Infrared transmission spectroscopy of direct bonded Si compound optics

Immersion gratings are made from monolithic hockey-puck sized Silicon boules. The massive size presents challenges when using conventional semiconductor equipment designed for DVD-sized Si wafers. An alternative to monolithic samples is to bond Si wafers directly to larger substrates. This presentation gives the history of Si direct bonding, and concludes with the introduction of a new optical technique to evaluate the quality of the bond interface.

Transcript

1. bonded Si optics gully monday, november 11, 2013
2. Key ideas History of Si bonding Literature How to directly bond Si-Si How to measure bonded Si optics – IR imaging – Cary5000 – Matrix method
3. Shimbo+ 1986 Stengl+ 1988 Lehmann+ 1989 Mitani+ 1990 Mitani & Göselle 1992 Feijoó+ 1994 Himi+ 1994 Reiche+ 1995 Göselle+ 1995 Göselle+ 1995 Reading the literature offers guidance on how to bond silicon pucks together.
4. Takagi+ 1996 Reiche+ 1996 Takagi+ 1998 Han+ 2000 Gracias+ 2001 Greco+ 2001 Litton+ 2001 Haisma+ 2007 Göselle & Tong 1992 Reading the literature offers guidance on how to bond silicon pucks together.
5. Bäcklund, Ljungberg, Söderbärg 1992 Journal of Micromechanical Engineering A suggested mechanism for silicon direct bonding from studying hydrophilic and hydrophobic surfaces They measure the bond energy in Joules per square meter (J/m2). The bond energy measurement is from “crack propagation”, W.P Maszara et al. 1998, Journal of Applied Physics. They find bond strength is: greater for hydrophilic Si for annealing 21 < T (°C) < 400 greater for hydrophobic Si for annealing T (°C) > 400 Hydrophilic bonds at room temperature are “reversible” The authors explain the physical processes going on.
6. Mitani and Gösele 1992 Journal of Electronic Materials Wafer BondingTechnology for Silicon-on-Insulator Applications: A Review An unconventional use of wafer bonding is the reversible room temperature bonding for the protection of polished wafer surfaces against organic and particle contamination. Annealing at T (°C) > 800.
7. For effective particle prevention, the initial bonding has to be performed in a clean room of class one or better quality right after chemical cleaning. Trapping of air can be prevented by propagating the bonding area radially from just one chosen contact point… …the most problems in wafer bonding is … particle-free bonding. This set-up allows wafer bonding without bubbles in a normal non-cleanroom laboratory environment by removing local particles between two silicon wafers, facing each other at a distance of about 1 mm, by deionized (DI) water flushing. The procedure of wafer bonding with using this micro-cleanroom setup completely eliminates particles and maintains particle-free surfaces once the wafers are cleaned and free from particles. So far, particle-free wafer bonding with up to 8 inch wafers has been realized in this micro-cleanroom setup. Mitani and Gösele 1992 cont…
8. 1 erg/cm2 = 0.001 J/m2 Mitani and Gösele 1992 cont…
9. The bubbles tend to disappear if the annealing temperature is high enough. Lots of discussion on the origin of bubbles: water? hydrocarbons? Mitani and Gösele 1992 cont…
10. Mitani and Gösele 1992 cont… FZ wafers are better than CZ wafers, because oxygen diffuses into the Si in the FZ wafers
11. Mitani, Lehmann, Gosele 1990 IEEE Solid-state sensor workshop Bubble formation during silicon wafer bonding: causes and remedies
12. Mitani, Lehmann, Gosele 1990
14. Hydrophilic bond chemistry Hydrophobic bond chemistry
15. High Resolution Electron Microscopy Of the bond interface It’s about 3 nm of Si-O-Si bonds Recall, this is after 1100 °C annealing The hydrophobic case has Si – F H – Si bonds, But occasionally has ‘oxide islands’ Anyways, 3 nm of bond is negligible on our optical performance. Their MIRS technique raises the question of whether in double pass the column is enough to introduce subtle spectral features. I think ‘no’. (they have ~ 18 passes)
17. UT-JPL bonded Si optics 2012-2013
18. Experiments so far
19. Ash 4.0 µm 0.0 1.0 2.0 3.0 4.0 VG03 1. 2. VG03
21. Experiments so far
36. Metrology for small gaps Cary5000 and Matrix method
37. Example 1: Double Side Polished (DSP), thick Si wafer at normal incidence •  Zeroth order assumption: •  A few subtle effects to consider: 1)  Reflection is a function of nSi , which is a modest function of λ and temp: nSi =n(λ, T) 2)  There will be multiple reflections within the substrate, since reflection loss is multiply retroreflected in the propagation direction 3)  λ << d Incoherent 4)  Absorption per unit length n Si ~ 3.48 at room temp R = (n −1)2 (n +1)2 ≈ 0.31 € T net = T2 = (1− R) × (1− R) ≈ 0.48 Si Air Air
38. 1000 1200 1400 1600 1800 2000 2200 2400 h (nm) 0.0 0.2 0.4 0.6 0.8 1.0 Relative Transmission Example 1: Double Side Polished (DSP), thick Si wafer at normal incidence Absorption sets in at λ = 1150 nm The red line is the measurement. It agrees very well with the prediction for multiple reflections within the substrate, shown in black - - - - Zeroth order only: Tnet ~ T2 Multiple reflections are a ~10% effect! Must include up to 3rd order terms!
39. 1)  The etalon is characterized by the Fresnel reflectivity of its sidewalls, and the gap size 2)  The reflectivity is encapsulated in the coefficient of finesse, FSi-Air ~2.5 3)  Multiple reflections within the substrate will interfere to make a λdependent Transmission spectrum Te (λ) 4)  d ~ λ Coherent 5)  Lossless: Re (λ) = 1 - Te (λ) Example 2: Fabry Pèrot etalon Air gap immersed in Si, normal incidence Re (λ) Air Si Si Te (λ) d
40. 1)  There will be dozens of permutations of reflections and transmission 2)  The intermediate interface can be treated as a black box with a etalon reflection and transmission: Re (λ) and Te (λ) 3)  The interface walls can be treated as a black box with Fresnel reflection and transmission: Rn (λ) and Tn (λ) 4)  Then apply wave transfer matrix technique Example 3: Fabry Pèrot etalon Air gap immersed in Si, normal incidence, with incoherent multiple reflections from Si puck walls Re (λ) Te (λ) Rn (λ) Tn (λ)
41. Example 3: Fabry Pèrot etalon Air gap immersed in Si, normal incidence, with incoherent multiple reflections from Si puck walls 2007 Saleh and Teich Fundamentals of Photonics
42. Example 3: Fabry Pèrot etalon Air gap immersed in Si, normal incidence, with incoherent multiple reflections from Si puck walls Re (λ) Te (λ) Rn (λ) Tn (λ) Rn (λ) Tn (λ) Rn (λ) Tn (λ) Rn (λ) Tn (λ) Example 1 revisited: DSP thick Si puck at normal incidence ** n = nSi = n(λ)
43. 1000 1200 1400 1600 1800 2000 2200 2400 Λ ￿nm￿ 0.1 0.2 0.3 0.4 0.5 0.6 Trans Model Fabry Perot Transmission; 3960 nm Air gap Part VG03 measured Transfer Matrix d￿3960 nm DSP Si measured DSP Si prediction
44. VG15 9/16/2013 1200 1400 1600 1800 2000 2200 2400 h (nm) 0.80 0.85 0.90 0.95 1.00 Transmission Measurement +/ï 0.3% Prediction VG09 9/16/2013 1200 1400 1600 1800 2000 2200 2400 h (nm) 0.80 0.85 0.90 0.95 1.00 Transmission Measurement +/ï 0.3% Prediction
45. VG08 9/16/2013 1200 1400 1600 1800 2000 2200 2400 h (nm) 0.80 0.85 0.90 0.95 1.00 Transmission Measurement +/ï 0.3% Prediction VG07 9/16/2013 1200 1400 1600 1800 2000 2200 2400 h (nm) 0.80 0.85 0.90 0.95 1.00 Transmission Measurement +/ï 0.3% Prediction