Semiconductor doping. Si solar Cell

Transcription

1 Semiconductor doping Si solar Cell Two Levels of Masks - photoresist, alignment Etch and oxidation to isolate thermal oxide, deposited oxide, wet etching, dry etching, isolation schemes Doping - diffusion/ion implantation Metallization - Materials deposition, PVD, CVD

2 What s a metal, a semiconductor? IV How do we dope a semiconductor

3 Electrons and holes Conduction Band E D E c E A Valence Band E v

4 Sheet Resistance, what is it? What is the Resistance of this bar of material with resistivity ρ? W R = ρ L/Wt L We can rearrange to get a film dependent quantity called the Sheet Resistance R s = ρ/t =R / (L/W) Notice L/W is unit less, but gives us the number of squares in the length of the bar. The units of R s are ohms, but they are often given as Ω /. t

5 Sheet Resistance - Four Point Probe If Probe spacing is: Larger than film thickness Smaller than distance to edge of film Probe points are small Using a four point approach is a standard technique for eliminating the effects of contact resistance R s =4.53 V/I and ρ=r s t where t is thickness

6 How do we get the doping? R s and t give us ρ, which gives us doping (but we must know t)

7 Another way to get doping - from C-V of a diode Formation of a p-n junction Formation of a Schottky junction

8 1/C 2 vs V C -2 Slope gives carrier Concentration Assumes an abrupt junction - Schottky, p + n or n + p x-intercept give V v bi What if the line isn t straight?

9 How about the thickness of our Oxide? Again, C = εa/w, so we should have another way to measure W. In practice, we must be careful about what C we use. Corresponds to oxide thickness What about trapped charge?

10 Inversion in an MOS structure accumulation (negative bias) no bias inversion (positive bias)

11 What about I-V Characteristics? Forward biased pn junction: Probability that carriers are over the barrier is like a Boltzmann factor But, there is also an electric field pushing carriers back so at V = 0 there should be no current. We can write this in a simpler form as: What about when light is shining on the device? Note, there is a sign difference with respect to the capacitance analysis

12 How can we tell the carrier type Thermovoltage V Hot Probe e e e e e e e e Hall Effect carrier type mobility sheet concentration

13 Other methods of getting at the carriers SIMS RBS Rutherford Backscattering Polaron profiler Spreading Resistance...

14 Doping - reminder Goal of Doping: Substitution of atoms with excess or deficiency of valence electrons e.g. B or P substituting for Si Diffusion doping (in fact most doping) is typically done in two steps: (Almost all doping is now ion implantation) Predeposition - Use a source to create the desired dose Drive in - Source at surface removed, additional diffusion to get desired distribution (in ion implantation the anneal also removes damage and activates the dopant).

15 Generic Predeposition Process Deliver Dopants to Partially Masked Substrates Diffusion (Hot) Ion Implantation (Cold) Structure: Dopants Mask: Oxide, Nitride, Photoresist Silicon

16 Dopant delivery Options for Diffusion Gas Source: C B δ Nasty Gases: AsH 3, PH 3, B 2 H 6 Very similar to Deal Grove Oxidation C s x j C o Liquid Source: SOG: Spin-On Glass Doped SiO 2 dissolved in solvents Apply exactly like Photoresist C i Solid Source: Glass Discs (B 2 O 3, P 2 O 5 ) Close-space Sublimation Vapors sublime/diffuse/react Which is Best?

17 Drive-in - estimating the profile Fick s law - You need the PDE, but you also need the boundary conditions! C(z,0) = 0, Z 0 dc(0,t)/dz = 0 C(,t) = 0 0 C(z,t)dz = Q T Solution: C(z, t) = 2 -z Q T 4Dt πdt = constant e We can model the drive in step from our homework, here after a P predep with p8545 we had a sheet resistance of 12Ω/ and depth of 1.1µm. This gave a carrier concentration of 5x10 19 / cm 3 and a surface concentration of 5.5x10 15 /cm 2 Characteristic Length Scale - Diffusion Length

18 What about the diffusion Coefficient? Use first three terms in Fair s vacancy model. D = D o + n n i D + n n i 2 D 2 I told you to assume n~n i ~10 19 /cm 3 Is this reasonable? From Campbell table 3.2 (1100C=1373K) D o = 3.9cm 2 /s e -(3.66/k1373) = 1.43 x cm 2 /s D - = 4.4cm 2 /s e -(4.0/k1373) = 9.13 x cm 2 /s D 2- = 44cm 2 /s e -(4.37/k1373) = 4.00 x cm 2 /s D = 1.56 x cm 2 /s

19 Simulations Suprem-IV is a process simulation tool developed at Stanford University

20 nanohub TCAD tools

21 Suprem simulation of boron predep and drive-in Boron Diffusion Log10(Boron) Depth in microns Boron Predep 1100C 30 min. Boron drivein 1100C 30 min. Boron drivein 1100C 60 min. Boron drivein 1100C 60 min 200 angstrom oxide cap Boron predep in gas at 5 x /cm 3 concentration followed by drive-ins. Effect of oxide cap on profile near the surface Boron Diffusion Why 5x10 20 /cm 3? 1) Damage threshold 2) Solubility limit 3) B partial pressure 1) Dimensional argument Log10(Boron) Depth in microns

22 Solid Solubility, what is it? 1100C 5x10 20 /cm 3

23 Oxide is an effective anti-diffusion barrier for Si VLSI? 1) For boron but not for phosphorus 2) For phosphorus but not for boron 3) It works well for both 4) It depends

24 Final Topic on Diffusion: Oxide How fast do dopants diffuse through oxide? Diffusivity important, Solubility important Consider D o of Boron Si prefactor 0.37cm 2 /s Activation Energy 3.46eV SiO 2 prefactor cm 2 /s Activation Energy 3.53eV Now D o of Phosphorous Si prefactor 3.9 cm 2 /s Activation Energy 3.66eV SiO 2 prefactor 0.19 cm 2 /s Activation Energy 4.03eV Oxide is often used as a diffusion mask- how thick does it need to be? Oxide is used for isolation - does it isolate? What is the thermal load? Oxide is also a gate dielectric with heavily B doped polysilicon gates - diffusion through gate is an issue M Metal O S Oxide Silicon Doped polysilicon

25 Suprem-IV Wet Oxide then Diffusion Oxide antidiffusion barrier 20 Log10(Boron) min wet O2 at 1000C, 30 min boron predep at 1100C 30 minute boron predep at 1100C 0 Effect of oxide cap on profile near the surface Depth in microns Substrate is P doped at 1 x /cm 3, Wet oxide growth at atmospheric pressure for 60 minutes at 1000C, Boron predep from 30 minutes at 1100C in gas with a concentration of 5 x /cc.

26 Simulation of predep and drive-in to find junction depth 1000 C P predep in p-type wafer doped at 1x10 17 /cm C drive in. How long to get a 4.0µm deep junction?