SuperNova Acceleration Probe Research and Development Efforts

SuperNova Acceleration Probe Research and Development Efforts
Chris Bebek UC Berkeley Lawrence Berkeley National Laboratory

Instrument R&D areas In the past year we have conducted a technical and scientific trade study covering a range of options for the SNAP instrumentation suite. We have arrived at a coherent instrument working concept and observation strategy constrained by reliability, satellite, thermal, orbit and telemetry issues described Mike Lampton and SNe characteristics that optimizes the science reach of SNAP. We have identified four risk areas and have an R&D program to address these: CCDs MTF/PSF Cold irradiation Fabrication HgCdTe QE Readnoise CCD readout HgCdTe readout 7 May 2002 Chris Bebek

How Science-Driven Requirements map onto Instrument Concept
Measurement Program ~50 Type Ia SNe per 0.03 in z from z=0.3 to 1.7 (2500 total). Follow up spectroscopy near peak luminosity. Template building (spectra vs époque for subset of z<0.7 SNe). Photometry U, B, V, (R)-band light curves. B-band to 1% at peak. B–V color evolution. Malmquist bias. Rise time. Peak to tail ratio. Spectroscopy UV metalicity features – strength and location. S and Si features SII 5350Å line, Dw = 200Å SII “W” shape, Dw = 75Å SiII 6150Å line, Dw= 200Å Ejecta velocity, Dl >15Å Calibration Instrument Use two plate scales to cover the wavelength range to obtain time efficient photometry and a large FOV. Observation cadence commensurate with SNe evolution. Allocation of time for photometry and follow up spectroscopy. Imager Wavelength coverage from 350 nm to 1700 nm. 9 filters. Cadence of repetitive exposures matched to SNe evolution rate. Exposure lengths generate required S/N versus magnitude. Spectrograph S/N = 20 Resolution ~ 140 (l/dl) 7 May 2002 Chris Bebek

Photometry illustration
1+z spaced B-band filters 7 May 2002 Chris Bebek

Spectroscopy illustration
Metalicity SII “W” SiII 7 May 2002 Chris Bebek

Instrument working concept
Guiders Cold plate Radiator Cables Thermal links Shield Spectrograph CCDs/ HgCdTe Shutter Filters FE Electronics Data/Monitoring/ Command 7 May 2002 Chris Bebek

Focal plane concept Coalesce all sensors at one focal plane.
Imager sensors on the front. 36 HgCdTe 2kx2k 18 mm 36 CCD 3.5kx3.5k 10.5 mm Filters 1 of 3 per HgCdTe 4 of 6 per CCD Spectrograph on the back with access ports through the focal plane. Common 140K operating temperature. Dedicated CCDs for guiding from the focal plane. Exposure times of 200 s with four/eight exposures in CCDs/HgCdTe. 20 s readout slow enough for CCD noise and 4 post exposure and 4 pre exposure reads of HgCdTe. rin=6.0 mrad; rout=13.0 mrad rin= mm; rout= mm 7 May 2002 Chris Bebek

Integral Field Unit Spectrograph Design
Slit Plane Detector Camera Prism Collimator SNAP concept: Small IFU, 2-arm spectrograph based on image slicer work performed for NGST. Image slicer 7 May 2002 Chris Bebek

Example scan 7 May 2002 Chris Bebek

Establishing detector requirements
Some sensor requirements are determined by SNe characteristics, eg, wavelength coverage. Some characteristics are innate to the sensors, eg, QE(l). Other requirements (eg, read noise, dark current) have been bounded by studying the desired S/N as a function of SNe epoch, exposure time, and number of exposures. Example study of achieved S/N with four 200 s exposures and: CCD RN = 4 e- DC = 0.02 e-/s/pix Npix = 4 HgCdTe Requirement S/N>30 Dmag S/N>30 S/N>20 S/N>15 S/N>10 S/N>3 7 May 2002 Chris Bebek

Derived Requirements for the Imager
Visible (CCD) NIR (HgCdTe) Units FOV 0.34 deg2 Plate scale (nominal) 0.10 0.17 asec Pixel size 10.5 18 mm Wavelength nm Quantum efficiency 70 65 % Operating temperature 140 K Read noise (multiple reads) 4 5 e Dark current 0.08 0.02 e/s/pixel Diffusion Radiation tolerance1 TID NIEL 2 2x107 krad MeV/g Note: Requirements for spectrograph use are similar. 7 May 2002 Chris Bebek

LBNL CCD work

LBNL CCD technology Back-illuminated thick CCD on a high-resistivity n-type substrate, operated fully depleted. Advantages: 1) Conventional CMOS processes without super thinning. 2) Full quantum efficiency to >1 mm => no fringing. 3) Good blue response with suitably designed rear contact. 4) No field-free regions for charge diffusion, good PSF. Drawbacks: 1) Enhanced sensitivity to radiation (x-rays, cosmic rays, radioactive decay). 2) More volume for dark current generation. 3) Dislocation generation. The technology has been transferred to DALSA and “rad hard, p-channel, high resistivity capability” is now listed on their web site. 7 May 2002 Chris Bebek

LBNL CCD evolution 100 mm LBNL-fabbed 2kx2k 100 mm DALSA 2kx2k
150 mm DALSA PIN diodes 150 mm DALSA 2kx2k & 3kx3k 7 May 2002 Chris Bebek

Near-IR vs Visible imaging
WIYN 3.5m with LBNL 2048 x 2048 CCD (Dumbbell Nebula, NGC 6853) Blue: H- at 656 nm Green: SIII at 955 nm Red: 1.02 mm 7 May 2002 Chris Bebek

LBNL 2k x 2k results Image: 200 x m LBNL CCD in Lick Nickel 1m. Spectrum: 800 x m LBNL CCD in NOAO KPNO spectrograph. Instrument at NOAO KPNO 2nd semester 2001 ( 7 May 2002 Chris Bebek

CCD issues Fabrication Pixel size Well depth Linearity Dark current
Persistence Read noise MOSFET operation Charge transfer efficiency Quantum efficiency Diffusion Intrapixel response Radiation Proton damage 60Co Damage when cold Fabrication 7 May 2002 Chris Bebek

Performance Pixel size Well depth Linearity Dark current Persistence
Read noise MOSFET operation Charge transfer efficiency Quantum efficiency Diffusion Intrapixel response Radiation Proton damage 60Co Damage when cold Fabrication 10.5 m work. 130 ke for 10.5 m pixel. Better than 1%. 2-5 e/hr/pixel. Erase mechanism is effective. e. Documented at operating temperature. CTI ~ 10-6 pre-irradiation. Extended red performance realized. On-going study. More robust than existing space devices when damaged warm. No surprises for T=300K dosing.(won’t discuss today). An activity during the next 3 months. Partially commercialized. R&D areas 7 May 2002 Chris Bebek

LBNL 2k x 2k Quantum Efficiency
7 May 2002 Chris Bebek

MTF/PSF issues We are ~2x undersampled.
Uniformity and simplicity of PSF determines the amount of image dithering that will be required. Diffusion – the fundamental ~gaussian spread of charge as it drifts from the photon conversion site to a pixel. For conventional CCDs, the rms spread is the depletion depth, ~10 mm. For LBNL CCDs, the rms spread is determined by thickness and the sq.rt. of the depletion voltage. We require 4 mm. E.g., t = 200 mm, Vsub = 100V, s = 3.2 mm. We are working on routine thinning to 200 mm and beginning the study of device robustness versus Vsub (we have routinely operated in the lab at 60V with excursion to 140V). Intra-pixel response – after diffusion drift, does the charge hit the “correct” pixel. 2D modeling of conventional CCDs and LBNL CCDs with its substrate voltage shows good termination of the field lines on pixels. Measurements – we are commissioning a pinhole projector to measure diffusion as a function of voltage and thickness and to map intrapixel response. 7 May 2002 Chris Bebek

Diffusion/intrapixel response measurements
We are commissioning a pinhole projector, ~4 m FWZ, to scan the backside of the CCD looking for charge collection variations in vicinity of pixel edges. We have scanned the front side so far and see the polysilicon gate structure. Backside scans any minute now. Front-side scans 7 May 2002 Chris Bebek

Proton radiation damage

Radiation environment
Integrated for three years, SNAP will be exposed to: A few krad (Si) TID. A few 107 MeV/g NIEL. Note: 1x MeV is 1.5x107 MeV/g NIEL. 1x MeV is 500 rad. 7 May 2002 Chris Bebek

Proton irradiation studies
We used 12 MeV protons at the LBNL 88” Cyclotron Two set of four device were irradiated at room temperature. Doses were 5x109, 1x1010, 5x1010 and 1x1011 p/cm2. We characterized the devices by measuring their CTE and dark current as a function of temperature. 12 MeV protons onto CCDs 7 May 2002 Chris Bebek

Dark Current Degradation
Dark current is measured with one thousand or more second exposures. The gaussian charge distribution in the active region of the CCD is compared with the gaussian change distribution in the overscan region. SNAP Fit gives expected Si bandgap, so no new dark current sources are developing. The plateau at right is not identified yet, but could be surface leakage currents. 7 May 2002 Chris Bebek

Charge transfer efficiency
CTE is measured using the 55Fe X-ray method at 128 K. The readout speed is 30 kHz, the X-ray density is 0.015/pixel. Degradation is about 110-13 g/MeV. Comparison to conventional CCDs after converting dose to NIEL (MeV/g). SNAP Caveat. We irradiated parts at 300K and unpowered. While we have compared apples with apples, our study will be complete only after performing damage at operating temperature and powered. Will try to complete this this summer. [1]L.Cawley, C.Hanley, “WFC3 Detector Characterization Report #1: CCD44 Radiation Test Results,” Space Telescope Science Institute Instrument Science Report WFC , Oct.2000 [2] T. Hardy, R. Murowinski, M.J. Deen, “Charge transfer efficiency in proton damaged CCDs,” IEEE Trans. Nucl. Sci., 45(2), pp , April 1998 SNAP 7 May 2002 Chris Bebek

CTE vs Temperature at 1x1011 p/cm2
Both serial and parallel CTE exhibit significant temperature dependence due to interactions with radiation induced trapping centers. 7 May 2002 Chris Bebek

Hole Traps Found in n-Type Si
Trap parameters measured using DLTS V V + V VV Proton Irradiation CiOi Sii Ci CiCs 7 May 2002 Chris Bebek

Fitted trap density versus dose
7 May 2002 Chris Bebek

CCD fabrication

100 mm wafer fabrication LBNL manufactured
We have fabricated 10.5, 12, and 15 mm devices in a variety of formats up to 2kx4k. These have ranged from 190 to 300 mm thick. Some of these devices are deployed in ground telescope. Recently, much effort has gone into developing careful handling procedures and equipment modifications to protect the backside of the wafer during manufacture. DALSA manufactured Our process technology transfer first done here. 15 mm devices up to 2kx2k have been successfully built. Devices as thin as 200 mm have been finished. 7 May 2002 Chris Bebek

150 mm wafer fabrication DALSA work
They have converted exclusively to 150 mm wafers. These wafers are must be thinned from ~675 mm to 200 mm for our use. Unthinned photodiode wafers have been fabricated with good results. A few thinned wafers have been fabricated. We found similar backside “damages” areas that we have already eliminated at LBNL. Unthinned CCD wafers have been fabricated that were of high quality in front illuminated studies. We have received one thinned (300 mm) CCD wafer that is now under backside illumination tests. LBNL work We are gradually transferring our backside handling knowledge to DALSA but expect this to take some time to fully implement. We view this as the second phase of our commercialization effort. In the meantime, we have acquired the one piece of 150 mm processing equipment that will allow us to perform the last steps of wafer processing – contacts, metalization, AR coating. DALSA will provide 675 mm thick CCDs where the front side is complete, device is thinned, and backside thin poly is deposited, This includes all the conventional CMOS process steps. We will continue to work with them on thinning issues with fully automated processing equipment. 7 May 2002 Chris Bebek

Example of backside “damage/particles” before remedies
MTI resist dispense chuck Aligner handler cups PE641 aligner chuck (waffle pattern) 7 May 2002 Chris Bebek

LBNL efforts on backside particles/damage
Backside processing issues and remedies to be transferred to DALSA. Back side scratches through ISDP layer fatal for fully-depleted operation Avoid where possible handlers made from materials that can scratch silicon Improved wafer carriers for MRC sputtering system Manual override of wafer alignment arm on MTI resist dispense arm Use of sacrificial SiO2 layer on wafer backside Not scratch immune but allows for undercut of particles during strip Particle removal via wafer scrubbing (most effective technique to date) Use of wear resistant materials on vacuum chucks and wafer handlers where possible (DuPont VESPEL effective but does shed particles) Avoid use of silicone parts (cannot remove with scrubbing) First wafers through equipment (coater, aligner) tend to have significantly higher particle counts Photoresist aerosol particles – too large to be removed with ashing, require addition of solvent to scrubbing soap solution LBNL experience: particles can be removed with mechanical action (scrubbing). Main concern is damage through thin backside poly layer. 7 May 2002 Chris Bebek

HgCdTe

Rockwell HgCdTe Rockwell HgCdTe devices are our only option at the moment. WFC3 MBE material with 1.7 mm cutoff is a perfect match to SNAP. NGST 2k x 2k format being developed is also a good match. Status (as of March) The dark current is OK. There is a QE “problem” in the 900 nm to 1100 nm region. There is a large read noise, ~30 e, not the design goal of 10 e. Long-term drifts and settling times are seen at some test sites. Rockwell claims they understand the MBE knobs that control QE. They are have grown new material (1k x 1k). It is presently being bumped and packaged. The large read noise is bad for SNAP We want to rt-N this down to 5 e, ie, four CDS reads take 10 e to 5 e. More reads have a big impact on observation time budget. Rt-N has only got to 17 e so far (there may be new info on this). 7 May 2002 Chris Bebek

MTF/PSF issues We have studied the impact of a “gutter” around each pixel as existed in the PACE devices (this is NOT present in the MBE devices). The impact of that dead region relative to a device without it was to double the number of exposures required to obtain equivalent photometry. Intrapixel response for the MBE HgCdTe has not been measured yet. We will acquire a device to measure this ourselves. We have ordered a mux to begin setting up a measurement system with pin hole projector. Intrapixel response may be just fine as it is or it may not be. If not, Rockwell has posited: Design changes of the implants near the PN junctions. This is essentially tuning up the electric fields to better capture the charge. Etching microlenses into the CdZnTe substrate to focus photons on the pixel sweet spot. 7 May 2002 Chris Bebek

CCD readout

CCD support electronics
Goals: Photons-to-bits focal plane Eliminate large cable plant to reduce system noise problems. Reduce power dramatically relative to conventional implementation. ASIC Challenges: Large voltages ~10 V clock swings ~20 V MOSFET biases ~32 V span within CCD, excluding depletion voltage Large dynamic range from 2 e- readnoise and 130 ke- well depth. Radiation tolerance (borrowed from GLAST for now) Total ionizing radiation dose: performance maintained up to 10 Krad (Si). Single-Effect Latch-up (SEL): immune to a minimum LET of MeV-cm2/mg. Singe-Effect Upset (SEU): performance maintained for a LET of at least 8 MeV-cm2/mg. Operation at 140K to reduce cable plant and associated problems – requires low power 7 May 2002 Chris Bebek

Readout Electronics Concept
CDS – Correlated Double Samples is used for readout of the CCDs to achieve the required readout noise. ADC – 16-bit dr, 12-bit res 100 kHz equivalent conversion rate per CCD. Sequencer – Clock pattern generator supporting modes of operation: erase, expose, readout, idle. Clock drivers – Programmable amplitudes. Supports 4-corner or 2-corner readout. Bias and power generation – Provide switched, programmable large voltages for CCD and local power. Temperature monitoring – Local and remote. DAQ and instrument control interface – Path to data buffer memory, master timing, and configuration and control. 7 May 2002 Chris Bebek

ASIC roadmap We are working with LBNL ASIC designers to address CCD clock generation, bias voltage generation, and analog signal processing in one or more ICs. Correlated double sampler Starting here since it has the most challenging analog issues. We have performed a survey of sub-micron CMOS processes. We have evaluated system noise for different technologies and signal processing schemes. We have measured pre and post irradiated test structures as a function of T. We will design a CDS circuit for fabrication over the next 4 months. ADC This could be part of CDS circuit, so we are thinking of implementations in parallel with CDS development. We are exploring a 12-bit pipeline ADC with three ranges. Clock drivers Pattern generator is an “easy” digital design. Amplitude control of large voltages will be challenging. Study of rad tolerance of 40 V sub-micron CMOS. 7 May 2002 Chris Bebek

CCD MOSFET noise Measured noise spectral density at low temperature for an LBNL CCD MOSFET. 7 May 2002 Chris Bebek

Noise comparison (PMOS)
PMOS noise spectral density for several vendors derived from their technology models. Model results have been validated by test data from others. 7 May 2002 Chris Bebek

Differential averager
Pmos Agilent 0.5mm IC = 1, Id = 100mA Nmos TSMC 0.25mm IC=0.1, Id = 100mA CCD noise + first stage noise C (100pF) Dt R t Out X1 CCD noise source (Noiseless) t -X1 Integrator voltage gain: 2 Integration time t Input referred noise (e) Conversion gain: 3.5mV/e Dt = 0 4ms (R=20KW) (1) 2.8 / (2) 2.9 t t 8ms (R=40KW) (1) 2.1 / (2) 2.2 10ms (R=50KW) (1) 1.92 / (2) 2 The 1/f noise of the input stage is reduced by the CDS The thermal noise from the input stage is negligible compared to the CCD 7 May 2002 Chris Bebek

TSMC 0.25 mm cold CMOS Threshold voltage NMOS Threshold voltage PMOS
~1mV/K ~1mV/K Mobility ratio NMOS After rad means >10 Mrad. Mobility ratio PMOS UTE = -1.3 UTE = -0.7 7 May 2002 Chris Bebek

Sub-micron CMOS comments
Sub-micron CMOS appears to perform well down to 100K. Vendor BSIM3 SPICE model predicts performance down to 150K region. Technology is extremely rad hard. LBNL has the “rad hard by design” methodology to build robust systems. As with CCDs, rad testing at cold temperature needs to be explored. Sub-micron 40V processes need careful radiation study because of the thick oxide used. [Our default plan is to use external JFETs as voltage boosters for clock and bias drivers.] 7 May 2002 Chris Bebek

HgCdTe readout

HgCdTe readout Inputs to readout architecture
Advertised single CDS noise is to be 10 e; we require 5 e. Therefore, we need four pre and four post exposure reads. To accomplish 8 reads in 20 s at 100 kpixel/s rate requires 16 taps per 2kx2k device. H2-RG has 32 taps. Desirable to do pre and post read averaging in hardware. There is a Rockwell initiative for the NGST mux readout for an ASIC operated cold adjacent to the sensor Five 16-bit ADCs (intended for 4 tap readout) Microprocessor based timing sequencer Data processing Can implement co-adding and averaging at the pixel and line level. Power including mux is ~2 mW per read port. R&D issues Trade study of using existing H2-RG mux or developing one with 16 taps. Ability to cascade multiple Rockwell ASICs to achieve more ADCs per H2-RG. Development of our own ASIC. 7 May 2002 Chris Bebek

R&D Summary CCD Study operation at high depletion voltage to minimize diffusion. Measure intrapixel response. Radiation damage at 140K. Refine fabrication process. Establish “production yield.” HgCdTe Read noise needs to be reduced. Track QE developments. CCD electronics Radiation measurements at 140K. Fabricate demonstration CDS/ADC in 0.25 mm CMOS during the next year. Radiation study of 40V sub-micron CMOS. HgCdTe electronics Refine commercial solution, if it exists, to SNAP needs. Or, develop our own readout. 7 May 2002 Chris Bebek

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