Metallization ( Part 2 )

Transcription

1 1 Metallization ( Part 2 ) Chapter 12 : Semiconductor Manufacturing Technology by M. Quirk & J. Serda Saroj Kumar Patra TFE4180 Semiconductor Manufacturing Technology, Norwegian University of Science and Technology ( NTNU ) TFE4180 Semiconductor Manufacturing Technology

2 2 Objectives 1. State the advantages and disadvantages of sputtering. 2. Describe the physics of sputtering and discuss different sputtering tools and applications. 3. Describe the benefits and applications for metal CVD. 4. Explain the fundamentals of copper electroplating. TFE4180 Semiconductor Manufacturing Technology

3 3 Expressions SSI Small Scale Integration devices on die (until late 1960s) MSI Medium Scale Integration. 50 5k devices on die (until mid 1970s) LSI Large Scale Integration. 5k 100k devices on die (until early 1980s) VLSI Very Large Scale Integration. 100k 1 mill. devices on die (until early 1990s) ULSI Ultra Large Scale Integration. > 1 mill. devices on die PVD Physical Vapor Deposition CVD Chemical Vapor Deposition Step coverage The thickness of a deposited film over steps or features relative to the thickness of on the top surface. Gap fill The filling of narrow spaces with a material without creating voids or other defects. Yield The ratio of the number of good units to the total number of units produced. TFE4180 Semiconductor Manufacturing Technology

4 4 Introduction Metallization is the process of depositing metal film over a dielectric film. Most commonly used for metal wires and plugs. Both PVD and CVD processes can be used to deposit metal. TFE4180 Semiconductor Manufacturing Technology

5 5 Metal Deposition Systems Evaporation Sputtering Metal CVD Copper electroplating TFE4180 Semiconductor Manufacturing Technology

6 6 Brief history Evaporation was the main method for metallization during the SSI and MSI era. Sputtering was the main method during the LSI and VLSI era, but is still commonly used. Metal CVD is the main method for the ULSI era. TFE4180 Semiconductor Manufacturing Technology

7 7 Evaporation Early PVD method. Poor step coverage, gap fill and yield. Limitation for depositing alloys. Process: 1. Place target material in a crucible inside a vacuum chamber. 2. Heat material using electron beam until it vaporizes. 3. Vapor condenses when it strikes a surface and forms a film. TFE4180 Semiconductor Manufacturing Technology

8 8 Simple rotating evaporator Rotating servo Wafer(s) Wafer carrier Evaporating metal Crucible Process chamber (bell jar) Hi-Vac valve Hi-Vac pump Roughing pump Figure in Q&S TFE4180 Semiconductor Manufacturing Technology

9 9 Step coverage Conformal step coverage Poor step coverage TFE4180 Semiconductor Manufacturing Technology

10 10 Sputtering PVD method that mostly replaced evaporation systems. Advantages of sputtering: 1. Ability to deposit and maintain complex alloys. 2. Capability to deposit high-temperature and refractory metals. 3. Ability to deposit controlled, uniform films on large wafers. Big improvement in gap fill, but still limited capability for step coverage making it insufficient for ULSI applications. TFE4180 Semiconductor Manufacturing Technology

11 11 Six Basic Sputtering Steps 1. Positive argon ions are generated in a plasma in a high vacuum chamber and accelerated toward a target material at a negative potential 2. During acceleration, the ions gain momentum and strike the target 3. The ions physically dislodge(sputter) atoms from the target which has the desired material composition 4. The dislodged(sputtered) atoms migrate to the wafer surface 5. The sputtered atoms condense and form a thin film on the wafer surface with essentially the same material composition as the target, following the stages for thin film growth 6. Excess material is removed from the chamber by vacuum pump TFE4180 Semiconductor Manufacturing Technology

12 12 DC Diode Sputtering System Gas delivery 1) Electric fields create Ar + ions. Argon atoms Plasma e- e- + Cathode (-) 3) Metallic atoms are dislodged from target. Metal target 2) High-energy Ar + ions collide with metal target. e- Exhaust Electric field 4) Metal atoms migrate toward substrate. 6) Excess matter is removed from chamber by a vacuum pump. 5) Metal deposits on substrate DC diode sputterer Substrate Anode (+) Figure in Q&S TFE4180 Semiconductor Manufacturing Technology

13 13 Physics of Sputtering Argon is used because it is heavy and is a chemically inert gas Positively charged argon ions in the plasma are strongly attracted to the negative potential of the cathode Momentum transfer: argon ions transfers to the target material to dislodge one or more atoms Sputtering ion energy: ev

14 14 Physics of Sputtering Cathode (-) Metal atoms Sputtered metal atom High-energy Ar + ion + 0 Rebounding argon ion recombines with free electron to form a neutral atom. Figure Quirk & Serda

15 15 Sputter yield depends on: 1. Incident angle of the bombarding ions 2. Composition and geometry of the target material 3. Mass of bombarding ions 4. Energy of the bombarding ions

16 16 Three types of sputtering Radio frequency (RF) Magnetron Ionized Metal Plasma (IMP)

17 17 RF sputtering RF field creates the plasma Frequency MHz The electrons and ions in the plasma are exposed to the RF field (electrons responds most strongly) The chamber and electrodes behaves like a diode Limited because it does not have a high sputtering yield, leading to a low deposition rate Not used in wafer fabrication

18 18 RF sputtering Matching network RF generator Electrode Blocking capacitor Microcontroller operator interface Gas flow controller Substrate Target Pressure controller Gas panel Chuck Turbo pump Exhaust Argon Figure Quirk & Serda Roughing pump

19 19 Magnetron sputtering Employs magnets around the target to capture and restrict the electrons Requires a substantial amount of power (3kW-20kW) Cooling of the cathode required Magnet DC power supply Argon inlet Cathode Target Heated wafer chuck Figure Quirk & Serda Vacuum Pump

20 20 Magnetron sputtering Step Coverage: Requires a vacuum environment with high purity argon to avoid contamination from residual gas The atoms dislodged from the target essentially pass on the wafer by following a line-of-sight path Atoms exiting at many different angles

21 21 Magnetron sputtering Collimated Sputtering: Uses to achieve increased coverage on the bottom and side of a contact or via Any neutral species sputtered from the target at a high angle is intercepted and deposited on the collimator A large portion of the sputter will not reach the wafer lowers the sputter yield and increases the cost of deposition

22 22 Magnetron sputtering Collimated Sputtering: Target Collimated Sputtering System Ar Collimator Figure Quirk & Serda Cross section of via showing coverage of resulting sputtered film.

23 23 IMP sputtering The sputtered metal is ionized in an RF plasma Positive metal ions travel in a highly directional, vertical path towards a wafer configured with a negative voltage bias Biasing the wafer enables a higher degree of film conformality in bottoms and corners Achieve good hole-fill for Ti and TiN Is used in production to deposit Ti, TiN and Cu

24 24 IMP sputtering DC supply Titanium target Electrode DC field High-energy Ar + ion + Sputtered Ti atom Plasma RF field e - e - + Ti + ion Induction coil Substrate Electrode DC bias supply RF generator Figure Quirk & Serda

25 25 Metal CVD Tungsten CVD (W) 1. Excellent step coverage and gap-fill, high-aspect ratio vias. 2. High electromigration resistance. Blanket Tungsten CVD Deposition.

26 26 Metal CVD

27 27 Metal CVD Via Ti PECVD SiO 2 Gap fill dielectric Aluminium TiN 1. Interlayer dielectric via etch Tungsten via fill 2. Collimated Ti deposition covers bottom of via Tungsten plug 3. CVD TiN conformal deposition 4. CVD tungsten deposition 5. Tungsten planarization Figure Quirk & Serda

28 28 Copper CVD Thin copper seed layer of Å. Necessary for copper electroplating. Critical to have a continuous seed layer, which is free of pinholes and voids. Precursors: -Cu(I) - CU(II)

29 29 Copper Electroplate Copper replace aluminium. Electroplating Fundamentals. Quantity of copper is proportional to current at wafer surface. Complicated to control because of current density. Use different type of voltage waveforms to assist in planting high-aspect ratio holes. Lots of issues to address: No copper plate on the backside of the wafer, avoid high vacuum or complex heating.

30 30 Copper Electroplating Outlet Copper atom attached to wafer - Cathode Substrate Outlet Copp er ion + Plating solution + + Copper anode Figure Quirk & Serda Inlet

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