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Engineered Substrates for High-Mobility MOSFETs

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Presentation on theme: "Engineered Substrates for High-Mobility MOSFETs"— Presentation transcript:

1 Engineered Substrates for High-Mobility MOSFETs
Nathan Cheung Dept of EECS, UC-Berkeley GSR: Eric Liu and Vorrada Loryuenyong FLCC Seminar 11/8/04

2 OUTLINE Motivations for SOI, SSOI, and GeOI substrates
Layer Transfer Technologies - Epitaxial Growth and Implantation - Plasma Activated Bonding - Delamination - Post-Delamination Surface Smoothing FLCC Research - GeOI layer transfer - Transfer Thickness Mechanisms - Thermal-Mechanical Stress Analysis FLCC Seminar 11/8/04

3 FLCC Seminar 11/8/04

4 Various mobility enhancement device structures
Strain Si Ge FLCC Seminar 11/8/04

5 Layer Transfer Approaches
Splitting By Internal Force H+ H peak Donor wafer SiO2 Handle wafer Wafer bonding Thermal exfoliation > 400°C M.K. Bruel, Electron. Lett. 31, 1201 (1995). Splitting By External Force Mechanically weakened Layer Donor wafer SiO2 Handle wafer Wafer bonding Edge initiated crack propagation En et al, SOI Conference Proc, 163 (1998) Yonehara et al, APL, 64, 2104 (1994) FLCC Seminar 11/8/04

6 Direct Wafer Bonding Chemical Cleaning: HF, H2SO4, H2O2
IR Transmission Image Through a Bonded Pair Plasma exposure Room temperature bonding Complete bonding over 4 inch diameter Annealing FLCC Seminar 11/8/04

7 Delamination Methods (1) Exfoliation of implanted hydrogen [ SOITEC, Amberwave] Si donor Transferred Si overlayer Handle wafer (2) Cleavage along implant damage region (gas jet) [Sigen] (3) Mechanical rupture of Porous Si (water jet) [Canon] FLCC Seminar 11/8/04

8 Layer Transfer Theory: Bonding strength > Cutting layer strength
Strengths of Bonding and Cutting Layers Separation Modes No transfer Partial transfer Full transfer gbond gcut Temperature (°C) g (J/m2) gcut > gbond gcut < gbond i = surface energy of interface i Donor Si Donor Si Donor Si SiO2 Receptor Si Receptor Si Receptor Si SiO2 Transferred Si 1 cm This explanation is consistent to the observations reported by Yonah cho. In the lower T range below the crossover temperature the mechanically initiated crack propagated entirely through the boned interface, resulting in no layer transfer. Around and slightly above the cross over T, the crack propagated mostly through the implant interface with parital derailing to the bonding interface. As a result, partial transfer was observed. After annealing substrantially above the crossover T, the crack advanced entirely at the implanted surface, rendering full Si layer transfer. Full layer transfer occurred after Si-SiO2 were annealed above 250C. The derailing results are similar to what has been observed by Yonah Cho. Yonah investigated the separation modes of the Ion cut of Si-SiO2 pairs by comparing the strengths of bonding and the strength of the cutting layers. She found that if the bonding is very weak, there would be no transfer or in other words the crack would propagate at the weak plane along the interface. If the bonding is much larger, the crack would propagate along the weak plane which is now the cutting layer. In the intermediate case the crack would derail between the interface and the cutting layer, resulting in partial transfer. gcut > gbond gcut ~ gbond gcut < gbond Cho et al, J. Phys. Lett., 92, 5980 (2003) Full transfer requires a full strength at the bonding interface layer Non-uniform bonding induces partial transfer FLCC Seminar 11/8/04

9 Advantages of Layer Transfer Approach
Donor wafer can be recycled Transferred thickness and buried oxide thickness are independently controlled (100), (110), and (111) Epi layers can be transferred Multi-stack structures can be achieved with various epi and transfer combinations FLCC Seminar 11/8/04

10 Some state-of-the-art results
GeOI SSOI SOI Sigen Canon Amberwave 300mm SOI 50nm Si range=1.2nm FLCC Seminar 11/8/04 SOITEC

11 Why transfer of Epi Donor Wafers ?
For SOI, Epi Si has less COP defects than bulk Si For GeOI, no 300mm Ge bulk wafers yet For s-SOI and SGOI , layer formed epitaxially on SiGe buffer layers FLCC Seminar 11/8/04

12 Plasma Activated Bonding
Surface damage 1-2 nm O H Si rich surface Defect layer High mobility of H2O High coverage of OH PLASMA FLCC Seminar 11/8/04

13 Annealing Temperature (oC)
Si/SiO2 Bonding Energy vs. Temperature Hydrophobic Si Hydrophilic Si 3000 2500 2000 1500 1000 500 100 200 300 400 600 700 800 900 Bonding Energy (mJ/m2) Annealing Temperature (oC) O2 plasma Si (100) Fracture Strength Cho et al, UCB, 2000. FLCC Seminar 11/8/04

14 Requirements for Direct Bonding
Surface micro-roughness ~ nm No macroscopic wafer warpage Minimal particle density and size - “soft “ particle size < 0.2 um *Deposited films will need CMP FLCC Seminar 11/8/04

15 Surface Smoothing by Hydrogen anneal
As-split surface Surface Smoothing by Hydrogen anneal After-anneal surface FLCC Seminar 11/8/04

16 Hydrogen Induced NanoCleave Thermal Separation rms ~ 0.8nm
Current et al, European Semiconductor, Feb 2000 FLCC Seminar 11/8/04

17 Ultra-Thin (<1KÅ tSOI) Non-Uniformity
10 20 30 40 50 60 70 80 100 200 300 400 500 600 700 800 Ultra-Thin SOI Layer Thickness Range (Max-Min) (Å) Device Layer Thickness (Å) 10% Typical Range <25Å Early 2002 Late 2002 Range Now Independent of Layer Thickness FLCC Seminar 11/8/04

18 Ge/Si3N4/Si and Ge/SiO2/Si substrates by ion-cut
Size: 1x1 cm2 Ge donor wafer Si Substrate Implanted Hydrogen GeOI Ge/Si3N4/Si Fabrication Method Processing Temp (ºC) Transfer thickness (nm) Mobility (as-cut) cm2/V-sec Bulk =300 cm2/V-sec Ge/Si3N4/Si Mechanical Ion Cut 205 439 240 Thermal Ion Cut ~250 450 280 Ge/SiO2/Si ~270 410 252 FLCC Seminar 11/8/04

19 (Size of AFM images: 5x5µm2; H ion dose: 6x1016 /cm2.)
Ge/Si3N4/Si surface roughness by AFM (b)GeOI by thermal cut; Tcut=360°C, RMS: 20.5nm (a)GeOI by mechanical cut; Tanneal=205°C, RMS: 17.5nm (Size of AFM images: 5x5µm2; H ion dose: 6x1016 /cm2.) GeOI transfer surface roughness > SOI transfer surface roughness (RMS<7 nm) Post-transfer smoothing is required FLCC Seminar 11/8/04

20 Ge/SiO2/Si by thermal ion-cut
Transferred Ge SiO2 400nm Ge 30nm dry ox Si substrate 100 200 300 400 Transferred Ge SiO2 Thickness,t, (nm) Scan distance (µm) Fabrication processes: Oxygen plasma activation for 15sec; Direct bonding; Post-bonding annealing: 130°C for 20h; 220 °C for 10h; Thermal-cut at T>270 °C ; FLCC Seminar 11/8/04

21 Ge/SiO2 (or Si3N4)/Si system by ion-cut shows that the cutting depth is deeper than the implantation zone ! Si Ge Vacancy Distribution Hydrogen Distribution Depth (Ǻ) Observed cutting depth SRIM2000 Ideal crack propagation Pgas Thermal Cut Mixed-mode crack propagation Pgas 1 Data is obtained in courtesy of ZhengXin Liu 2 FLCC Seminar 11/8/04

22 What controls Transfer Layer thickness ?
SOI: H+ 175 keV 5.0 x 1016 cm-2 600C Ion-cut location 19 nm 10 nm 29 nm [100] [011] Höchbauer et al, J. Appl. Phys, 89, 5980 (2001) The out of plane tensile strain enhances the development of H platelets parallel to the surface and therefore makes the location of highest ion implantation damage more favorable for platelets to nucleate. This out of plane tensile strain facilitates the occupation of the bond-centered sites between two neighboring silicon atoms by hydrogen atoms, thus forming (100) hydrogen planes parallel to the surface. The first example is done by Hochbauer and his colleague. These figures show a XTEM image of the as-implanted state and after ion cutting was performed of the samples implanted with H+ at 175keV. The image shows a 0.29um broad region of dense damage . The densely damaged region of the implantation zone is composed mainly of planar defects. The majority of these defects are aligned parallel to the surface in the 100 plane. A small amounts of the platelets lie in the 111 planes and appear throughout the region where the 100 platelets appear. The image of the newly cleaved surfaces on the right handed side clearly show that the ion-cut does not take place in the center of the platelet distribution but does occur at shallower depths. The thickness of the platelet damage region in the donor wafer is nearly twice as thick as the exfoliated layer. The graph of the far right handed side show the surface blister depth as a function of the dopant level. These observations showed that the ion-cut takes place in the region of maximum damage or slightly shallower. During annealing hydrogen out diffusion in the near surface side of the hydrogen implantation distribution occurs, causing a shift of the hydrogen concentration peak towards a larger depth. Expectedly, the recovery of implantation damage in the shallow part of the damage zone combined with the formation of hydrogen defect complexes causes the damage peak to shift towards a larger depth. Note: Because of H2+ fragmentation at the surface this implantation process is equal to the implantation of atomic H ions at half energy and twice dose. Ion-cut take place at shallower depth than the center of the hydrogen platelet distribution The ion-cut location is found to occur at the depth of maximum damage. FLCC Seminar 11/8/04

23 Transfer thickness of Ion-Cut is different with substrate stress
Implanted Si (100) H+ 8  1016 cm-2 28 keV Transferred layer of implanted Si is thicker than non-implanted Si Si (100) Small Area Transfer Large Area Transfer Donor Si Glass Donor Si Glass H Peak Implanted Si (100) Thickness Non-implanted Si (100) thickness Si Thickness (m) Transferred Si Looking at the separation mode of nonimplanted and implanted samples, it was observed that ion implantation enhance the separation mode such that large area Si layer transfer could be achieved. The important observation is that the thermal stress effect is larger than the implantation effect such that the cutting is occur due to the thermal stress. However, the implantation effect enhance the separation mode to be confined in the si layer. In addition, the thickness measurement shows that the transferred layer is thicker than non implanted transfer layer. Transferred Si Top views of transferred layers Distance (m) FLCC Seminar 11/8/04

24 Thickness Measurement Data Stress Measurement Data
Transfer Thickness versus Implantation Dose Thickness Measurement Data Stress Measurement Data Transferred Thickness (nm) 2 4 6 1 3 5 100 200 300 H Peak Damage Peak Implantation dose  1016 cm-2 Implantation at 40 keV -100 σo SU-8 -200 -300 Si Compressive Stress (Mpa) -400 -500 -600 -700 -800 2 4 6 Ion implantation introduces defects and damage into Si, which causes expansion of the Si lattice in the implanted region. As a result compressive stress is induced and the wafer develops a curvature. The stress measurement in the dose range between 2 and 6 x10^16 cm-2 were made. This figure shows the measured average stress at the implantation damage as a function of H dose. In all cases compressive stress were observed to incrase with increasing implantation dose. Compressive stress induces further crack surface displacement due to stress-induced moment, casuing the crack plane to become further away than the prediction based only on the P and M due to thermal stress. The analysis needed to quantify the implied effect is still under progress. Implantation dose  1016 cm-2 Transferred thickness is a function of ion implantation dose Compressive stress induced by implantation was determined by measuring wafer curvature Compressive stress leads to additional shear forces at the crack tip FLCC Seminar 11/8/04

25 Layer Transfer Without Hydrogen Implantation
SiO2 Si (100) Si (111) Crack tends to propagate into brittle substrate Crack propagation driving force is inversely proportional to fracture toughness of materials. Si(100)1: 0.91 MPam1/2 Si(111)1: 0.82 MPam1/2 SiO22: MPam1/2 Non-uniform thermal stress result in non-uniformity of the transferred surface Full Transfer Partial Transfer Transfer Donor Si Donor Si Donor Si SiO2 SU-8 Glass Glass Glass Original donor wafer The followings are digital images of the experimental results observed at the top surface. It was found that the thermal stress in the substrate induced by thermal mismatch in additional to the elastic mismatch could induce the crack propagation underneath the interface layer into the substrate. Amazingly is that for oxide grown on si the cutting occurs within oxide layer. Comparing the separation modes between 3 different samples. The cleavage in oxide render the full transfer in which the crack propagate entirely in the oxide layer. On the other hand in Si samples crack propagate along the interface, with partial derailing into the si, rendering the partial transfer of si onto glass substrate. Si111 offers larger area transfer than si The separation mode could be explain by the driving force available for crack propagation. Oxide with the lowest toughness and hence exhibit the most relaxation by crack growth. This is consistent with the observation that the entire crack path in oxide occrs at the oxide layer. In Si, however the stress relaxation by crack propagation is slightly more difficult to fulfill due to its higher fracture toughness and hence the crack may partially derail to the bonding interface. Any non-uniformity of the transferred surface is expected to be a possible non uniformity of the thermal stress in the su-8. SiO2 on SU-8 SU-8 SU-8 1 cm Transferred Si Top views of transferred layers 1Chen et al., American Ceramic Society Bulletin, 59, 469 (1980) 2Lucas et al., Scripta Metallurgica et Materialia, 32, 743 (1995) FLCC Seminar 11/8/04

26 Layer Transfer – A Mechanical Fracture Perspective
Stress intensity factors Effect of KII on KI crack propagation loading Opening mode, KI Out-of-plane shear mode, KIII Shear mode, KII KII = 0 KII > 0 KII < 0 To understand the crack propagation direction into Si, I’d like to introduce a little bit of background on brittle fracture. In the brittle fracture of materials with existing cracks, stress intensity factor is an important fracture mechanics parameter used to predict the stress field around the crack tip. When the stress state near the crack tip reaches the fracture toughness of the material the crack grows. Generally there are three modes to describe different crack surface displacement. Mode I or opening or tensile mode where the crack surfaces move directly apart is the most common load type encountered in engineering design. In practical applications, however, surface cracks under mixed mode conditions can be encountered frequently. Cracks may experience mixed mode loading due to mainly two factors. Under mixed mode loading, the crack driving force would be greater in a direction other than along the plane of the interface and there is a possibility that the crack may be forced off the interface. Mixed remote loading i.e. normal and shear remote loads acting on a component having perpendicular crack to the normal loading. And 2) mechanical and/or thermal loads combined with arbitrary restraint conditions. Restraint conditions include a modulus mismatch which changes the nature of the stress singularity at the crack tip both for interface cracks and for cracks terminating at an interface. This elastic mismatch causes slight differences in how quickly the crack reaches steady-state with regards to depth. As shown in this figure, the crack exists along the plane of weakness with both normal and shear stresses acting at the crack tip. There would be two directions in which a surface crack may propagate when it reaches an weakness plane. Depending upon the stress level and the relative strengths of weakness plane (e.g. interface) and matrix, the crack may either propagate under pure mode I loading into the substrate or it may kink and grow parallel to the interface. In other word, shear stress component could divert the crack path out of the weakness plane. In an isotropic material or in a system with symmetric stress field around the crack tip, a crack propagates along a path that locally maximizes the energy release rate KII = 0. In contrast when a crack is subject to some shear. One consequence of this is that the crack driving force is greater in a direction other than along the original plane and there is a possibility that the crack may be forced off the weak plane. The direction in which the crack leaves the interface depends on the sign of KII. A negative value of KII causes the crack to kink into the film whereas a positive value of KII causes the crack to kink into the substrate. As a result, an interfacial crack propagating under a film subjected to a residual tensile stress may crack the substrate. Desired condition for uniform layer transfer FLCC Seminar 11/8/04

27 Transferred Thickness Experimental Data vs. Analytical Data
Analytical Model 250 Analytical SU-8/Si (111) SU-8/Si (100) SU-8/SiO2 0.005 1/Ecleaved material Transferred Thickness (nm) 0.020 M h Film d P h λh Neutral plane Substrate Substrate λ KII = 0 Σ = Efilm/Esubstrate Ki – Stress intensity factor (mode i) o – Thermal stress I – Moment of inertia of the transferred beam Model by Drory et al, Acta Metall., 36, 2019 (1988) FLCC Seminar 11/8/04

28 Derailing Mechanism of Mixed-Mode Crack Propagation
Map of Failure Mechanism Mixed-Mode Crack Propagation 1.0 Substrate cracking Substrate σo Film h d λh No cracking Steady state cracking Partial cracking Normalized interfacial toughness, i2/o2h 0.5 Interfacial delamination Stress intensity factor of the kink crack inclined at  to the main crack 1 2 Normalized substrate toughness, S2/o2h Where kI and kII are the stress intensity factors acting on the main crack and, Interfacial delamination Partial substrate cracking Steady state substrate cracking The derailing mechanism could be understood as the following. If the crack surface is under the mixed mode, cracks would have 2 possibility to propagate, either continue propagate along its own weakness plane or extend out of this plane into the matrix. This depends on the relative fracture toughness of interface and the matrix the crack propagate into. The right-handed figure illustrates the map of failure mechanism. If the interface is very weak, crack is likely to remain in the interface. However, if the substrate is brittle or fracture toughness is low, crack is likely to extend into the substrate. When the driving force for crack propagation is large or when the stress intensity at the crack tip is large compared to the fracture toughness, catastrophic substrate cracking could occurs until the crack reaches the steady state depth where KII = 0, resulting in large area transfer. Under certain conditions the crack can be drawn out of the interface and arrest within the substrate, rendering partial cracking of substrate. Based on this map and the fact that the fracture resistance or toughness is proportional to surface energy of the fracture surfaces, the sio2 with lowest surface energy exhibit the most relaxation of the stress by crack growth. This is consistent with the observation that the entire crack path in SiO2 occurs at the cutting interface. In Si substrates, however, the stress relaxation by crack propagation is slightly more difficult to fulfill due to its higher surface energies, and hence the cracks may partially derail to the bonding interface. FLCC Seminar 11/8/04 Thouless et al. (1991)

29 Direct Stress σxx (MPa)
Thermal Stress Simulation of Ge-SiO2-Si systems by finite element analysis y (800 °C) Annealing Ge (100 nm) Ge SiO2 (100 nm) SiO2 x Si (500 μm) Si SiO2 Ge Annealing T (ºC) Direct Stress σxx (MPa) Ge layer has a tendency to buckle due to the compressive direct stress. The interfacial stress may exceed the fracture tensile strength of SiO2 (approximately 110MPa) FLCC Seminar 11/8/04

30 Max Direct Stress σxx (MPa)
Thermal Stress Simulation of Patterned Ge-SiO2-Si systems by finite element analysis y x Si (500 μm) Ge (50 nm) SiO2 (100 nm) 50 nm 300ºC annealing 500ºC annealing 800ºC annealing Max Direct Stress σxx (MPa) 50nm w (nm) Compressive Tensile SiO2 Si Ge 500ºC Direct Stress σxx distribution after annealing FLCC Seminar 11/8/04

31 Impact of Fin Orientation
Source: Professor T-J. King (UCB) Electron Mobility Hole (110) S D PMOS (100) NMOS FLCC Seminar 11/8/04

32 Manufacturing Equipment Issues
High-throughput, low-cost Epi Reactors CMP or smoothing of SiGe and s-Si High Current Hydrogen implanters Plasma Activated Bonders Mechanical Delamination Machines Plasma Bonder Gas Jet Delamination H+ Plasma Implanter FLCC Seminar 11/8/04

33 Summary Ultra-thin (<10nm) SOI, GeOI, and s-SOI pose new challenges to meet stringent uniformity and roughness specifications Mechanical stress distribution (bonding-induced and implantation-induced) are key factors in transfer thickness control. Improved process recipes are needed to ensure thermal stability of sSOI and GeOI structures Challenges for process control , metrology, and low-cost manufacturability FLCC Seminar 11/8/04


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