1. Film Deposition Haedo Jeong PG65211

2. 2 What is thin film? ♦ Thin film vs Bulk 1. 사전적 의미 - Bulk : 충분한 크기의 3 차원 체적을 가진 물체의 형태. - Bulk : 충분한 크기의 3 차원 체적을 가진 물체의 형태. - Thin film : 아주 넓은 면적을 가진데 비해 두께는 극도로 얇은 형태. - Thin film : 아주 넓은 면적을 가진데 비해 두께는 극도로 얇은 형태. 2. 차이점 : - Bulk : Bulk 모양의 단결정 고체는 각 방향으로의 기계적, 전기적 성질이 균일. - Bulk : Bulk 모양의 단결정 고체는 각 방향으로의 기계적, 전기적 성질이 균일. 표면 대 체적비가 작으므로, bulk 의 물리적 성질은 재료내부의 특성에 의해 지배적으로 결정됨. - Thin film : Bulk 와는 확연히 다른 성질을 보임. - Thin film : Bulk 와는 확연히 다른 성질을 보임. 표면 대 체적비가 아주 크므로, 물리적 성질은 재료내부보다는 표면의 특성에 의해서 지배적으로 결정됨.

3. 3 1.JIS Definition (1975) : - Thin film : 단원자 또는 단분자층에서부터 두껍게는 대략 5 ㎛ 두께까지를 thin film 으로 정의 - Thin film : 단원자 또는 단분자층에서부터 두껍게는 대략 5 ㎛ 두께까지를 thin film 으로 정의 2. KIST Definition (1991) : - Thin film : 기판층 (substrate layer) 에 형성된 수 ㎛ 이하의 두께를 갖는 것으로 독립적인 기능을 보유한 막 - Thin film : 기판층 (substrate layer) 에 형성된 수 ㎛ 이하의 두께를 갖는 것으로 독립적인 기능을 보유한 막 3. Classification of Materials : ♦ Definition of thin film PlateSheetFoil Thick Film Thin Film > 1mm 0.1~1mm 10-100  m > 5  m <5  m What is thin film?

4. 4 MEMS 소자에서 박막의 용도 What is thin film?

5. Evaporation & sputtering : low pressure CVD & epitaxy : reduced or atmospheric pressure Thin Film Deposition

6. 6.1 Evaporation Evaporation One of the oldest methods of depositing metal. Aluminum and gold are heated to the point of vaporization, and then evaporate to form a thin film Performed under vacuum to control the composition.

7. A basic vacuum deposition system A vacuum chamber, a mechanical roughing pump, a diffusion pump or turbomolecular pump, valves, vacuum gauges, and other instrumentation. The roughing valve is opened first, and the mechanical pump lowers the pressure to an intermediate vacuum level of approximately 1Pa. If a higher vacuum level is needed, the roughing valve is closed, and the foreline and high-vacuum valves are opened. the roughing pump now maintains a vacuum on the output of the diffusion pump. 6.1 Evaporation(cont.)

8. A liquid-nitrogen cold trap is used with the diffusion pump to reduce the pressure to approximately 10 -4 Pa. Ion and thermocouple gauges are used to monitor the pressure at a number of point, and several other valves are used as vents to return the system to atmospheric pressure. 6.1 Evaporation(cont.)

9. The ideal gas law : 6.1.1 Kinetic Gas Theory

10. Even a small amount of oxygen or other elements will result in formation of a contamination layer. The rate of formation of layer is determined from the impingement rate of gas molecules 6.1.1 Kinetic Gas Theory(cont.)

11. If we assume that each molecule sticks as it contacts the surface, then the time required to form a monolayer Pressure and temperature also determine another important parameter called the mean free path,. 6.1.1 Kinetic Gas Theory(cont.)

12. At P=10 -4 Pa, a 4 Å molecule has of approximately 60 m. Thus, aluminum molecules tend to travel in a straight line from the source to the target. Sputtering uses argon at a pressure of approximately 100Pa ( =60  m). Thus, the material being deposited tends to scatter and arrives at the target from random directions. 6.1.1 Kinetic Gas Theory(cont.)

13. The simplest evaporator consists of a vacuum system containing a filament that can be heated to high temperature. Small loops of a metal such as Al are hung from a filament formed of a refractory metal such as tungsten. 6.1.2 Film Evaporation

14. Increasing the temperature until Al melts and wets the filament. Filament temperature is then raised to evaporate the aluminum. The wafers are mounted near the filament and are covered by a thin film. 6.1.2 Film Evaporation(cont.)

15. Although easy to set up, contamination level can be high, particularly from the filament material. Evaporation of composite materials cannot be easily controlled. The material with the lowest melting point tends to evaporate first, and the deposited film will not have the same composition as the source. Thick films are difficult to achieve since a limited supply of material is contained in the metal loops. 6.1.2 Film Evaporation(cont.)

16. A high-intensity beam of electrons, with an energy up to 15 keV, is focused on a source target. The energy from the E-beam melts a region. Material evaporates from the source and covers the wafers. 6.1.3 Electron-Beam Evaporation

17. The growth rate using a small planar source : 6.1.3 Electron-Beam Evaporation(cont.)

18. For batch deposition, a planetary substrate holder consisting of rotating sections of a sphere is used. Each substrate is positioned tangential to the surface of the sphere. 6.1.3 Electron-Beam Evaporation(cont.)

19. The wafers are heated to improve adhesion and uniformity. The source sits in a water-cooled crucible, and its surface only comes in contact with the electron beam. Purity is controlled by the purity of the source. The relatively large size of the source provides a virtually unlimited supply of material, and the deposition rate is easily controlled by changing the current and energy of the beam. 6.1.3 Electron-Beam Evaporation(cont.)

20. One method of monitoring the deposition rate uses a quartz crystal. The resonant frequency shifts in proportion to the thickness. Accuracy of better than 1 Å/sec. Dual beam with dual targets may be used for composite materials. 6.1.3 Electron-Beam Evaporation(cont.)

21. X-ray radiation can be generated for acceleration voltage > 5 ~ 10 keV. Substrates may suffer some radiation damage from both energetic electrons and X- rays. Sputtering has replaced E-beam evaporation in many steps. 6.1.3 Electron-Beam Evaporation(cont.)

22. Because of the large, evaporation techniques tend to be directional, and shadowing and poor step coverage can occur. 6.1.5 Shadowing and Step Coverage

23. The shadowing can occur with closely spaced features. In the fully shadowed region : little deposition. In the partially shadowed region : variation in thickness. To minimize these effects, the planetary substrate holder continuously rotates the wafers. 6.1.5 Shadowing and Step Coverage(cont.)

24. 6.2 Sputtering By bombarding a target with energetic ions, typically Ar +. Atoms at the the target are knocked loose and transported to the substrate. Using DC power source : Al, W, Ti (the target acts as the cathode in a diode system.) Using RF power source: silicon dioxide, aluminum oxide (dielectrics).

25. Sputtering system 6.2 Sputtering(cont.)

26. There is a threshold energy. The sputtering yield is the ratio of the number of atoms liberated by each incident atom, and it increases rapidly with energy of the incident atoms. Usually operated to ensure a sputtering yield of at least unity. 6.2 Sputtering(cont.)

27. Alloys can be deposited (Ex : Al-Cu-Si). Results in the incorporation of some argon into the film, and heating of the substrate up to 350  C can occur. Provides excellent coverage of the sharp topologies. Sputter etching can be used to clean the substrate prior to film deposition, and often used to clean contact windows. Removes residual oxide and improves the contact. 6.2 Sputtering(cont.)

28. 6.3 Chemical Vapor Deposition By thermal decomposition or reaction of gaseous compounds. Deposited directly from the gas phase onto the substrate. Polysilicon, silicon dioxide, silicon nitride, and refractory metals such as tungsten(W). Can be performed at pressures for which the mean free path is quite small, and the use of relatively high T can result in excellent conformal step coverage.

29. A continuous atmospheric-pressure reactor (APCVD) Has been used for deposition of the silicon dioxide passivation layer as one of the last steps. 6.3.1 CVD Reactors

30. The reactant gases flow through the center and are contained by nitrogen curtains at the ends. The substrates can be fed continuously, and large-diameter wafers are easily handled. However, high gas-flow rates are required. 6.3.1 CVD Reactors(cont.)

31. The hot-wall, low pressure system (LPCVD) Polysilicon, silicon dioxide, and silicon nitride. 6.3.1 CVD Reactors(cont.)

32. The gases introduced into one end of a tube and are pumped out to the other end. 300 ~ 1150 o C, and 30 ~ 250 Pa. Excellent uniformity, and several hundred wafers in a single run. Disadvantage : the deposited film coats the tube.  Periodically cleaned or replaced. In widespread use. Vertical furnaces are also utilized. 6.3.1 CVD Reactors(cont.)

33. A plasma reactor (PECVD) Advantage : plasma permits the reaction to take place at low T. 6.3.1 CVD Reactors(cont.)

34. The wafers lie on a grounded plate, which serves as the bottom electrode. The wafers can be heated up to 400 o C using high-intensity lamps or resistance heaters. The top electrode is a second aluminum plate placed in close proximity to the wafer. Gases are introduced along the outside, flow radially, and are pumped through an exhaust in the center. An RF signal is applied to the top plate. Capacity is limited. A major problem is particulate matter that may fall from the upper plate. 6.3.1 CVD Reactors(cont.)

35. The furnace-plasma system Can handle a large number of wafers. An electrode assembly holds the wafers parallel to the gas flow. The plasma is established between alternating groups of electrodes supporting the wafers. 6.3.1 CVD Reactors(cont.)

36. Photon-enhanced CVD Optical excitation, usually with laser sources, can be used to assist or replace the thermal energy required for CVD reactions.

37. In an LPCVD system using thermal decomposition of silane: 25~100 Pa, either 100 % silane or 20 to 30 % silane diluted with nitrogen. Rate = 10 ~ 20 nm/min. @ 600 ~ 650  C 6.3.2 Polysilicon Deposition

38. Can be doped by diffusion or ion implantation or during deposition (in situ). Diborane greatly increases the deposition rate, whereas phosphine or arsine substantially reduces the rate. Diffusion occurs much more rapidly in polysilicon than in single-crystal silicon, and the polysilicon is typically saturated with the dopant to achieve as low a resistivity as possible for interconnection (0.01~0.001  ·cm). Ion-implanted polysilicon exhibits a resistivity about ten times higher. 6.3.2 Polysilicon Deposition(cont.)

39. Can be doped or undoped. Phosphorus-doped oxide can be used as a passivation layer or as the insulating medium in multilevel metal processes. SiO 2 containing 6 to 8 % phosphorus will soften and flow between 1000 and 1100  C. 6.3.3 Silicon Dioxide Deposition

40. This “P-glass reflow” process is used to improve step coverage. SiO 2 with lower concentrations of phosphorus will not reflow properly, and higher concentrations can corrode aluminum if moisture is present. Oxide doped with 5 to 15% of various dopants can be used as a diffusion source. 6.3.3 Silicon Dioxide Deposition(cont.)

41. Deposition of SiO 2 over Al must occur below the eutectic point of 577  C. The oxide may be doped with phosphorus using phosphine : 6.3.3 Silicon Dioxide Deposition(cont.)

42. Oxide passivation layers can be deposited by APCVD or LPCVD. Deposition of SiO 2 prior to metallization can be performed at higher T  a wider choice of reactions  better uniformity and step coverage : 6.3.3 Silicon Dioxide Deposition(cont.)

43. Decomposition of the vapor produced from a liquid source, tetraethylorthosilicate(TEOS). Deposition based on TEOS provides excellent uniformity and step coverage. 6.3.3 Silicon Dioxide Deposition(cont.)

44. Oxide doping may be accomplished in the LPCVD systems by adding phosphine, arsine, or diborane. Comparison of various CVD oxides 6.3.3 Silicon Dioxide Deposition(cont.)

45.  An oxidation mask in recessed oxide processes.  A final passivation layer because it provides an excellent barrier to both moisture and sodium. Composite films of oxide and nitride are being investigated for very thin gate insulators, and they are also used in electrically programmable memory. 6.3.4 Silicon Nitride Deposition

46. Thermal growth is possible. Silicon is exposed to ammonia at temperatures between 1000 and 1100  C, but growth rate is very low. ( 700 ~ 900  C, AP ) ( 700 ~ 800  C, LPCVD) 6.3.4 Silicon Nitride Deposition(cont.)

47. Plasma systems may be used. Silane will react with a nitrogen discharge to form plasma nitride(SiN): Silane reacts with ammonia in an argon plasma: LPCVD films are hydrogen-rich, containing up to 8 % hydrogen. Plasma deposition does not produce stoichiometric silicon nitride films, containing as much as 20 to 25 % hydrogen. 6.3.4 Silicon Nitride Deposition(cont.)

48. LPCVD films have high tensile stresses, and films thicker than 2000 Å may crack. Plasma-deposited films have much lower tensile stresses. The resistivity (10 16  ·cm) and dielectric strength (10 MV/cm) of the LPCVD nitride films are better than plasma films. Resistivity of plasma nitride can range from 10 6 to 10 15  ·cm, depending on the amount of nitrogen, while the dielectric strength ranges between 1 and 5 MV/cm. 6.3.4 Silicon Nitride Deposition(cont.)

49. Mo, Ta, Ti, and W have low resistivity and can form silicides. Cu is deposited by electro- and electrolus plating techniques. Tungsten can be deposited by thermal, plasma, or optically assisted decomposition of WF 6 : 6.3.5 CVD Metal Deposition Not CVD

50. Mo, Ta, and Ti can be deposited in an LPCVD system through reaction with hydrogen. 6.3.5 CVD Metal Deposition(cont.)