The Pinned Photodiode

Transcription

1 1 FEE (Front End Electronics) 2016 June 2, 2016 The Pinned Photodiode Nobukazu Teranishi University of Hyogo Shizuoka University RIKEN

2 Image Sensor (IS) Market 2 - IS sales amount has grown mainly by camera phone in this 10 years. Camcorder But, it became diminished in Q4, PMPTV - IS spreads into various applications, DSLR Others includes scientific, Compact DSC Game industrial, Automotive Bpcs Security 4 PC/WEB Cam Sales amount CMOS image sensor CCD image sensor Tablet Camera phone Medical Broadcast Others Year Application (2014) (Source: TSR)

3 Pixel Shrinkage Trend 3 Minimum Pixel Area (um 2 ) Shrinkage speed becomes slower recently. In 2015, 1 um pixel began to be mass produced Microlens Inner Microlens Shifted Microlens Lightpipe Pinned PD (PPD) 50% shrinkage in 3.5 years Mass Production Year CCD CMOS BSI 10 Stack DTI 15

4 Contents 4 1. Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

5 Contents 5 1. Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

6 PPD Structure and Advantages 6 Pinning PPD TG FD Layer x x x xx P + P+ N N P + P-Well 0V Potential Signal N-Substrate x: GR center OFF ON 1. The P + pinning layer prevents the interface from being depleted, and stabilizes the PD electrically. Low dark current Large saturation High sensitivity Electronic shutter 2. Complete charge transfer No image lag No transfer noise

7 Contents 7 1. Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

8 Potential Cause of Image Lag in Conventional PN PDs (1) TG PD FD P + N+ N + P + Subthreshold current P V PD ψ TG 0V log I The driving force is ψ TG V PD, or V GS. V (ψ TG : TG channel potential) Step 1. At first, TG operates in the saturation region. Step 2. A few ns later, it enters the subthreshold region. GS (m = 1 + C D / C G ) (~ VPD ) TG V T 8 The subthreshold region causes image lag and transfer noise.

9 Causes of Image Lag in Conventional PN PDs (2) 9 Time evolution of V PD is governed by the equation of continuity; is derived as C PD : PD capacitance m: 1+C D /C G I 0 : constant The n th frame lag, n lag (n), is obtained with = when >>1 and n>>1 (n sig : signal electron number)

10 10 Image Lag in Conventional PN PDs (3) Saturation 1st Frame 2nd Frame Long tail image lag 3rd Frame Frame Number The subthreshold model matches the measurements! (N. Teranishi et al., IEDM, 1982)

11 Potential Causes of Image Lag in PPDs (1) TG PPD x P + x x FD P+ N + P + N P - - (A) Small electric field C Barrier - - On On (B) Barrier at the PD edge 0V Off On (C) Pocket at the TG edge - Off - On (D) Pump back when the signal is large - - (E) Traps at the TG interface On the next slide. 11

12 Causes of Image Lag in PPDs (2) 12 (E) Traps at the TG interface If the electron transfer path touches the interface, some electrons are captured by traps. - Some of them are detrapped in the following frames, causing lag. - Some of them are annihilated, causing non-linearity. TG PPD x P + x x FD P+ N P + N P x : Traps at the TG interface Output electrons Ideal case With traps Signal electrons at PPD - The signal electron annihilation exhibits this kind of non-linearity. - A buried transfer path is needed to suppress these phenomena.

13 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

14 Transfer Noise in Conventional PN PDs (1) 14 (1) Average Noise The equation of continuity is : Noise, δ (3) The procedure to calculate the transfer noise is Step 1: Obtain V PDa (t). Step 2: Obtain V n (t). Step 3: Obtain the variance, <V n2 >. + (2) C PD : PD capacitance m: 1+C D /C G I 0 : constant

15 Transfer Noise in Conventional PN PDs (2) 15 Transfer noise, <V n2 >, is obtained as (4) Not an exponential decay, and the convergence is slow.

16 Transfer Noise in Conventional PN PDs (3) 16 When (5) Caution: - This convergence is very slow, and the initial noise decay is also slow. - If the TG ON period is 1 μs, we should not use this limit. We should use (4) and calculate the value at t = 1 μs, considering the initial condition.

17 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

18 Dark Current Reduction Mechanism by SRH (1) 18 Schockley-Read-Hall Process pn n U v N i th t E n p 2ni cosh Assuming 1. If depleted, n, p n i U v th U v N t th 2n N t i n p 2 ni E cosh 2 t 2 E kt 2. If not depleted, p n i n, U N pn n p 2 i v th i N t t E kt When E t = E i where U is maximum, then, n i (2) v th t n 2 i p i (3) (1) PPDs configure this non-depleted situation! U: Recombination Rate (Sze: Semiconductor Devices, Chap. 1 Eq.(59)) Large dark current! Small dark current!

19 Dark Current Reduction Mechanism by SRH (2) 19 (1) Estimate the interface dark current reduction ratio, assuming that: - Hole density (p) at the P + pinning layer: cm -3 - Intrinsic carrier density, n i : cm -3 U U Not depleted Depleted v v th th N N t t n n i 2 i 2 p p 2n i ~ 10 7 (2) Dark current comparison by image sensors. Non-PPD (1982) PPD (2012) Unit Scheme CCD FSI CMOS Pixel size 23 x x 1.12 μm Dark current 1, e - /s/μm 2 at %

20 Example of Dark Current Reduction (1) 20 If the dark current is reduced, the dark current FPN and dark current shot noise will also be reduced. Conventional PD PPD The dark current FPN is suppressed, therefore, picture quality is much improved.

21 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

22 A Question About the Dark Current Reduction Mechanism 22 The N-type PD is depleted. Incident Light TG P + N N PD FD Even if the P+ pinning layer neutralizes the interface states, the N-type PD is still depleted nearby the P+ pinning layer. The assumption of spatial uniformity, which is implicitly used in SRH, is not realistic! To understand the effects and limitations of PPDs, a new, correct model is needed. 4Tr CMOS Sensor Cross Section

23 A New Model Including Non-Spatial-Uniformity 23 Modified Diffusion Current Model: 1-Dim (Along ) Put the GR centers at x= - x GR in the neutralized region. Assume still stationarity, but no more spatial uniformity. No electric field in the neutralized region. Low injection. Use the same notation of Sze s Semiconductor Devices. P + N PD TG N FD P + -Pinning layer Neutralized Depleted Neutralized GR Centers -x GR -x p N-PD x n x V New Model

24 New Diffusion Current Model with GR Center 24 Introduce the GR centers effect into the diffusion equation: 2 n p n p n p0 D ( ( ) 0 (3) n GD 0 ) 2 n n p n p x xgr x n At x = - x GR, the GR centers force n p toward n p0, the equilibrium. G: Intensity of the GR Centers. Unit is 1/cm. GL n is a dimensionless parameter for the GR centers intensity. L : Diffusion Length n D n Boundary Conditions: Same as in the diffusion current model without GR centers n at x p n p0 n p n p0 e qv n kt p (4) at x x (5)

25 Derived Solution 25 Diffusion Current (Dark Current) with GR Centers J ( GR) n ( x ) J ( x ) EDCF p where qd (0) nn p0 qv kt J n ( x p ) ( e 1) Ln Diffusion Current (Dark Current) without GR Centers GLn 1 EDCF ( xgr x p ) Ln GL 1 GL e L n D n n (0) n n EDCF: Extra Dark Current Factor n : Diffusion Length p GL n : Dimensionless Parameter for the GR Centers Strength (6) (7) (8)

26 Characteristics of the New Diffusion Current Model 26 When (x GR x p )/L n = 0, GR centers become not neutral; EDCF = GL n + 1 EDCF When GL n, then, EDCF 1 e No divergence; instead, saturation. Temperature dependence: ( GR) x GR -x p L n = 0 J n J ( x (0) n 1 GR x e E p g ) L kt n GR Centers Intensity, GL n When GL n 0, J n (GR) J n (0) Reasonable When (x GR x p )/L n, EDCF 1. The GR centers effect becomes negligible.

27 Characteristics of New Diffusion Current Model (2) 27 When (x GR x p )/L n 0, EDCF increases, because the GR centers position approaches the depletion region. EDCF GL n = GR Centers position, (x GR x p )/L n When GL n 0, J n (GR) J n (0) Reasonable. When (x GR x p )/L n, EDCF 1. The GR centers effect becomes negligible.

28 Is the P + Pinning Layer Thickness Sufficient? 28 How large is the diffusion length, L n, in the P + pinning layer? The surface dead zone depth, L 1, might be a good alternative for L n. L 1 is derived from the spectral response, to be ~0.08 μm. P + pinning layer thickness μm The GR centers at the silicon surface possibly contribute to the dark current! We should reduce the GR centers. (SONY ICX658ALA data sheet Pixel size: 6.35 x 7.4 um) Surface dead zone Sensitive zone (Depleted zone) Incident light xx L 1 L 2 Depth Deep dead zone (PD thickness is limited by P + substrate, or VOD barrier) x: GR center x Light intensity

29 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

30 Macroscopically Flattening 30 Itonaga et al. (Sony), IEEE IEDM, 2011 No isolation grooves/ridges and no substrate etching as in STI Less process damage, less stress and no STI side surface. STI Structure of FLAT, comparing with STI Dark Current

31 Atomically Flattening 31 Kuroda et al. (Tohoku Univ.); Highly Ultraviolet Light Sensitive and High Reliable Photodiode with Atomically Flat Si Surface Atomically Flat (100). Atomic step is 0.135nm. N + PN PD - Low Dark Current, High QE for UV at PD. - Low 1/f noise at MOS Tr. Atomically flat surfaces reduce GR centers/traps. Typical (100) after RCA Cleaning. AFM Images

32 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Conclusion

33 Vertical Overflow Drain (VOD) Shutter 33 For Anti-blooming and electronic shutter The VOD is used in CCD image sensors TG is used as LOD (lateral overflow drain) in CMOS image sensors. 1/60 s 1/125 s 1/250 s Blooming 1/500 s 1/1000 s 1/2000 s Electronic Shutter (Object is rotating at 720 rpm.)

34 VOD Structure and Mechanism 34 Pinning Layer PPD TG VCCD P + P+ e - N N-PD P + P-Well(Barrier) e - P + Pinning Layer N-PD C 1 P-Well (Barrier) N-Substrate V Well Excess electrons N-Substrate V PD All electrons Low Voltage Pixel cross section High Voltage Potential profile along the blue line

35 High Speed Shutter (1) 35 Definitions High speed shutter: Short exposure time / sharp shutter High speed camera: High frame rate Motivations of high speed shutter High speed motion capture, ToF, fluorescence life time imaging. Replace the streak tube and gated image intensifier. Shutter speed is limited by: (1) Photo-generated carrier collection time into the PD storage region. (2) Driving pulse delivery time, C R. (3) Carrier transferring time from the PD storage to analogue memory in the pixel. The VOD shutter mechanism with PPD has a merit on item (2).

36 High Speed Shutter (2) --- Load Capacitance VOD shutter Substrate capacitance K C Si0 Sub d S where, K Si : Si dielectric constant ε 0 : Permittivity in vacuum S: Area, d: distance (depletion thickness) For example, 1/3 inch S = 28 mm 2 d = 7.5 μm C sub = 400 pf LOD shutter Gate capacitance, C gate, + parasitic capacitance of wires, C wire K WL C N SiO2 0 Gate pixel t where, N pixel : Pixel number Ksio 2 : SiO 2 dielectric constant W: Channel width, L: Channel length, t: Gate SiO 2 thickness For example, N pixel = 1.3 M, W = L = 0.4 μm, t = 6 nm C Gate = 1,200 pf C Wire =? The load capacitance of the VOD shutter is smaller than that of the LOD shutter.

37 High Speed Shutter (3) --- Parasitic Resistance 37 Two methods for driving pulse delivery: (a) From the periphery (b) From the backside N-substrate A small parasitic resistance and small variations of the parasitic resistance are achieved with (b) backside feeding. A skew smaller than the measurement accuracy limit (0.2 ns). (E. Tadmor et al., 2014 IEEE Sensors)

38 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Visible Light Photon Counting Image Sensors 8. Conclusion

39 Visible Light Photon Counting Image Sensor 39 SPAD (Single photon avalanche diode) 4-Tr CMOS + High conversion gain + CMS (Correlated multiple sampling) n=1 n=2 n=3 n=0 n=4 128 samplings (N. Dutton et al., VLSI Symposium 2014) n=5 n=6 n=7 QVGA (320x240 pixel) SPAD, 20 fps, (MW. Seo, S. Kawahito et al., IEEE EDL 2015) at room temperature, at night In 2015, several organization reported High avalanche gain makes following low noise < 0.3 e- rms. circuit noise negligible. ref. DEPFET (Max Plank) uses CMS. Large dark count. Small fill factor

40 Contents Introduction 2. Pinned Photodiode (PPD) Structure and Effects 3. Image Lag 4. Transfer Noise 5-1. Dark Current Reduction 5-2. New Diffusion Current Model Including Non-Uniformity 5-3. Recent Approaches for Dark Current Reduction 6. Vertical Overflow Drain (VOD) Shutter with PPD 7. Conclusion

41 Conclusion The PPD is a primary technology for CCD and CMOS image sensors. It exhibits low noise, low dark current, no image lag, large saturation, high sensitivity, and allows electronic shutter operation. 2. Conventional non-ppds have long tail lag and transfer noise. 3. A new diffusion dark current model considering the GR centers is proposed. If the P+ pinning layer is thin compared with diffusion length, they contribute to the dark current. GR Eg kt The temperature dependence is J ( ) e. 4. Both macroscopiccally and atomically flatness of the silicon surface reduce the dark current. n 5. VOD shutters with PPDs are capable of high speed shutter operation.