1. Thin Film Deposition

2. 고려대학교 MNB Sensor Lab 61

3. 고려대학교 MNB Sensor Lab 62  Thin Films Used in the Fabrication of IC and MEMS Devices A large variety of thin films are used in VLSI and MEMS device fabrication Thin Film Deposition

4. 고려대학교 MNB Sensor Lab 63 Thin Film Deposition Physical Vapor Deposition (PVD) >> Vacuum evaporation >> Sputtering >> Reactive PVD  Types of Depositions -The majority of metal films used in integrated circuits are formed by physical vapor deposition(PVD). -PVD methods use mainly physical processes(either evaporation or sputtering) to deposit the films. -Films is formed by atoms directly transported from source to the substrate through gas phase. >> Vacuum evaporation >> Sputtering >> Reactive PVD

5. 고려대학교 MNB Sensor Lab 64 Chemical Vapor Deposition (CVD) >> Atmospheric pressure chemical vapor deposition (APCVD) >> Low pressure chemical vapor deposition (LPCVD) >> Metal-organic chemical vapor deposition (MOCVD) >> Chemical vapor epitaxy (CVE) >> Plasma enhanced chemical vapor deposition (PECVD) >> Photo-induced chemical vapor deposition (PHCVD) - Chemical vapor deposition is the process in which a film is deposited by a chemical reaction or decomposition of a gas mixture at elevated temperature on the surface of substrate or in its vicinity.

6. Thermal Oxidation

7. 66 Korea University MNB Sensor Lab Basic Bonding Unit Amorphous silica Quartz crystal lattice  Oxide Structure Silicon Dioxide (SiO 2 )

8. 67 Korea University MNB Sensor Lab - Surface passivation (final covering of all exposed areas of silicon). - Dopant barrier (dopant masks) - Device dielectric(gate oxide, field oxide, dielectric layer in capacitors.) Silicon substrate SiO 2  Silicon Dioxide Layer Uses

9. 68 Korea University MNB Sensor Lab Thermal oxidation is performed by subjecting the wafer to an oxidizing ambient(dry oxygen or water vapor) at elevated temperature. Diffusion Reaction O 2, H 2 O SiO 2 Silicon Oxidation Reaction

10. 69 Korea University MNB Sensor Lab - Best quality oxide, stoichiometric, high density, pinhole free. - Growth rate : very slow Thermal Oxidation  Dry Oxidation - Less dense, poorer quality oxide layer - Growth rate : much greater  Wet Oxidation

11. 70 Korea University MNB Sensor Lab Thermal Oxidation  Examples 90nm generation gate oxide (Intel) Side view of the a-SiO 2 /c-Si(001) interface structure, the arrows indicate the locations of the Si-O-Si bridges.

12. 71 Korea University MNB Sensor Lab Physical Vapor Deposition

13. 72 Korea University MNB Sensor Lab -Source material evaporates -Evaporant vapor transports to and impinges on the surface of the substrate -Evaporant condenses on and is adsorbed by the surface Thermal Evaporator  Vacuum Evaporation  Mass Deposition Rate per unit area of source surface: Maximum R m reaches at high chamber vacuum (P ~ 0)

14. 73 Korea University MNB Sensor Lab Spherical surface with source on its edge  Uniform Coating -Angle Independent –uniform coating ! -Used to coat instruments with spherical surfaces Uniformity on a Flat Surface It is typical to double this number to give some process margin

15. 74 Korea University MNB Sensor Lab Evaporation methods require a clean, high-vacuum environment so that species ejected from the sources are not scattered by gaseous molecules in their path. Larger r means  Evaporation System -bigger chamber -higher capacity vacuum pump -lower deposition rate -higher evaporant waste Container material also evaporates, which contaminates the deposited film

16. 75 Korea University MNB Sensor Lab Very low container contamination  Electron-beam evaporation

17. 76 Korea University MNB Sensor Lab PVD - Comparison

18. 77 Korea University MNB Sensor Lab DC-powered sputter deposition equipment A chamber filled with inert gas (Ar) DC voltage (~ kV) is applied to the diode Free electron in the chamber are accelerated by the electric field These energetic free electrons inelastically collide with Ar atoms. -Excitation of Ar → gas glows -Ionization of Ar → Ar + + 2 nd electron -2 nd electrons repeat above process >> gas breakdown >> discharge glow (plasma) Plasma An ionized gas consisting of a chemical soup of many types of species- positive and negative ions, electrons, neutrals, atoms, molecules, clusters, etc.  Glow discharge - plasma Sputter Deposition

19. 78 Korea University MNB Sensor Lab  Self-Sustained Discharge Near the cathode, electrons move much faster than ions because of smaller mass -Positive charge build up near the cathode, raising the potential of plasma -Less electrons collide with Ar -Few collision with these high energetic electrons results in mostly ionization, rather than excitation -Dark zone (Crookes Dark Space or Cathode dark space ) Cathode sheath voltage drop

20. 79 Korea University MNB Sensor Lab Crookes dark space Condition for Sustain Plasma : L Film

21. 80 Korea University MNB Sensor Lab Sputter deposition is a process in which surface atoms and/or molecules of a material (target) are physically ejected by energetic bombarding from glow discharge or ion beam in a momentum transfer process. The ejected atoms and/or molecules are then deposited on the surface of a different material (substrate) and condensed into solid thin films.  Sputtering Process In fact, sputtering deposition rate R : -Large LP to sustain plasma -Small LP to maintain good deposition rate and reduce random scattering

22. 81 Korea University MNB Sensor Lab DC-powered sputter deposition equipment plasma  DC Diode Sputtering Deposition

23. 82 Korea University MNB Sensor Lab  DC Magnetron Sputtering Using low chamber pressure to maintain high deposition rate. Using magnetic field to confine electrons near the target to sustain plasma. Apply magnetic field parallel to the cathode surface. Electrons will hope (cycloid) near the surface (trapped) Hoping radius r : V d : voltage drop across dark space (~ 100 V) B : Magnetic field (~ 100 G) For electron r ~ 0.3 cm For Ar+ ion r ~ 81 cm

24. 83 Korea University MNB Sensor Lab Current density (proportional to ionization rate) increases by 100 times Required discharge pressure drops 100 times Deposition rate increases 100 times As a result,

25. 84 Korea University MNB Sensor Lab  Problems of DC Sputtering DC sputtering will only work if a DC current can flow. This is no problem if the target and the substrate are both conducting. If one is not - usually the target such as SiO2 - there is a problem! The solution is to quickly reverse the polarity before the positive ions hitting the insulating target generate a positive repulsive charge! Upon polarity reversal, electrons will hit the target and neutralize any previous charge ! DC sputtering cannot be used for depositing dielectrics because insulating cathode will cause charge build up during Ar + bombarding → reduce the voltage between electrodes → discharge distinguishes

26. 85 Korea University MNB Sensor Lab  RF (Radio Frequency) Sputtering At low frequency (< 100 KHz), both electrons and ions can follow the switching of the voltage → DC sputtering At high frequency (> 1 MHz), heavy ions cannot no long follow the switching → ions are accelerated by dark-space (sheath) voltage → electron neutralizes the positive charge buildup on both electrodes. However, there are two dark spaces : → sputter both target and substrate at different cycle The ion of the plasma (e.g. Ar) knocks off a “target” atom or molecule (e.g. SiO 2 ) that deposits on the substrate. Its energy can be up to several hundreds eV! Once the RF reverses polarity, plasma ions will strike the substrate! This ion has a high probability of knocking off the deposited atom. If the system is symmetric the two effects will cancel!

27. 86 Korea University MNB Sensor Lab  Electrode-Size Effects Voltage across target electrode V T : voltage across target sheath V s : voltage across substrate sheath A T : area of target electrode A s : area of substrate electrode Larger dark-space voltage develops at the electrode with smaller area → make target electrode small It minimizes ion bombardment of grounded fixtures.

28. 87 Korea University MNB Sensor Lab RF-powered sputter deposition equipment  RF Sputtering System

29. 88 Korea University MNB Sensor Lab Comparison

30. Chemical Vapor Deposition

31. 90 Korea University MNB Sensor Lab Chemical Vapor Deposition (CVD) Deposit film through chemical reaction and surface absorption (1) Vaporization and transport of reactants into reactor. (2) Transport of reactants by diffusion from the main gas stream through the boundary layer to the substrate surface. (3) Adsorption of reactants on the surface. CVD film growth steps

32. 91 Korea University MNB Sensor Lab (4) Surface processes, including chemical decomposition or reaction, surface migration to attachment sites, site incorporation, and other surface reactions. (5) Desorption of byproducts from the surface. (6) Transport of byproducts by diffusion through the boundary layer and back to the main gas stream. (7) Transport of byproducts by forced convection away from the deposition region.

33. 92 Korea University MNB Sensor Lab APCVD Hot-wall LPCVD  CVD Reactor Types Cold-wall reactor (vertical). PECVD

34. 93 Korea University MNB Sensor Lab LP CVD Thermal energy for reaction activation System works at vacuum (~ 0.1 –1.0 torr), resulting in high diffusivity of reactants → reaction-rate limited. Wafers can stacked closely without lose uniformity as long as they have the same temperature. Temperature is controlled around 600 - 900ºC by “flat” temperature zone through using multiple heaters. Low gas pressure reduces gas-phase reaction which causes particle cluster that contaminants the wafer and system.

35. 94 Korea University MNB Sensor Lab Use rf-induced plasma (as in sputtering case) to transfer energy into the reactant gases, forming radicals (decomposition). Low temperature process (< 300 ºC). For depositing film on metals and other materials that cannot sustain high temperature. Surface reaction limited deposition; substrate temperature control (typically cooling) is important to ensure uniformity. PE CVD

36. 95 Korea University MNB Sensor Lab Types of CVD Reactions

37. 96 Korea University MNB Sensor Lab

38. 97 Korea University MNB Sensor Lab

39. 98 Korea University MNB Sensor Lab Quality : Physical and chemical properties - Composition (stoichiometry) - Film density, pinhole density - Contamination levels, Impurity level - Grain size, boundary property, and orientation - Defect density - Mechanical properties : stress, adhesion - Electrical properties : breakdown voltage, conductivity (resistivity) Uniformity : - Across wafer uniformity - From wafer to wafer uniformity - Run-to-run uniformity Homogeneity : - The composition of the film - Films with density variation can be also termed nonhomogeous. Several Issues Related to Thin Films and their Deposition

40. 99 Korea University MNB Sensor Lab Conformity : - Uniform thickness as the film crosses nonplanar topography. - A conformal coating covers every surface to the same thickness. Step coverage of metal over nonplanar topography (a)Conformal (i.e. ideal) step coverage (b) poor step coverage

41. 100 Korea University MNB Sensor Lab Filling Thin film filling issues Examples of poor filling and coverage - Aspect ratio(AR) : A deep, narrow contact hole would have a large aspect ratio and would be harder to fill. Deposition Directionality Directional : good for lift-off, trench filling Non-directional : good for step coverage

42. 101 Korea University MNB Sensor Lab Adhesion - How well the film sticks to the surface Stress - The film possesses tensile stress : causes the surface of the substrate to bend upward. - The film possesses compressive stress : causes the surface of the substrate to bend downward.

43. 102 Korea University MNB Sensor Lab Characteristics and Applications of CVD reactors

44. 103 Korea University MNB Sensor Lab Common Deposition Methods for Thin Films in IC and MEMS fabrication

45. 104 Korea University MNB Sensor Lab

46. 105 Korea University MNB Sensor Lab - Epitaxy is the growth of single-crystal semiconductor layers upon a single- crystal semiconductor substrate. - The word is coined from the Greek words epi(meaning "on") and axis(meaning "ordered or arrangement". - Epitaxy(or "epi") thus offers a way of adding material without terminating the single-crystal structure of the substrate. Epitaxy

47. 106 Korea University MNB Sensor Lab Vapor phase epitaxy (VPE) : a special case of CVD in which the film is deposited epitaxially. SiCl 4 + 2H 2 Si + 4HCl SiH 4 Si + 2H 2 Horizontal pancake barrel susceptors

48. 107 Korea University MNB Sensor Lab Comparison of Typical Thin Film Deposition Technology