SWIR InGaAs focal plane arrays in France

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1 SWIR InGaAs focal plane arrays in France A. Rouvié 1, O. Huet 1, S. Hamard 1, JP. Truffer 1, M. Pozzi 1, J. Decobert 3, E. Costard 1 M. Zécri 2, P. Maillart 2, Y. Reibel 2, A. Pécheur 2 1 SOFRADIR, Campus de Polytechnique, 1 avenue Augustin Fresnel, Palaiseau, France 2 SOFRADIR, Actipole, 364 Route de Valence B.P.21, Veurey-Voroize, France 3 III-Vlab, Route de Nozay, Marcoussis, France ABSTRACT SWIR detection band benefits from natural (sun, night glow, thermal radiation) or artificial (eye safe lasers) photons sources combined to low atmospheric absorption and specific contrast compared to visible wavelengths. It gives the opportunity to address a large spectrum of applications such as defense and security (night vision, active imaging), space (earth observation), transport (automotive safety) or industry (non destructive process control). InGaAs material appears as a good candidate to satisfy SWIR detection needs. The lattice matching with InP constitutes a double advantage to this material: attractive production capacity and uncooled operation thanks to low dark current level induced by high quality material. The study of InGaAs FPA has begun few years ago with III-VLab, gathering expertise in InGaAs material growth and imaging technology respectively from Alcatel-Lucent and Thales, its two mother companies. This work has led to put quickly on the market a 320x256 InGaAs module. The recent transfer of imagery activities from III-VLab to Sofradir allows developing new high performances products, satisfying customers new requirements. Especially, a 640x512 InGaAs module with a pitch of 15µm is actually under development to fill the needs of low light level imaging. Keywords: InGaAs, SWIR, VisSWIR, VGA 1. INTRODUCTION Short Wavelength Infra Red (SWIR) imaging presents a tremendous interest in various applications such as night vision systems, security surveillance systems, enhanced vision detectors for safety systems, passive/active imagery cameras for laser designators and hot spot or see-spot detection. Most of these applications are based on an extended spectral range revealing specific features due to unusual contrasts in visible. Among the available technologies able to detect wavelengths from visible to SWIR range, InGaAs material appears as an excellent candidate. Indeed, the physical material properties associated to the growth and technical process maturity thanks to telecommunications developments lead to the capability to grow very high quality structures able to work at ambient temperature with dark currents in the femto-amper range. In January 2013, the III-V imaging activities from Thales/III-VLab (QWIP and InGaAs) and from Sagem (InSb) have been transferred to Sofradir, well known for its leadership on MCT infrared technology. As a consequence, Sofradir portfolio of technologies now offers an unbeatable breadth of solutions addressing any customer need from visible to very long-wave infrared. Concerning InGaAs SWIR activity, stimulated by the success of Cactus modules (320x256@30µm and 640x512@25µm), the new synergies have lead to the development of an InGaAs VGA module with a pitch 15µm filling the needs of customers, called SNAKE640. This paper firstly describes the global structure of our focal plane arrays and detailed some specific performances such as dark current, quantum efficiency and FTM. In the second part, we present our well established products (CACTUS family) and also the last new comer 15µm.

2 2.1 Structure and implementation Detection array 2. TECHNICAL DESCRIPTION The developed design is common for all array formats. The active region is composed by PIN photodiodes and surrounded by common N-contact pixels to apply uniform polarization voltage. The typical PIN InGaAs/InP heterostructure is grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on 3 and 4 inches n+ type InP substrates. Zn-diffusion is achieved through circular apertures in the silicon (SiN) layer, creating p+ regions in the n-inp window layer. The planar junction combined to the SiN passivation layer ensures high stability and reliability to the structure. Common contact Active region Common contact Window layer InP n- SiN passivation layer Absorption layer N-contact layer InGaAs n- InP n+ Zn-diffused region Figure 1: Views of the 640x512 array and schematic cross section of InGaAs photodiodes Thanks to the high material quality, carriers diffusion lengths are superior to pixel dimensions. Consequently, under reverse bias voltage, although electric field is concentrated below the diffusion region, all carriers which are photogenerated in the InGaAs active layer are collected. High filling factors of 100% are then achieved. Simultaneously, as minority carriers concentration gradually increases from depleted region to the interpixel area, crosstalk effect is naturally limited and large FTM values are guaranted. Hybridization process Detection arrays are individually hybridized to Read Out Integrated Circuit (ROIC) with an Indium bump based process. As shape and size uniformity of Indium bumps are key parameters to optimize the hybridization process and then obtain high connection pixel yields, a special attention has been given to optimize the technological process, particularly for 15µm pixel pitch arrays. The resulting Indium bumps are presented on Figure 2 for 30µm and 15µm pixel pitches.

3 30µm pixel pitch 15µm pixel pitch Figure 2 : 30µm and 15µm-pitch Indium bumps Thanks to the reflow process, high quality hybridization is possible as illustrated on Figure 3 which represents 30µm and 15µm pitches hybrids microsections. On these SEM pictures, we can see the Indium bumps connecting ROIC pixels to photodiodes from detection array. Excellent pixel connection yields higher than 99.9% are then achieved. InP detection array InP detection array Indium bump Indium bump Si ROIC Si ROIC 30µm pixel pitch 15µm pixel pitch Figure 3 : MEB image of 30µm and 15µm-pitch hybrid microsection Following hybridization step, detection arrays are then mechanically thinned. An adapted antireflection coating is deposited to optimize quantum efficiency. This step will be detailed in paragraph Performances Dark current Since the beginning of InGaAs array development, a particular work has been dedicated to understand the origin of dark current in photodiodes structures. In good agreement with literature [1]-[2], temperature measurements coupled to geometrical studies have lead to the following conclusions. When the crystalline quality is not optimized, InGaAs material presents a high defects or dislocations density, through which carriers can be generated. In this case, dark current is linked to a Shockley Read Hall (SRH) mechanism which takes place in the depleted volume of the reverse biased junction. Dark current is therefore proportional to the photodiode area (Figure 4).

4 I dark photodiode area p+ n-inp Depleted region Volume SRH mechanism n-ingaas n+inp Figure 4 : dark current origin for medium quality material When the crystalline quality is optimized, the contribution of volume SRH mechanism is negligible. For low reverse bias voltages (<0.5V), dark current is due to the diffusion current of minority carriers (holes) from the InGaAs volume toward depleted region; dark current is here proportional to the photodiode perimeter. For high bias voltages (> 2V), dark current is associated to surface SRH mechanism which takes place at the interface between InGaAs and InP window layer. Due to atomic exchanges between P and As during epitaxial growth [3], this interface presents defects through which carriers can be generated when local electric field is applied. In this case, dark current has also perimeter dependence (Figure 5). I dark photodiode perimeter p+ p+ n-inp Diffusion current Depleted region Surface SRH mechanism n-ingaas n+inp Lowbiasvoltages < 0,5V Biasvoltages > 2V Figure 5 : dark current origin for high quality material For the growth of InGaAs arrays, InP/InGaAs interfaces have been designed to limit atomic substitution and then defect density in the InGaAs bulk layer. Dark currents measured on test cells for photodiode diameters varying from 4µm up to 18µm demonstrate clear perimeter dependence and activation energies confirm the diffusion mechanism for low bias voltages and SRH generation mechanism for high bias voltages. For classical 6µm-diameter PIN photodiodes, dark current is found to be as low as 11fA per pixel at -0.1V and ambient temperature. The temperature decrease to 0 C, which can easily be managed with a ThermoElectric Cooler, enables to reach 1fA per pixel at -0.1V. Quantum efficiency and visible extension Classical back side illuminated InGaAs detectors are sensitive in the SWIR spectral band from 0.9µm to 1.7µm. The cut on wavelength is imposed by the absorption of the thick InP substrate. To limit the absorption of visible photons, InP substrate can be thinned by mechanical and chemical processes down to few hundred of nanometers (Figure 6). This technique allows visible photons to reach InGaAs absorption layer and then to be detected.

5 InP InP InGaAs InGaAs InP n+ InP n+ InGaAs InP substrate n+ Figure 6 : Visible extension schematic principle The antireflection coating based on SiO 2 /TiO 2 materials and classically used for pure SWIR detectors has to be adapted to VisSWIR detection band. After optimization, the VisSWIR-coating presents a reflection coefficient lower than 6% over the large VisSWIR spectral range [400nm; 1700nm]. It enables to reach quantum efficiencies as high as 40% at 500nm, 75% at 800nm and over 80% from 900nm to 1600nm (Figure 7). EQ (%) VisSWIR SWIR Lambda (nm) Figure 7 : Measured quantum efficiency on SWIR and VisSWIR FPA To our knowledge, this result represents the higher quantum efficiencies ever reported for VisSWIR structures, particularly in the visible spectral range. MTF measurements MTF measurements have been made following the slanted edge method described in [4] on 15µm pixel pitch module. Figure 8 presents the measurement results, the theoretical MTF for a 15µm pixel pitch detector and the Nyquist frequency defined by f Nyquist = 1/ 2.pitch. The measurement points are not unconvoluted from the optics MTF. On Figure 8, the measurement points are compared with the theoretical MTF of an ideal 15µm pixel pitch detector (blue curve) and of an ideal 15µm pixel pitch detector associated with F/2 optics (orange curve). It appears that our measurement results are close to the theoretical characteristics with a MTF of 0.55 at Nyquist frequency. This point confirms the optimized behavior of our photodiode planar structure.

6 1 0.8 MTF Measurement Detector Optics Det x Opt 0.2 f Nyquist Frequency (cy/mm) Figure 8 : MTF on 15µm pixel pitch module compared to theoretical MTF 3.1 CACTUS family CACTUS PRODUCTS AND DEVELOPMENTS The InGaAs activity has begun mid-2006 at III-Vlab and thanks to the experience acquired in thermal imaging and the high quality material, our laboratory has been able to propose a product called CACTUS 320 in This InGaAs array is a 320x256 format with a 30µm pixel pitch hybridized with a commercial ROIC (ISC9809) based on a Capacitive Trans Impedance Amplifier (CTIA) structure. The hybrid is integrated in a hermetically sealed module with a Thermo-Electric Cooler (TEC) to control and stabilize hybrid temperature (Figure 9). Interconnection ceramic Housing Integration Sapphire window Hybrid Figure 9 : CACTUS320 module integration 27.5 mm x 20 mm x 8.7 mm

7 CACTUS 640 The production of CACTUS 640 module has begun in The 640x512 InGaAs array with a 25µm pixel pitch hybridized with a commercial ROIC (ISC0002) based on a Capacitive Trans Impedance Amplifier (CTIA) structure. Like CACTUS 320 module, the hybrid is integrated in a hermetically sealed module with a Thermo-Electric Cooler (TEC) to control and stabilize hybrid temperature (Figure 10). Figure 10 : CACTUS 640 module The CACTUS 320 and CACTUS 640 modules are designed for most demanding low flux applications from 0.9µm to 1.7µm (optional visible extension) such as night vision, surveillance, airborne gimbals and various scientific and industrial applications. Since 2007, more than 350 modules have been delivered as shown on Figure 11. Modules orders Other formats SWIR640 VisSWIR320 SWIR Year Figure 11 : number of modules orders since Last developments: SNAKE 640 Thanks to the success of CACTUS family modules, developments had been launched to reach the VGA format with a 15µm pixel pitch. These developments were supported by the French MoD and lead to the realization of a VGA InGaAs whose performances are listed in the table on Figure 12.

8 I dark 11 fa J dark 4.8nA.cm -2 Shot noise (dark) 33 e- ROICnoise withcds 40 e- NU +/-σoffset 1.3 % NU +/-σgain 1.0 % Operability(resp.) 99.8 % 500nm 40 % 800nm 75 % > 80 % Made with achromaticvisswir optics (F/2) By Optec (Italy) Figure 12 : Performances summary of prototype InGaAs VGA 15µm The visible-like picture, shown on Figure 12, is due to the VisSWIR detection range of the InGaAs 15µm-VGA module combined to the indoor light sources (neon tube) whose emission spectrum is mainly visible. Benefiting from the experience of this prototype development, a new module has been designed to answer the needs of a wide range of applications. This new module is called SNAKE640 and its planned performances are summarized in Figure 13. We can here highlight the Snapshot integrating mode available with ITR, IWR and NDR reading mode, the low noise gain of 35e- and the high frame rate of 300Hz. Concerning the electro-optics performances, dark current, noise and quantum efficiency are expected to be the same as the prototype beacause the design of the InGaAs detection array remains unchanged. Array format 640x512 Pixel Size 15 µm Windowing mode down to 1 row x 24 columns Full Well / ROIC Noise 40ke- (35e-) 100ke- / 150ke- (75e-) 1.5Me- (600e-) Integrating mode Snapshot Reading mode ITR, IWR, NDR Master Clock 9MHz Output rate 18MPixel/s Number of output channels 2, 4 or 8 (programmable) Frame rate 60 Hz 2 outputs 120 Hz 4 outputs 300 Hz 8 outputs Power Consumption < 160mW 2 outputs Figure 13 : Performances summary of SNAKE640, the new VGA 15µm The first SNAKE640 modules will be available for sampling at the end of 2013.

9 4. CONCLUSION The CACTUS320 and CACTUS640 constitute attractive products for the traditional SWIR spectral range and the visible extension. The high performances of these modules in terms of dark current, quantum efficiency and MTF have brought a large sales growth. Based on this success, development towards VGA format with 15µm pixel pitch have been launched and ended with a first prototype realization with limited functionalities. The recent transfer of QWIP and InGaAs detector activities to Sofradir boosted the development of a new VGA snapshot product with a 15µm pixel pitch; the low noise ROIC hybridized to low dark current InGaAs array and the advanced ROIC functionalities allow this module to answer the needs of a broad range of applications. REFERENCES [1] Forrest, S., "Performances of In x Ga 1-x As y P 1-y photodiodes limited with dark current by diffusion, generation recombination and tunneling" IEEE Journal of Quantum Electronics, QE-17 (2), (1981) [2] Boisvert, J., Isshiki, T., Sudharsanan, R., Yuan, P., and McDonald, P. Performance of very low dark current SWIR PIN arrays, Proc. SPIE 6940 (2008) [3] Decobert, J., and Patriarche, G., Transmission electron microscopy study of the InP/InGaAs and InGaAs/InP heterointerfaces grown by metalorganic vapor-phase epitaxy, Journal of Applied Physics 92 (10), (2002) [4] Reichenbach, S., Park, S., Narayanswamy, R., Characterizing digital image acquisition devices, Optical Engineering 30 (2), (1991)