Thermal doping review example

1. Thermal doping review example This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide.

2. Thermal Doping Example Pattern a wafer and place an oxide film on top of the exposed silicon. Silicon wafer This section was protected by the mask Dopant will diffuse into the unprotected silicon as function of time and temperature in the furnace Oxide film Dopant containing film. Cross section cut view that is not to scale

3. Thermal Doping Example Side of wafer that will have the functional device Sources of dopants Solid wafer made of the dopant material Solid Source Wafer side that will house the functioning device. Wafer side that will house the functioning device. Cross section view of oven rack to hold wafers and solid dopant sol-gel film sol-gel film Spin-on Dopant film

4. Thermal Doping Example Sources of dopants Solid as vapor source Solid dopant placed in platinum boat for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

5. Thermal Doping Example Sources of dopants Liquid as vapor source for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

6. Thermal Doping Example Sources of dopants Pure vapor source for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

7. Thermal Doping Example Pattern a wafer and place an oxide film on top of the exposed silicon. Place a dopant containing film on the wafer and heat for some time. Silicon wafer Oxide film Dopant containing film. Cross section cut view that is not to scale

8. Thermal Doping Example Region of interest Silicon wafer Dopant containing film. Oxide film Cross section cut view that is not to scale

9. Oxide film Silicon wafer Dopant containing film. Cross section cut view that is not to scale

10. HEAT Cross section cut view that is not to scale

11. DEGLAZE then CLEAN Cross section cut view that is not to scale

12. HEAT Cross section cut view that is not to scale

13. Cross section cut view that is not to scale

14. Mask thickness (microns) Diffusion time (hours) Thermal Doping Example How thick does the protective oxide have to be? Practical factors Oxide film needed to be thick enough to mask diffusion process 1 If your furnace is at 1100 degrees C, it will be at least 3.5 hrs before the boron gets through the1 micron thick oxide protective cover. for educational use only. Fig 3.7 p 53, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

15. 3 3 N 20 20 Boron atoms/ cm Boron atoms/ cm 1.1 x 10 1.1 x 10 0 Impurity concentration (atoms/cm3) Thermal Doping Example How much dopant will dissolve in the silicon? Practical factors The real issue is how many dopant atoms will replace silicon atom. You can dissolve more P and As atoms into crystal than can substitute for silicon atoms. At 900 C and maximum Boron concentration (solubility)at the surface is about Therefore, = for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

16. -6 86.2 x10 o ev/ K E E E D D A A A 0 0 D 0 k (1173) k T Atom B 10.5 3.69 ev P 10.5 3.69 ev Al 8.0 3.47 ev Ga 3.6 3.51 ev As 0.32 3.56 ev D(1173) = D(1173) = -6 P P 86.2 x10 -15 2 1.48 x10 cm /sec Thermal Doping Example How much does temperature influence the dopant transport into the silicon? Practical factors [ ] - e D(T) = These plots can be modeled as exponential functions for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002 10.5 3.69 ev From the model, what is the diffusion coefficient for P at 900 C? [ ] - e D(1173) = (900 C equals 1173 K) [ ] 3.69 B and P - (1173) e 10.5 As Come on! Work it out, its good for you. ?

17. Thermal Doping Example Practical factors What are the model equations for the diffusion of dopant from an infinite source? Concentration profile through the diffusion region as a function of distance and time. X =0 at the outside edge of the wafer t =0 before the diffusion starts. Total dopant that was added to substrate. Distance into wafer were the concentration of the n and p materials is identical. Junction depth

18. Thermal Doping Example Practical factors What are the model equations for the diffusion of dopant from a constant or fixed source? Concentration profile through the diffusion region as a function of distance and time. X =0 at the outside edge of the wafer t =0 before the diffusion starts. Concentration at the surface as a function of time? Put X =0 and solve for all values of time. Distance into wafer were the concentration of the n and p materials is identical. Junction depth

19. Function values 2 -x 2 e X /4(Dt) 1/2 [ ] Thermal Doping Example Practical factors What is erfc and how do I use it.? The error function and its complement are popular functions because they are solutions to differential equations that deal with diffusion problems. erfc( ) Values for the function are available from tables or plots like this one, or approximation functions like this one also found in common mathematics software packages.

20. The gaussian curve on the right is also often used as a substitute for the erf complement. For most of the model curves shown the plots have similar shape and functional response. Thermal Doping Example Practical factors for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002 for educational use only. From, R.C. Jaeger, Introduction to Microelectronic Fabrication, 2nd Ed., Prentice Hall, 2002

21. Thermal Doping Example Practical Problem You have a n-type silicon wafer that has a resistivity of 0.36 ohm-cm. You want to use boron to form the base region in the wafer for an npn transistor. You perform a solid-solubility limited boron “predeposition” at 900 C for 15 minutes followed by (after deglaze and clean) a 5 hour “drive-in” at 1100C. Find the boron surface concentration , the junction potential and the dose. (I) just after the “predeposition” step. (II) just after the “drive in” step.

22. Get N from the solubility graph for Boron at 900 C. 0 E A 2 x N 0 4 D t D T 0 k (1173) Thermal Doping Example Practical Problem Find the boron surface concentration, the junction potential, and dose. (I) just after the “predeposition” step. (a) Boron surface concentration just after the “predeposition” step. 1) 2) Find the value for diffusion coefficient at 900 C. 900 C 1173 K For Boron, B, the model becomes ,B [ ] - e D(1173) = ,B B 3) Find the number of boron atoms, N ( x, t ) when x = 0 and t = 15 minutes (900 seconds). 1/2 [ ] erfc N ( x , t ) =

23. 2 x 4 D t T 1/2 2 [ ] [ ] Q p D t T Thermal Doping Example Practical Problem Find the boron surface concentration, the junction potential, and dose. (I) just after the “predeposition” step. (b) Boron junction depth in the original resistivity of 0.36 ohm-cm n doped wafer. 1) Determine the number of Boron atoms that correspond to the same resisitivity. (dopant concentration vs resistivity plot) 2) Use the concentration profile model as a function of distance and time and solve for the junction depth distance. (c) Boron dose for this process. 1) Integrate the area under the concentration profile model for the pre-deposition or the “drive-in” process. (II) just after the “drive-in” step. (a) Boron surface concentration just after the “drive-in” step. 1) Use the concentration profile model as a function of distance and time and solve when x = 0. - e N ( x , t ) =

24. 2 x 4 D t T 1/2 2 [ ] [ ] Q p D t T Thermal Doping Example Practical Problem Find the boron surface concentration, the junction potential, and dose. (II) just after the “drive-in” step. (a) Boron surface concentration just after the “drive-in” step. 1) Use the concentration profile model as a function of distance and time and solve when x = 0. - e N ( x , t ) = (b) Boron junction depth, just after “drive in” step, in the original resistivity of 0.36 ohm-cm n doped wafer. 1) Solve concentration profile model as a function of distance and time for junction depth.

25. 2 x 4 D t T 1/2 2 [ ] [ ] Q p D t T Find the boron surface concentration, the junction potential, and dose. (II) just after the “drive-in” step. (a) Boron surface concentration just after the “drive-in” step. 1) Use the concentration profile model as a function of distance and time and solve when x = 0. - e N ( x , t ) = (b) Boron junction depth, just after “drive in” step, in the original resistivity of 0.36 ohm-cm n doped wafer. 1) Solve concentration profile model as a function of distance and time for junction depth.