Introduction to Deep Reactive Ion Etching

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


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


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


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


After Kovacs et al. (1998)

Wet and dry etching features


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)


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


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


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


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


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.


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


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


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).


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)


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)


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


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.


UNC Alcatel DRIE system ?

http://www.alcatelmicromachining.com


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.


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; [email protected] 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

Introduction to Deep Reactive Ion Etching

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