optics (electrostatic and magnetic lens). Aberrations (spherical, chromatic, diffraction, astigmation). EBL systems (raster/vector scan, round/shaped beam) Interaction of electrons with matter (scattering, x-ray, Auger). Proximity effect and how to reduce it. Re
23、sist contrast and sensitivity. Several popular resist materials. High resolution EBL, resolution limit. Grey-scale EBL for 3D structure fabrication. Anti-charging techniques.,12,Gray-scale electron beam lithography,E-beam exposure with variable doses to create variable depths after development.,AFM
24、of a grating in resist,Fresnel zone lens (plate) in silicon,PMMASilicon,RIE pattern transfer into Si,5m,13,Which resist is best for gray-scale EBL?,Contrast curve of PES with 10keV electron beam. Sensitivity was found to be 200C/cm2, with contrast only 0.8.,Contrast curve of PMMA,Bryce, “Poly(ether
25、sulfone) as a negative resist for electron beam lithography”, APL, 90, 203110 (2007).,Ideal resist has positive tone with very low contrast (ideally 1) and high sensitivity. High contrast leads to very narrow process/dose window (tiny dose change large pattern depth change). When using negative resi
26、st, make sure that the electrons can reach resist bottom (otherwise, resist at bottom will be dissolved by developer, lifting off all the resist above).,PES,Negative tone,14,Micro and nano optical elements in silicon and silica by regular and gray scale e-beam lithography,15,Electron beam lithograph
27、y (EBL),Overview and resolution limit. Electron source (thermionic and field emission). Electron optics (electrostatic and magnetic lens). Aberrations (spherical, chromatic, diffraction, astigmation). EBL systems (raster/vector scan, round/shaped beam) Interaction of electrons with matter (scatterin
28、g, x-ray, Auger). Proximity effect and how to reduce it. Resist contrast and sensitivity. Several popular resist materials. High resolution EBL, resolution limit. Grey-scale EBL for 3D structure fabrication. Anti-charging techniques.,16,Charging during e-beam writing,Even though most resists are ins
29、ulating, charging is not an issue for typical resist thickness of 500nm, because most electrons penetrate deep into the conducting substrate at 30kV. (more serious charging for lower kV) When electron beam lithography must be performed on insulating substrates (quartz, SiO2/Si), negative charge buil
30、dup can occur on substrate surface, causing beam deflection, and thus pattern distortion.,electron beam,Glass substrate,ma-N 2403 negative resist,electron beam,negative charge accumulation,Initial Condition,Future Condition,repulsive electric potential lines,17,To eliminate charging effect, one can
31、coat a conducting layer on top of or beneath the resist. Typically 10nm metal is enough, such as Al, Ti, Cr or Au; conducting polymer may also work. Lighter metal (Al) causes less (forward) scattering of electron beam than the heavier one (Au), so is preferred. Al and Ti can be removed easily by dil
32、uted HF (1:100 diluted). Some resist (such as SU-8) is sensitive to UV/x-ray light generated during e-beam (10keV beam energy) evaporation of the metals (thermal evaporation or sputtering is OK).,Anti-charging technique: coat conducting layer,GOOD (no charging),BAD (severe charging),Coat metal,no an
33、ti-charging layer,18,Anti-charging technique: variable pressure (VP) EBL,It is the same idea as VP-SEM, i.e. introducing gas (H2O, N2, Ar, He) into the chamber. Gas molecules are ionized by electron impact; these positive gas ions migrate to negatively charged surface and balance surface charge. Pri
34、mary electron beam will be scattered to some extent by collision with gas molecules, forming a beam “skirt” around the focused primary beam at sample surface. Higher energy e-beam has less “skirt”; but it is shown that “skirt” doesnt noticeably reduce resolution. (For SEM only) Gas also amplifies se
35、condary electron (SE) signal by “gas cascade” effect.,VP-SEM images of a pattern on glass substrate with 30keV e-beam writing under high vacuum 0.4 Torr (water vapor pressure) 1 Torr The dashed red line indicates the pattern dimension as written. The pattern exposed with 1Torr pressure shows no sign
36、ificant distortion or displacement.,“Variable pressure electron beam lithography (vp-ebl): A new tool for direct patterning of nanometer-scale features on substrates with low electrical conductivity”, Nano Lett. 6(5), 963-968(2006).,19,Anti-charging technique: critical energy EBL,Bulk insulating mat
37、erial is positive charged when 1 and negatively charged when 1, so 1 electron (including SE and BSE) ejected for each incident electron. E1 is too small, E2 depends on resist thickness and substrate material. For 65nm PMMA on glass, E2=1.3keV. Such low keV can penetrate such thin resist.,Total (ejec
38、ted) electron yield () vs beam energy for a typical resist.,How to find E2? Reduce magnification, here from 500 to 200 at acceleration voltage: 0.5keV 1.3 keV 2.0 keVElectron accumulated during imaging at 500. When zoom out to 200, more electrons ejected at the charged region, so is brighter.,positi
39、vely charged,negatively charged,Virtually no charge,“Nanoscale Patterning on Insulating Substrates by Critical Energy Electron Beam Lithography”, Nano Lett. 6(9), 2021-2025 (2006).,Positive charge,Negative charge,20,Anti-charging technique: critical energy EBL,Line deflection determined by SEM measu
40、rement at various voltages based on methods reported by Craighead. (Two parallel single-pass reference lines were first patterned with a 1m gap, followed by charge pads written at 30C/cm2. Finally, a third single-pass line was patterned between the pad and the reference line),No deflection at 1.3keV
41、,Low voltage difficult to get high resolution, here only 60nm. But the sensitivity becomes very high, only 10C/cm2, (250C/cm2 for 30kV).,21,Electron beam lithography (EBL),Overview and resolution limit. Electron source (thermionic and field emission). Electron optics (electrostatic and magnetic lens
42、). Aberrations (spherical, chromatic, diffraction, astigmation). EBL systems (raster/vector scan, round/shaped beam) Interaction of electrons with matter (scattering, x-ray, Auger). Proximity effect and how to reduce it. Resist contrast and sensitivity. Several popular resist materials. High resolut
43、ion EBL, resolution limit. Grey-scale EBL for 3D structure fabrication. Anti-charging techniques. Electron projection and multi-beam lithography,High throughput electron-based lithography: overview,Electron beam lithography using single beam is too slow for mass production. Three directions to incre
44、ase throughput drastically: Use broad beam and mask, like photolithography. It is called electron projection lithography (EPL), with SCALPEL as best known EPL method. Start from a broad beam, divide into many sub-beams, like zone-plate x-ray lithography. Using an array of mini-electron guns/sources
45、to generate multi-electron beams. Those techniques can also be used for ion, will be addressed later. Currently, multi-e-beam direct write is competing with EUV lithography for next generation lithography.,ITRS (2006) Projections for Lithography Technology,ML2: mask-less lithography - EBL, SPM (scan
46、ning probe microscope) tip based lithography,MEMS-based electron emitters array,Not mature, far away from mass production tool. Same principle as field emission display, go to see http:/en.wikipedia.org/wiki/Field_emission_display,Dark and light regions differentiated by their scattering strength at
47、 the SCALPEL aperture.,Harriott, JVST B, 15(6), 2130-2135 (1997),SCALPEL: Scattering with angular limitation projection electron beam lithography,SCALPEL: mask,Mask is like that for x-ray lithography, consisting of membrane and “absorber”, which are both almost completely electron-transparent at 100
48、keV electron energy. This means that very little of the incident energy is actually absorbed by the mask, minimizing thermal instabilities in the mask. Si3N4 membrane with thickness order 100nm; “absorber” typically W. Such a membrane mask is nothing robust. As a result, like x-ray lithography, mask
49、 problems make electron projection lithography unsuitable for mass production, despite the great investment the semiconductor tool company has put in it.,World first 200mm mask fabricated by HOYA,Coulomb interaction between electrons limit resolution,Coulomb repulsion is important because all electr
50、ons not scattered by “absorber” pass the small aperture where the current density/repulsion is highest.,SCALPEL summary,Electron energy 100keV, negligible diffraction. Reduction is possible, such as 4 (i.e. pattern on mask 4 that on resist). Target resist sensitivity is 5C/cm2, which is over 100 fas
51、ter than PMMA. Shot noise and line-edge-roughness would be an issue. Up to around year 2000, EPL had been considered as a promising next generation lithography to replace photolithography, but no more for today, partly because its resolution is no longer impressive.Problems: Alignment method. Pattern distortion/resolution limit due to repulsive force among electrons.,
