ISAT 436 Micro-/Nanofabrication and Applications

1. ISAT 436Micro-/Nanofabrication and Applications Ion Implantation David J. Lawrence Spring 2004

2. Semiconductor Doping Techniques • Dopants can be introduced into semiconductors in several ways: • Doping during ingot growth. • Doping during epitaxial growth. • Diffusion of dopants into a wafer. • Ion implantation of dopants into a wafer. • Diffusion and Ion Implantation are the only processes that enable the introduction of dopants into a wafer from its surface after all crystal growth has been completed.

3. Ion Implantation • Ion implantation is an alternative to diffusion for producing n- or p-type material by introducing dopants into a semiconductor through its surface (see Chapter 5 of Jaeger, pp. 109-124). • With ion implantation, dopant atoms are injected into the wafer not by the application of thermal energy, but by ionizing the atoms, accelerating the ions to a high velocity, and implanting them into the wafer by virtue of their high kinetic energy. • Ion implantation has several advantages over diffusion, to be discussed later.

4. B B B B B B B n-type silicon Converted to p-type by implantation of boron. Ion Implantation • For example, we can implant boron ions into n-type silicon to form a p-n junction: “Not to scale!”

5. Ion Implantation • The essential components of an ion implanter are shown in Figure 5.1 on page 109 of Jaeger. • Ion Source: A high voltage is used to produce ions of the desired dopant. Arsine, phosphine, diborane, other gases and also solids can be used as the dopant source. • Mass Spectrometer: An “analyzer magnet” is used to bend the beam of desired ions through a right angle. The desired ions then go through an aperture. Undesired impurities will be bent through different angles and will miss the aperture.

6. Ion Implantation • High-Voltage Accelerator: An acceleration tube uses a high voltage (up to 5 MV) to accelerate the ions to their final speed. • Scanning System: Varying voltages applied to deflection plates are used to raster scan the ion beam across the wafer surface. This allows uniform implantation of an entire wafer. • Target Chamber: This chamber contains the wafers to be processed.

7. Ion Implantation • The entire ion beam path in the implanter is maintained under vacuum. Why? • The silicon wafers are electrically connected to “ground” so that electrons can readily flow to or from the wafers to neutralize the implanted ions. • The target wafers can be maintained at relatively low temperatures during implantation. This minimizes any diffusion of the implanted atoms.

8. Ion Implantation • The total dose Q of implanted ions per unit area is controlled by adjusting the beam electric current I and the implantation time T. • The total dose per unit area is given by: where A is the total implanted area, q is the proton charge, I is in amperes, and n = 1 for singly ionized ions, n = 2 for doubly ionized ions, etc. (See Jaeger, pages 110-111.)

9. Ion Implantation • If the beam current is constant, the previous equation simplifies to: • Total Dose per Unit Area

10. Ion Implantation • The implanted dopant profile can be approximated by a Gaussian distribution function (see Jaeger, page 111). • This distribution is described mathematically by: • See Figure 5.2 on page 111. • Rpis called the projected range and is equal to the average distance an implanted ion travels into the silicon before it stops. • The peak concentration Np occurs at x = Rp . • The spread of the implanted atom distribution is characterized by the standard deviation, DRp , which is called the (vertical) straggle.

11. Ion Implantation • The area under the impurity distribution curve is equal to the implanted dose Q : • If the implanted ions are completely contained in the silicon wafer, this simplifies to:

12. Ion Implantation • Typical implanted doses range from 1010 /cm2 to 1018 /cm2 . • Implantation of large doses ( > 1015 /cm2 ) can be time consuming. • The projected range of a given ion depends on: • energy of the ion, • atomic mass of the ion, and • atomic mass of the target wafer. • Lindhard, Scharff, and Schiott developed a theory for range and straggle calculations, called the LSS Theory. • See Jaeger, pp. 112-114.

13. Ion Implantation • Figure 5.3(a) on page 113 of Jaeger gives the results of LSS calculations of the projected range Rp of B, P, As, and Sb implanted into silicon or SiO2 . • Rp is roughly proportional to the acceleration energy. Since this is a “log-log” plot, a slope of one is required for a linear relationship. • For a given energy, lighter elements strike the silicon with higher velocity and therefore penetrate more deeply into the wafer.

14. Ion Implantation • Figure 5.3(b) on page 113 of Jaeger gives the results of LSS calculations of the vertical straggle DRp of B, P, As, and Sb implanted into silicon or SiO2 . • For all cases except boron, the straggle increases linearly with acceleration energy over much of the energy range. • Figure 5.3(b) also gives values for the transverse straggle DR^ , which will be covered later.

15. Ion Implantation • The LSS results shown in Figure 5.3(a and b) on page 113 of Jaeger are based on the assumption that the target material is amorphous, having no long-range order. This is true for SiO2 and amorphous silicon thin films, but it is not true for crystalline silicon wafers. • The regular arrangement of silicon atoms in a single crystal wafer results in a regular arrangement of open spaces. • If the incoming ions happen to be directed along certain special crystal directions, they will “channel” much more deeply into the material than the LSS theory predicts. • See pp. 118-119 of Jaeger.

16. Ion Implantation • Channeling can be eliminated by tilting the silicon target wafer relative to the incoming ion beam. • This makes the silicon atom arrangement appear to be nearly random to the incoming dopant ion beam. • See Figure 5.8 on page 119 of Jaeger.

17. Ion Implantation • During the ion implantation process, the impact of implanted ions can knock silicon atoms out of their lattice positions. • The implanted region of the substrate is damaged. • If the implanted dose is high enough, the implanted layer will become amorphous. • Implantation damage can be removed by an “annealing” step at a temperature from 600 to 1000°C. • Implantation damage can be prevented by heating the wafer during implantation. • See pages 120-121and page 123 of Jaeger.

18. Ion Implantation vs. Diffusion • What are the advantages of ion implantation compared to thermal diffusion? • Low temperature process minimizes dopant movement by diffusion. • Wide variety of thin film materials can be used to “mask” (block) the implantation. • Better control of dopant dose. • Maximum doping need not be at the wafer surface. • Multiple implants can be used to create complex doping profiles needed for sophisticated devices.

19. Ion Implantation vs. Diffusion • What are the disadvantages of ion implantation compared to thermal diffusion? • Implantation damages the wafer, so annealing is required. • High doses can require long implant times, reducing wafer throughput. • Ion implanters are expensive, with production machines costing more than $2 Million.