and for a complementary error function profile is x ;=2v Dt erfc (23.16) ffected by the diffusion by using an oxide mask and making a cut in it where specific diffusion is to s are So far we have considered just vertical diffusion. In practical IC fabrication, usually only small Hence, we also have to be concerned with lateral diffusion of the dopant so as not to affect adjacent devices. Two-dimensional numerical solutions exist for solving this problem [Jaeger, 1988]; however, a useful rule thumb is that the lateral junction, y, is 0.8 Another parameter of interest is the sheet resistance of the diffused layer. This has been numerically evaluated for various profiles and presented as general-purpose graphs known as Irvin's curves For a given profile type, such as n-type Gaussian, Irvin's curves plot surface dopant concentration versus the product of sheet resistance and junction depth with substrate doping as a parameter. Thus, given a calculated diffusion profile one coul estimate the sheet resistivity for the diffused layer. Alternatively, given the measured junction depth and sheet resistance, one could estimate the surface concentration for a given profile and substrate doping. Most processing books [e.g. Jaeger, 1988] contain Irvin's curves Ion Implantation Diffusion places severe limits on device design, such as hard to control low-dose diffusions, no tailored profiles, and appreciable lateral diffusion at mask edges. Ion implantation overcomes all of these drawbacks and is an alternative approach to diffusion used in the majority of production doping applications today. Although many different elements can be implanted, IC manufacture is primarily interested in B, P, As, and Sb Ion Implant Technology A schematic drawing of an ion implanter is shown in Fig 23. 3. The ion source operates at relatively high voltage (20-25 kV) and for conventional dopants is usually a gaseous type which extracts the ions from a plasma The ions are mass separated with a 90 degree analyzer magnet that directs the selected species through resolving aperture focused and accelerated to the desired implant energy. At the other end of the implanter is the target chamber where the wafer is placed in the beam path. The beam line following the final accelerator and the target chamber are held at or near ground potential for safety reasons. After final acceleration the beam is bent slightly off axis to trap neutrals and is asynchronously scanned in the X and Y directions over the wafer to maintain dose uniformity. This is often accompanied by rotation and sometimes translation of the target The implant parameters of interest are the ion species, implant energy, and dose. The ion species can consist of singly ionized elements, doubly ionized elements, or ionized molecules. The molecular species are of interest in forming shallow junctions with light ions, i.e, B, using BF=. The beam energy is (23.17) where n represents the ionization state(1 for singly and 2 for doubly ionized species), q the electronic charge, and V the total acceleration potential (source acceleration tube)seen by the beam. The dose, Q, from the implanter is dt (23.18) where I is the beam current in amperes, A the wafer area in cm?, t, the implant time in sec, and n the ionization state c2000 by CRC Press LLC© 2000 by CRC Press LLC and for a complementary error function profile is (23.16) So far we have considered just vertical diffusion. In practical IC fabrication, usually only small regions are affected by the diffusion by using an oxide mask and making a cut in it where specific diffusion is to occur. Hence, we also have to be concerned with lateral diffusion of the dopant so as not to affect adjacent devices. Two-dimensional numerical solutions exist for solving this problem [Jaeger, 1988]; however, a useful rule of thumb is that the lateral junction, yj , is 0.8xj . Another parameter of interest is the sheet resistance of the diffused layer. This has been numerically evaluated for various profiles and presented as general-purpose graphs known as Irvin’s curves. For a given profile type, such as n-type Gaussian, Irvin’s curves plot surface dopant concentration versus the product of sheet resistance and junction depth with substrate doping as a parameter. Thus, given a calculated diffusion profile one could estimate the sheet resistivity for the diffused layer. Alternatively, given the measured junction depth and sheet resistance, one could estimate the surface concentration for a given profile and substrate doping.Most processing books [e.g., Jaeger, 1988] contain Irvin’s curves. Ion Implantation Diffusion places severe limits on device design, such as hard to control low-dose diffusions, no tailored profiles, and appreciable lateral diffusion at mask edges. Ion implantation overcomes all of these drawbacks and is an alternative approach to diffusion used in the majority of production doping applications today. Although many different elements can be implanted, IC manufacture is primarily interested in B, P, As, and Sb. Ion Implant Technology A schematic drawing of an ion implanter is shown in Fig. 23.3. The ion source operates at relatively high voltage (ª20–25 kV) and for conventional dopants is usually a gaseous type which extracts the ions from a plasma. The ions are mass separated with a 90 degree analyzer magnet that directs the selected species through a resolving aperture focused and accelerated to the desired implant energy. At the other end of the implanter is the target chamber where the wafer is placed in the beam path. The beam line following the final accelerator and the target chamber are held at or near ground potential for safety reasons. After final acceleration the beam is bent slightly off axis to trap neutrals and is asynchronously scanned in the X and Y directions over the wafer to maintain dose uniformity. This is often accompanied by rotation and sometimes translation of the target wafer also. The implant parameters of interest are the ion species, implant energy, and dose. The ion species can consist of singly ionized elements, doubly ionized elements, or ionized molecules. The molecular species are of interest in forming shallow junctions with light ions, i.e., B, using BF2 +. The beam energy is E = nqV (23.17) where n represents the ionization state (1 for singly and 2 for doubly ionized species), q the electronic charge, and V the total acceleration potential (source + acceleration tube) seen by the beam. The dose, Q, from the implanter is (23.18) where I is the beam current in amperes, A the wafer area in cm2 , tI the implant time in sec, and n the ionization state. x Dt N N j B = Ê Ë Á ˆ ¯ ˜ - 2 1 0 erfc Q I nqA dt t I = Ú0