introduction to deep reactive ion etching
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Introduction to Deep Reactive Ion Etching
Felix LuAQT/Duke
April 22, 2008
After Hibert (2002)
After Gale
http://www.micromagazine.com/archive/05/12/0512MI35d.jpg
http://www.oxfordplasma.de/images/scetch/icp_r_ww.gif
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Topics
• Motivation and applications for Deep RIE• Requirements using DRIE• Deficiencies of standard wet/dry etch
processes• Optimization of etch rate, smoothness and
selectivity• The Bosch and Cryo DRIE processes• Summary
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Motivation & Applications for DRIE
DRAM micrograph at left shows cross section of ~60:1-deep trench capacitor. SEM images at right show Al2O3 thicknesses proving 100% step coverage. [http://www.micromagazine.com/archive/02/06/lead.html]
http://www.semiconductor-technology.com/contractor_images/sts/3_gaas-via.jpg
http://www.clarycon.com/Resources/Slide2s.jpg
Trench capacitors
After Walker
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
SOIMUMPS backside etching
– Backside etching of SOI MUMPS die for releasing and metallization of mirror surfaces
– Need to etch ~500-700 µm through Si substrate.
SOI (mirror)
Au pads
SOI (mirror)
Au pads
Evaporated Au
SOI (mirror)
substrate
Au pads
BACK
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Desirable characteristics for etching high aspect ratio features
• Relatively high etch rate– standard RIE ≈1 µm/min; DRIE � ~2 to >20 µm/min [1]
• Cost effectiveness– higher density of reactants
• Anisotropic etch independent of crystal orientation– Vertical sidewalls/ability to control taper (≈90 deg vertical sidewalls, [2])– Control lateral etch rate
• ion motion normal to surface & protected sidewalls• High mask etching selectivity (120-200:1 for SiO2 [1])
– Thin mask more convenient– Ion bombardment not dominant, balancing of chemical sputtering and
ion bombardment• Relatively high smoothness on sidewalls and bottom
– Depends on application requirements– Control of diffusion profiles for ions and radicals– May have a tradeoff with etch rate
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
After Kovacs et al. (1998)
Wet and dry etching features
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Extending the RIE process
http://www.ee.byu.edu/cleanroom/everything_wafers.parts/v_groove
RIE � dry etch, anisotropic, independent of crystal orientationDeep vertical etching achievable [3] – SLOW (~0.5-1 µm/min)
After Bruce Gale, U of Utah.
User controllable
Increase reactants:
Increase gas flow (pressure)
More collisions (less directional)
Decrease in anisotropy
Increase etch rate:
< 20:1
increase ion energies
Increase RF power
More radicals
Less mask selectivity
Higher energy ions
(Ayon, 1999)
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
High density plasma requirements for faster etching
• Typical RIE : Capacitively Coupled Plasma –ion energy and density of radicals COUPLED.
• Inductively Coupled Plasma (ICP) : control for plasma confinement
• Substrate bias: control for ion bombardment
• Radicals � chemical reaction (higher selectivity)
• ICP desirable because:– High density of radicals (~10×) [6] without
high density of high energy ions. [7]– Ion bombardment at low levels [7] for http://www.ece.neu.edu/edsnu/hopwood/icp-labpage.html
ICP reactorHigh Density Plasma
Ion Assisted Chemical Etching
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Ion assisted chemical etching
After Coburn and Winters (1979)
Artifact from measurement
XeF2 flux decay
~5-6× sum of individual etch rates ~sum of XeF2 and
Ar+ etch rates
Enhanced etch rate not explained by summing XeF2etch rate and Ar+ etch rate.
-123
°C
Spontaneous etch rate of XeF2 at 50 mTorr on Tungsten
Bensaoula (1986)
Ion enhanced etch rate at low temperature.
Ion assisted chemical etching
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Ion enhanced chemical etching modelsIons & electrons
SF6
Damage to Si surface and/or SF6
FF
S F FF FEnhanced dissociation and/or adsorption
Volatile product
F
FDamage may also enhance removal
“Damage Enhancement model”[Coburn and Winters (1978)]
Chemisorbed FF- ion
Volatile product
“Reactive Spot model”[Tachi (1985)]
Implanted ions provide the energy to chemically sputter the substrate material.
DRIE processes take advantage of ion assisted chemical etching
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Overview of DRIE processes
A.k.a. “Cryo process” A.k.a. “Bosch”, “Pulsed” or “Time multiplexed” process
Condensed n-CF2 polymer
F-
Si
Mask
/
Condensed SiOxFy
F-
Si
Mask
SF6 / O2 plasma
Sidewall protection because fluorine radicals spontaneously etch Si.
@ ~ -110°C
Significantly reduced spontaneous etch rate
SF6 C4F8
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
DRIE parameters
• High plasma density at low pressure– low pressure reduces ion scattering – maintains ion trajectory as mostly vertical– better control of etch profiles– improves transport of species into deep trenches– Low P � fast pumping or low flow rate
• Low flow rate reduces etch rate
• SF6 used as isotropic etchant due to low toxicity compared to F2.
• O2 typically used with SF6 to :– Combine with SFn and CFn so that F does not combine with them
� keeps F concentration high.– Passivates surfaces where mask has eroded– Reacts with polymer film to keep it from getting too thick.
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Fluorine reactivity with Si and SiO2as a function of Temperature
SiSiO2
C1 Ea’(eV)2.86×10-22
6.14×10-23 0.1630.108
[After Roth (2001)]
Etch Rate = C1nFT1/2 e-eE’a/kT
Constant with weak T dependence
Density of F atoms (3×1021/m3)
At -110°C , >100× drop in Si etch rate by F radicals.
Etch rate using fluorine radicals
1.E-071.E-061.E-051.E-041.E-031.E-021.E-01
1.E+001.E+01
-250
-200
-150
-100 -5
0 0 50 100
150
200
250
300
350
Substrate temperature (°C)
etch
rat
e (µ
m/m
in)
SiSiO2
condensation of SF6
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Cryo process data
Should not go below -130 °C as SF 6 will condense on wafer [7]After Tachi (1987)
Decreasing T
SF6 DRIE
Sidewall etching (R) effectively goes to zero at T < 90°C.
Si etch rate increases by > 2× with decreasing T.� presumably due to Ion assisted enhancement.
SiO2 etch rate decreases by ~5× with decreasing T. � Presumably due to F not efficiently reacting with SiO2 compared with Si
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Bosch process details
– High mask selectivity over Si etching (at least 50:1 if not 100:1) possible
• “soft” teflon like polymer ( low energy ion bombardment for removal)
• Low energy bombardment does not significantly erode masking materials.
• Harder (more polymerized teflon based polymers) polymers would require larger ion bombardment energies and the masking selectivity suffers. [2]
– Alternating of etch and passivation steps allows easier and dynamic optimization of process.
– Using the two steps simultaneously causes extinction of the amount of radicals by chemical recombination. [2]
– This alternating sequence allows of RIE lag• the duty cycle of the step varied to adjust for
trench widths (which are proportional to the amount of passivation at the bottom of the trench).
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Bosch process artifactsBosch DRIE 20 µm via
With
pol
ymer
With
out p
olym
er
After Lietaerwww.alcatelmicromachining.com/amms_en/download/docs/news/doc148.pdf
After Hibert (2002)
Bosch Process scalloping
Scallop period is determined by duty cycle.
After Qu (2006)
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
DRIE artifacts
After Walker (2001)
Smaller opening � fewer ions � lower etch rate.
Chambers et al., Surface Technology Systems, Advanced Packaging, 2005
http://ap.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsection=Display&ARTICLE_ID=225422
Via etching
Aspect Ratio Dependent Etching (ARDE)
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
DRIE factors and tradeoffs
• Maximize smoothness?– Bosch – reduce duty cycle (thus etch rate) for smaller
scallops.– Cryo – intrinsically smoother than Bosch structures
• Maximize mask selectivity?– Less ion bombardment, more chemical activity
• Maintain 90°walls?– Balance ion bombardment with chemical etching
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Comparison of Bosch and CryoDRIE processes
After Walker (2001)
Cryo etch rate > 5µm/min [20]
Bias is higher for Bosch � consistent with lower mask selectivity.
Bosch process alternates between etch and passivation steps – which allows tuning of duty cycle to accommodate deep features.
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
UNC Alcatel DRIE system ?
http://www.alcatelmicromachining.com
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
Summary
• DRIE main advantages over other wet and dry processes are fast etching speed with freedom to tune selectivity, smoothness, and have vertical sidewalls.
• A remote high density plasma is independently controlled along with a substrate bias to balance radicals and ion bombardment.
• The combination and judicious tuning of chemical and physical etching produces an enhanced etching rate with “smooth” vertical sidewalls
• Cryo process has smoother sidewalls, however Bosch process allows dynamic optimization to account for RIE lag.
Felix Lu / Applied Quantum Technologies / Duke University / April 2008
References1. Gregory T. A. Kovacs, Nadim I. Maluf, Kurt E. Petersen, “Bulk Micromachining of Silicon”, Proceedings of the IEEE, Vol 86,
No. 8, August 1998, p. 1536 2. Franz Laermer and Andrea Urban, Robert Bosch Gmbh, “Milestones in Deep Reactive Ion Etching”, Transducers’05, 13th
international conference on solid state sensors, actuators, microsystems, Seoul, Korea, june 5-9, 2005, p. 11183. Roger Shile, MEMSTALK posting; rshile@thermomicro.com Tue Mar 6 20:19:47 2001 ]4. Bruce K. Gale, Dry etching. (presentation slides), Fundamentals of Micromachining, BIOENG 6421, The University of Utah5. A. A. Ayon, R. Braff, C. C. Lin, H. H. Sawin, and M. A. Schmidt, Characteriation of a time multiplexed inductively coupled
plasma etcher, Journal of the Electrochemical Society, 146, (1) 339-349 (1999)6. Scott Smith, Ph.D. Thesis, “inductively coupled plasma etching of III-N semiconductors”, 1999, NCSU7. Martin J. Walker, “Comparison of Boasch and cryogenic processes for patterning high aspect ratio features in silicon”, ©
2001 by the Society of Photo-opical Instrumentation Engineers, P. O. Box 10, Bellingham, Washington 982278. J. W. Coburn and Harold F. Winters, “Ion- and electron-assisted gas surface chemistry – An important effect in plasma
etching”, J. Appl. Phys. 50 (5) May 1979, p. 31899. A. Bensaoula, A. Ignatiev, J. Strozier, and J. C. Wolfe, “Low Temperature ion beam enhanced etching of tungsten films with
Xenon Difluoride”, Appl. Phys. Lett. 49 (24) 15 Dec 1986, p. 166310. Shin’ichi Tachi, Kazunori Tsujimoto, and Sadayuki Okudaira, “Low temperature reactive ion etching and microwave plasma
etching of silicon”, Appl. Phys. Lett. 52 (8) 22 Feb 198811. J. Reece Roth, Industrial Plasma Engineering, CRC Press 200112. Hongwei Qu, Ph.D. Thesis, “DEVELOPMENT OF DRIE CMOS-MEMS PROCESS AND INTEGRATED
ACCELEROMETERS”, U. of Florida, 200613. Cyrille Hibert, “State of the Art DRIE processing”, CMI Annual Review, 18 May 200414. Cyrille Hibert, “Dry Etching in MEMS fabrication”, CMI Comlab Review, 4 June 200215. Sami Franssila, “Introduction to Microfabrication”, John Wiley 200416. Shin’ichi Tachio and Sadayuki Okudaira, “Chemical Sputtering of silicon by F+, Cl+, and Br+ ions: Reactive spot model for
reactive ion etching”, J. Vac. Sci. Tenol. B 4 (2) mar/Apr 1986, p. 45917. S. A. McAuley, H. Ashraf, L. Atabo, A. Chambers, S. Hall, J. Hopkins, and G. Nicholls, “Silicon micromachining using a high
ensity plasma source”, J. Phys. D: Appl. Phys: 45 (2001) 2769-277418. Ranganathan Nagarajan, Krishnamachar Prasad, Lioa Ebin, Balsubramaniam Narayanan, “Development of dual etch via
tapering process for through-silicon interconnection”, Sensors and Actuators A 139 (2007) 323-32919. Daniel L. Flamm, Mechanisms of silicon etching in fluorine and chlorine containing plasmas”, Pure & Appl. Chem. Vol. 62,
No. 9, pp. 1709-1720, 199020. L. Sainiemi, and S. Franssila, “Mask Material effects in cryogenic deep reactive ion etching”, J. Vac., Sci. Technol. B 25 (3)
May/ Jun 2007, p. 801
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