Sputtering ( Microelectronics & IC Technology )
- 2. CONTENTS • Introduction • Sputtering Mechanism • Sputter Yield • Different Techniques of Sputtering DC Sputtering RF Sputtering Magnetron Sputtering Reactive Sputtering Bias Sputtering • Advantages and Disadvantages • Conclusion
- 3. INTRODUCTION 3 Sputtering is a mechanism by which atoms are dislodged from the surface of a material as a result of collision with high energy particles. It is a type of Physical Vapor Deposition (PVD). It can be described as a sequence of these steps: 1) Ions are generated and directed at target material. 2) The ions sputter atoms from the target 3) Sputtered atoms get transported to the substrate through a region of reduced pressure 4) The sputtered atoms get condensed on the substrate, forming a thin film.
- 4. SPUTTERING MECHANISM • An electrical circuit where the Target is connected to the cathode and the Substrate is connected to the anode around a pressurised inert gas like Argon is created. • This creates a glow discharge which is a self- sustaining type of plasma created by the breakdown of the inert gas (Argon). The plasma gets ionised as high electrical bias is created within the electrodes around the plasma & the ions travel at high velocity towards the cathode i.e., the Target. • When the ions hit the Target, they dislodge or eject some of the atoms sitting on the surface of the Target. • These Target atoms then get deposited on the surface of the substrate forming a layer of the Target atoms over it.
- 5. SPUTTER YIELD 5 Sputter yield is the ratio of number of sputtered atoms to number of incident ions. 𝑆 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑝𝑢𝑡𝑡𝑒𝑟𝑒𝑑 𝑎𝑡𝑜𝑚𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑎𝑡𝑜𝑚𝑠 Sputter Yield depends on : • 𝑆 ∝ 1 𝑏𝑖𝑛𝑑𝑖𝑛𝑔 𝑒𝑛𝑒𝑟𝑔𝑦 • 𝑆 ∝ 𝐸𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑖𝑜𝑛𝑠 • 𝑆 ∝ 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑖𝑜𝑛𝑠 • Reduction in deposition of pressure gives better yield.
- 6. DC SPUTTERING 6 Here a D.C electric field is impressed across the electrodes which are located in the inert gas. The deposition rate depends on the pressure of the inert gas and the DC voltage. As pressure is increased at fixed voltage, the mean free path is decreased & more ions are generated. If pressure is too high, the sputtered atoms undergo increased collisional scattering and are not efficiently deposited. Energy is gained via elastic collisions until E > 15 eV for ionisation. 𝐴𝑟 + 𝑒− → 𝐴𝑟+ + 2𝑒− The Optimum condition is shown in the shaded region of the graph. This range of pressure is suitable for DC Sputtering.
- 7. RF SPUTTERING • The RF Sputtering helps to deposit insulating thin films (resistivity > 106 Ω-cm). • It is usually carried out with 1 to 3 kV peak-to-peak RF potential. • At AC signal above 50 kHz, electrons oscillating in the glow region acquire enough energy to cause ionising collisions, reducing the need for 2° electrons to sustain the discharge. • Also, RF voltages can be coupled through any kind of impedance so that the electrodes need not be conductors. So it is possible to sputter any material irrespective of its resistivity. • Typical RF frequencies employed is 13.56 MHz.
- 8. MAGNETRON SPUTTERING 8 • This technology uses powerful magnets to confine the "glow discharge" plasma to the region closest to the target plate. • That vastly improves the deposition rate by maintaining a higher density of ions, which makes the electron/gas molecule collision process much more efficient. • Uses either DC or RF power. • A magnetic field of 100-200 gauss is used, resulting in field lines which confine electrons to the region embraced by them • The discharge region is in the shape of a torroid or elongated ring, with extensive sputtering of the target in these regions. • Substrates are usually placed on a rotating anode in order to obtain uniform film deposition.
- 9. REACTIVE SPUTTERING 9 • In Reactive Sputtering, thin films of compounds are deposited on substrates using metallic targets in the presence of a reactive gas in a chamber, usually mixed with the inert gas to tune atrial pressure. • The most common compounds reactively sputtered (and the reactive gases employed) are: Oxides: Al2O3, SiO2 Nitrides: TaN, TiN, AlN Carbides: TiC, SiC Oxycarbides and Oxynitrides of Ti, Ts, Al and Si • Irrespective of which of these materials is considered, during reactive sputtering the resulting film is either a solid solution alloy of the target metal doped with the reactive element, a compound or some mixture of the two.
- 10. BIAS SPUTTERING • In Bias sputtering, electric fields near the substrate are modified in order to vary the flux and the energy of the incident charged species and is achieved by applying either a negative RF or DC bias to the substrate. • A target voltage of -1000V to -3000V, bias voltage of -50V to -300 V are typically used. • This technique is use in all sputtering configurations (DC, RF, Magnetron and Reactive) to tune resistivity, stress, dielectric properties, optical properties, etc rate, density and adhesion of the deposited film.
- 11. ADVANTAGES & DISADVANTAGES 11 ADVANTAGES • Large size targets thus simplifying the deposition with uniform thickness over large wafers. • Film thickness is easily controlled by fixing operating parameters and simply adjusting the deposition time. • Device damage from X-rays generated by electron beam is avoided. • In-situ cleaning is easily done with sputter etch. • Adhesion is excellent. DISADVANTAGE • High capital expense are required • Rate of deposition of some materials (such as SiO2 are relatively low. • Ionic bombardment damage occurs in organic solids • Possibility of incorporating impurities due to low-medium vacuum range as compared to Evaporation
- 12. CONCLUSION Sputtering is a Physical Vapor Deposition technique and it is widely used for depositing thin metal layers on semiconductor wafers. In Sputtering, ions are generated by a plasma discharge usually within an inert gas (Ar). It is also possible to bombard the substrate with an ion beam from an external ion source. This allows to vary energy and intensity of ions reaching.
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