H. Shin, W. Zhu, V. M. Donnelly, and D. J. Economou University of Houston. November 2, AVS 58h International Symposium, Nashville, TN, USA

Transcription

1 Near-Threshold Ion-Enhanced Silicon Etching H. Shin, W. Zhu, V. M. Donnelly, and D. J. Economou o ou University of Houston Acknowledgements: DOE Plasma Science Center, NSF and Varian Semiconductor Equipment Associates November 2, 2011 AVS 58h International Symposium, Nashville, TN, USA

2 Outline 2 Control of ion energy distribution (IED) Experimental set-up for near-threshold etching Results and Discussion Etching threshold, etching rate & yield Sub-threshold etching Photo-assisted etching Summary

3 Control of IED 3 In 2010 AVS, we presented how to control IED using a boundary electrode*. Detailed IEA configuration Biasable boundary electrode ICP coil IED Faraday shield 0.02 Variable DC synchronous bias 20µs 80µs ON OFF V p during active glow * H. Shin et al, PSST (2011) Controllable ion energy by synchronous DC bias BE bias during afterglow 7 mtorr 14 mtorr mtorr 50 mtorr Energy (ev) The IEDs were obtained by a differentially pumped repelling field energy analyzer. We can make IED with two peaks apart from each other placing E th in the middle.

4 Time resolved electron temperature 4 T e (ev) 5.0 ON 7mTorr 14mTorr mTorr mTorr OFF Electrons are cooled 1.5 by diffusion time ( s) At higher pressure, T e is lower during active glow, but higher during afterglow. We attribute the narrower IED width to the low T e Late afterglow biasing is more beneficial for narrow IED. Broadening of IED is due to collisions in pre-sheath and a function of T e. T e and V p changes while biasing. Time averaged IED is a convolution of IEDs during the bias. PS2-TuA9, W. Zhu

5 Benefits of such control of IED 5 We reviewed what affects IED and learnt how to control it. Using the precise control of IED with a narrow width, we investigated ion-assisted plasma etching in a plasma environment. Such precise control of IED is applicable to high selectivity etching and precise etching (e.g. atomic layer etching). Conventional plasma reactors have a broad IED and high plasma potential. Next we present a surprising and important discovery of sub-threshold etching in plasma thanks to our control of IED.

6 Setup for etching study 6 Biasable boundary electrode rf coil for ICP Faraday shield p-type Si sample Spectrometer photodiode IR laser 1.31µm Periscope for OES Cooling water Si 2881 Å was used to monitor etching. Etched depth was measured by IR laser interferometry. Ion flux was measured for etching yield. A load-lock is ready for clean and reproducible environment.

7 mission Rela ative ER or Si e Si removal with a well controlled IED mTorr 28mTorr 50mTorr 60mTorr ER(E) at 50mTorr Y(0eV)= mTorr E th Y(40eV)=0.41 Y(30eV)= E 1/2 (ev 1/2 ) ER (Å/m mtorr 1. E th not by extrapolation 2. Universal etching rate relation 3. Y(E) 4. Subthreshold etching UNIVERSITY of HOUSTON Plasma Processing Lab

8 No spontaneous chemical etching 8 30nm SiO 2 mask p type Si 50mTorr 1% Cl 2 Ar pulsed plasma with synchronous bias, 40V. p type Si is known not to etch by Cl or Cl 2 spontaneously [Mogab and Levinstein (1979), Ogryzlo et al (1990), Flamm (1990)] The sub threshold h was 2/3 of Ion assisted itdetching thi at 40V, but no undercutting. The sub threshold etching is NOT due to spontaneous chemical (isotropic) etching. UNIVERSITY of HOUSTON Plasma Processing Lab

9 Pure Cl 2 plasma also shows 9 the sub threshold etching Energetic Ar metatstables (11.55 and ev for 3 P 2 and 3 P 0 ) could have lead to surface reaction. This excludes possibilities of the Ar metastables as a source for the spurious etching. 882 Si int tensity at % Cl 2 CW plasma at 40mTorr BE bias for 50µs at 10kHz Boundary electrode bias 1/2 UNIVERSITY of HOUSTON Plasma Processing Lab

10 Low energy (<E th ) ions etching to minimize charge exchange V B V A V p C Sample (V C ) -5V A B Current (ma) 1 Voltage (V) Ions Radicals Photons -2-4 Radicals Photons Relative Etch hing Rate Sample bias was used to repel ions The low energy ions can create low energy electrons when they are neutralized by an Auger process. V p hardly changed during this measurement Fast neutrals by charge exchange can be safely ignored at low pressure. We did turn off ALL the ion flux but still have the same weird etching. UNIVERSITY of HOUSTON Plasma Processing Lab

11 Photo assisted etching (PAE) V (I=-3.5mA) 2.5 0V (I=-0.1mA) Si -30V (I=0.7mA) (arb. units) Intensity Si Si SiCl The sub-threshold etching remains the same under no ion bombardment Wavelength (Å) With negative 30V (ion-assisted etching with E=30+V p ), the etching is more. With positive 30V (no ions), the etching is the same as with no bias. The sub-threshold etching is due to photons which always exist in plasma. UNIVERSITY of HOUSTON Plasma Processing Lab

12 Validating proof of PAE 12 Si emission at 7mTorr w/o grid correcte w/ grid E 1/2 (ev 1/2 ) Comparison of the etching rate with grids (3% Cl 2 in CW) to the earlier measurement without the grids (1% Cl 2 in PP). With our best effort of calibration, the sub-threshold etching with the grid coincides well with the one without the grid validation of the grid experiment and reconfirm of no ion effect on the sub-threshold. This is the first time to report the photo-assisted etching (PAE) in a plasma environment.

13 Further investigation of PAE 13 (b) Etching for 12min in Ar plasma with 3% Cl 2 in a CW mode (300W). Same plasma and neutrals but different light illumination using an opaque and transparent (>170nm) quartz roof. Etched depth was compared using a Etched depth was compared using a step profilometer.

14 VUV is Responsible for PAE 14 Etched de pth (nm) Opaque (p-si) Transparent(p-Si) Position (#) Under the opaque roof, the p-type Si etching rate is much smaller due to smaller light illumination. n-type Si showed more overall etching but less effect of photo- assisted etching. With the quartz roof, the etching rate is 105 Å/min, which is only a fraction (<9%) of what would ve been expected at the same conditions (>1200 Å/min) (3% Cl 2, 300W). This implies the photo-assisted etching is dominated by the photons blocked by quartz, VUV photons below 170nm.

15 Efficiency of VUV for etching 15 Streller et al, Journal of Electron Spectroscopy and Related Phenomena (1996) It was reported by Streller et al that VUV <130nm is much more efficient to etch GaAs in Cl 2 system in their study using synchrotron. Strong VUV lines (104.8nm and nm) exist in Ar plasma. 52mW/cm 2 in Ar ICP over 50 and 250nm [Woodworth et al JVST A (2001)]

16 SEM of the sub threshold etching 16 DG 0.1 L/0.1S 0V DG 0.1 L/0.1S 0V ~110nm ~15nm Ar 50mTorr 10min Xe 50mTorr 10min Under no ion-assisted etching regime, we see micro-trenches by PAE. Chec Like ions, VUV photons glanced off the sidewall lead to microtrenches (???).

17 SEM of sidewalls in halogen etching 17 Cl 2 HBr Cl 2 HBr 400W ICP; 80W rf biasing; 100sccm 400W ICP; 20W rf biasing; 175sccm Cl 2 Mahorowala et al JVST B (2002) Vyvoda et al JVST B (2000) Longer time of Cl 2 etching

18 2.5min Ar 90nm 5min 105nm 120nm 210nm Xe & Ar comparison 50mTorr 40V DG0.1L/0.1S 1S 2.5min 110nm Xe 80nm 5min 180nm 360nm

19 Summary 19 Using our ability to control IED for exploring plasma etching near threshold, we showed a definitive evidence of photo-assisted etching (PAE) in plasma etching environment. PAE is dominated by VUV photons. The PAE could be an impediment to etching with atomic precision processing for smaller device fabrication in the future. The PAE could also be a cause of some of etching artifacts in chlorine containing plasma etching (e.g. sloped sidewalls and micro-trenching). UNIVERSITY of HOUSTON Plasma Processing Lab

20 Thank you

21 BACKUP

22 22 ion intensit ty (a.u.) Si 2881 emiss V p by synchronous active glow bias % pulsed 100sccm Ar/Cl2/TRG p-type Si, synchronous boundary bias HV=1500, G=10^9 activeglow bias 1.0 active glow sync bias after glow sync bias ~12V=Active glow V Ar Active glow sync bias does not produce more etching until afterglow ion peak reaches the threshold. V p during the active glow can be deduced Bias voltage (V)

23 Current measured at the sample 200 Bias during afterglow I (ma) 100 Ati Active glow +20V 0 0V Time ( s)

24 Si 2881 emission intensity ( a.u.) % pulsed 100sccm Ar/Cl2/TRG p-type Si, synchronous boundary bias HV=1500, G=10^9 activeglow bias active glow sync bias after glow sync bias ~12V=Active glow V p Bias voltage (V)

25 IED is determined by 25 temporal evolution of V and T temporal evolution of V p and T e Ar Te Kr Te Xe Te Ar Vp Kr Vp Xe Vp T e (ev V) V p (V) time ( s) time ( s) T e and V p decays slower in Xe plasma (slower diffusion cooling). V p is similar but T e is highest in Xe plasma. This results in broader width in Xe plasma.

26 IEDs of Ar, Kr and Xe pulsed plasmas Ar Kr Xe 0.3 IED Energy (ev) Ar has the narrowest width of IED Xe has the lowest V p during active glow T e and V p changes while biasing. Broadening of IED is due to collisions in pre-sheath and a function of T e. Time averaged IED is a convolution of IEDs during the bias duration.

27 Etching with different buffer 27 gases PMT current (A A) 9.0x10-8 Ar 120(11W) 8 Kr 110(4W) Xe 110(3W) 8.0x x mTorr 60x10-8 G=10 8 ; HV=1500V 6.0x10 5.0x x x x x BE bias (V) or ion energy (ev) Uncertain

28 Preliminary Result of Different Etchant/Buffer Gas 28 ion (V) Si emiss G=10^7, HV=1500V 50mTorr Cor_1% Cl2 Cor_1% Br % Br emission (V) Si G=10^7, HV=1500, 50mTorr Cor_1% Br2/Xe Cor_1% Br2/Ar Bias Bias 0.5

29 SEM images of etched patterns for XX min. with 0V afterglow bias in different carrier gas plasma with 1% Cl2 at 50mTorr: (Top) in pulsed Ar plasma with 1% Cl2 (a) 100nm line and 100nm space (b) 500nm line and 100nm space; (Bottom) in pulsed Xe plasma with 1% Cl2 (c) 100nm line and 100nm space (d) 500nm line and 100nm space

30 IR interfer rometry ) Si emissio on (a.u Plasma off Time (min) 0 During etching with 30V BE bias, 50mTorr 1% Cl 2

31 References Mogab and Levinstein (1979), Ogryzlo gy et al (1990), Flamm (1990) : no spontaneous etching of p-type Si by Cl or Cl2 Photochemical etching F. A. Houle T. J. Chuang (1982) Ehrlich et al(1981) Okano et al Jackman Strellar et al More 31