Formation of pn junction in deep silicon pores September 2002 By Xavier Badel, Jan Linnros, Martin Janson, John Österman Department of Microelectronics and Information Technology KTH, Stockholm

 OUTLINE 1. Introduction 2. Experiment 3. Results 4. Summary X. Badel, KTH, Stockholm

 Introduction 1. Introduction Application: dental X-ray imaging... Requirement: Spatial resolution=10LP/mm; Low X-ray dose... Detector principle: silicon based detector with CsI columns Challenging process: Form pn junctions in pore walls. X. Badel, KTH, Stockholm

 Experiment: Pore formation 2. Experiment DRIE:Electrochemical Etching: - Photolithography -  10s Etching (SF 6 plasma) -  10s Passivation (C 4 F 8 plasma) - Etch rate: 2  m/min - n-type silicon (N d = cm -3 ) X. Badel, KTH, Stockholm - Initial patterned surface: inverted pyramids - Dissolution of n-type silicon (N d = cm -3 ) involving holes and aqueous HF - Etch rate: about 0.5  m/min

 Experiment: Pore formation 2. Experiment Setup and other examples of electrochemical etching: X. Badel, KTH, Stockholm

2. Experiment  Experiment: Doping methods Boron diffusion from a solid source: - diffusion 1 at 1150ºC for 1h45’ : Na = cm -3 ; thickness =6  m. - diffusion 2 at 1050ºC for 1h10’ : Na = cm -3 ; thickness =2  m. LPCVD of boron doped poly-silicon: T=600ºC; P=150 mTorr; t=1h30’; Gases: SiH 4 and B 2 H 6 ; Na = cm -3 ; thickness = 400 nm. X. Badel, KTH, Stockholm

2. Experiment  Experiment: Techniques for analyses X. Badel, KTH, Stockholm SEM: Scanning Electron microscopy SCM: Scanning Capacitance Microscopy 2D imaging of the doping Principle: measure dC/dV (related to the doping) via a probe scanning the surface. SSRM: Scanning Spreading Resistance Microscopy 2D imaging of the doping Principle: measure the current (related to the resistance/doping). SIMS: Secondary Ion Mass Spectrometry Dopant profiling in planar samples and through the wall thickness

 Results: Doping by diffusion 3. Results Diffusion 1: 1150ºC, 1h45’ Profile along A A 5 µm AFM SSRM X. Badel, KTH, Stockholm Thickness at the pore bottoms: 3  m. Thickness on a planar wafer (SIMS): 6  m. Transport of boron down to the pore bottom may be limited.

 Results: Doping by diffusion 3. Results Diffusion 1: SIMS profiles at different positions along the pore depth: X. Badel, KTH, Stockholm - No B in the substrate (profiles c, g). Walls fully doped. - [B] in pores < [B] in a planar wafer (about instead of cm -3 ).

 Results: Doping by diffusion 3. Results Diffusion 2: 1050ºC, 1h10’. SIMS profiles at different positions along the depth: X. Badel, KTH, Stockholm - [B] in pores  [B] in a planar sample; no significant variation along pore depth. - Boron atmosphere in the pores maybe more uniform at 1050ºC than at 1150ºC. - Boron layers on each side of the walls.

 Results: Doping by LPCVD 3. Results On a DRIE matrix: On a EE matrix, close to a defect: - Deposition on the DRIE matrix seems to be conformal. - Deposition is disturbed by defects of the walls. - SIMS measurement on a planar wafer: Na= cm -3 ; thickness=400 nm. X. Badel, KTH, Stockholm

 Results: Doping by LPCVD 3. Results SCM at a pore bottom of a DRIE matrix after deposition: typical signature of a pn junction SCM AFM A Profile along A X. Badel, KTH, Stockholm

 Results: Detector efficiency 3. Results Calculated efficiency for depth=300 µm and wall=4.1 µm : 60%. X. Badel, KTH, Stockholm “Ideal” matrix: Pore spacing = 50 µm; Pores as deep as possible; Trade-off on the wall thickness: CsI(Tl) Si B: poly-Si CsI(Tl) Si B: poly-Si

 Summary 4. Summary X. Badel, KTH, Stockholm 1. Diffusion - Transport of boron into the pores is limited at high temperature (diffusion at 1150°C for 1h45’). - Doping improved in the case of diffusion at lower temperature (1050°C for 1h10’). - p+/n/p+ structure in the walls revealed by SIMS, SEM and SSRM. 2. LPCVD - Homogeneous coverage of the pore walls. - Presence of the pn-junction revealed by SCM. 3. Next - Need of contacts on the p + layers for I-V characterization and final detector. - Expected efficiency of about 60%.