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ADVANCED HGCDTE TECHNOLOGIES AND DUAL- BAND DEVELOPMENTS Philippe TRIBOLET Sofradir, ZI, BP21, 38113 Veurey-Voroize, France [email protected] Gérard DESTEFANIS, Philippe BALLET, Jacques BAYLET, Olivier GRAVRAND, Johan ROTHMAN CEA Leti-MINATEC - 17 rue des Martyrs - 38054 Grenoble Cedex 9- France ABSTRACT The Molecular Beam Epitaxy (MBE) approach was under investigation for several years to prepare both the very large array fabrication and the 3rd generation developments. This large step in Infrared (IR) detector mass production is also necessary for producing third generation of IR detectors such as bicolor and dual band FPAs which use more complex multi hetero-junctions architectures. These new advanced HgCdTe technologies necessary for third generation developments have been validated and their producibility have been improved. As far as dual band IR detectors are concerned, the technologies are developed and a full TV format (24μm pixel pitch) is currently under development with a first application in bicolor within medium waveband. Future improvements including avalanche photodiodes (APD), will lead to more compact systems as well as a low cost approach. 1 INTRODUCTION During the last decade, the progress made by Infrared (IR) technologies [1][2][3][5][6] has been very significant and has opened the way to using infrared for many new Defense applications. As a matter of fact, IR systems are now challenging visible systems in terms of image quality and identification as well as reliability [7] [8] The high level of performance reached allows IR systems to improve their ability to operate in bad weather conditions answering the needs of a larger range of applications. However, the present generations of IR detectors remain limited in terms of identification, compactness and reliability, as well as their ability to operate in all-weather conditions [9][10]. To establish a “true” third generation IR detector, these limitations need first to be addressed. That is the case for the new technologies under development at Sofradir [11][17][18][19][20]. They aim to reduce production costs, include smaller pixel pitches with larger formats, use of avalanche photodiodes (APD) [24]as well as dual-band detectors [25][26]. Based on these new technologies, IR systems will be able to answer new operational requirements including better identification range as well as light weight requirement and cost reduction. In France, the choice regarding the key technologies for the third generation have been made considering the different parameters from the performance to the system cost criteria. Among all the material candidates, only few are answering the needs like HgCdTe (Mercury Cadmium Telluride / MCT) material which is still the best candidate. 2 NEW IR DETECTOR APPROACHES Taking advantage of the experience of the present generation, IR system trends rely on the use of large 2D InfraRed Focal Plane Arrays (IRFPA) able to detect in different wavebands as well as to offer a high resolution cooled with low input flux ability. In the same time, the trends for cooled IR detectors are to reduce the cost as well as increasing the reliability and the performance. Infrared Technology and Applications XXXIV, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE Vol. 6940, 69402P, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.779902 Proc. of SPIE Vol. 6940 69402P-1
Transcript
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ADVANCED HGCDTE TECHNOLOGIES AND DUAL-BAND DEVELOPMENTS

Philippe TRIBOLET Sofradir, ZI, BP21, 38113 Veurey-Voroize, France

[email protected]

Gérard DESTEFANIS, Philippe BALLET, Jacques BAYLET, Olivier GRAVRAND, Johan ROTHMAN CEA Leti-MINATEC - 17 rue des Martyrs - 38054 Grenoble Cedex 9- France

ABSTRACT

The Molecular Beam Epitaxy (MBE) approach was under investigation for several years to prepare both the very large array fabrication and the 3rd generation developments. This large step in Infrared (IR) detector mass production is also necessary for producing third generation of IR detectors such as bicolor and dual band FPAs which use more complex multi hetero-junctions architectures. These new advanced HgCdTe technologies necessary for third generation developments have been validated and their producibility have been improved. As far as dual band IR detectors are concerned, the technologies are developed and a full TV format (24µm pixel pitch) is currently under development with a first application in bicolor within medium waveband. Future improvements including avalanche photodiodes (APD), will lead to more compact systems as well as a low cost approach.

1 INTRODUCTION

During the last decade, the progress made by Infrared (IR) technologies [1][2][3][5][6] has been very significant and has opened the way to using infrared for many new Defense applications. As a matter of fact, IR systems are now challenging visible systems in terms of image quality and identification as well as reliability [7] [8] The high level of performance reached allows IR systems to improve their ability to operate in bad weather conditions answering the needs of a larger range of applications.

However, the present generations of IR detectors remain limited in terms of identification, compactness and reliability, as well as their ability to operate in all-weather conditions [9][10]. To establish a “true” third generation IR detector, these limitations need first to be addressed.

That is the case for the new technologies under development at Sofradir [11][17][18][19][20]. They aim to reduce production costs, include smaller pixel pitches with larger formats, use of avalanche photodiodes (APD) [24]as well as dual-band detectors [25][26].

Based on these new technologies, IR systems will be able to answer new operational requirements including better identification range as well as light weight requirement and cost reduction. In France, the choice regarding the key technologies for the third generation have been made considering the different parameters from the performance to the system cost criteria. Among all the material candidates, only few are answering the needs like HgCdTe (Mercury Cadmium Telluride / MCT) material which is still the best candidate.

2 NEW IR DETECTOR APPROACHES

Taking advantage of the experience of the present generation, IR system trends rely on the use of large 2D InfraRed Focal Plane Arrays (IRFPA) able to detect in different wavebands as well as to offer a high resolution cooled with low input flux ability. In the same time, the trends for cooled IR detectors are to reduce the cost as well as increasing the reliability and the performance.

Infrared Technology and Applications XXXIV, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton,Proc. of SPIE Vol. 6940, 69402P, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.779902

Proc. of SPIE Vol. 6940 69402P-1

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Basically to prepare the next generation for high performance IR systems, some advanced function are necessary in order to offer a major step in performance. As part of these advanced functions, there is the increased sensitivity for the MW and for the LW bands. This could be done by offering avalanche photodiode mode (APD mode) for the MW and low NETD function for the LW by using new read out integrated circuit (ROIC) functions. In parallel, the development of dual-band infrared detectors has been the core of intense research and technological developments during the last ten years. Thus, based on these advanced HgCdTe technologies, we prepare a new generation of cooled high performance IR detectors. They will be able to detect very low IR light levels as well as to be sensitive in different IR bands with better sensitivity and resolution. The different types of detectors for this new generation will be as follows:

- low IR light level and high resolution with very small pixel pitch for more compact and high definition systems with more than 1 Million pixels,

- low NETD and high resolution for high performance LW IR systems, - dual band IR detector sensitive in MW/MW or in MW/LW with TV format and more and using a small pixel

pitch (20µm and less). These dual band detectors will include the low IR light level mode as well as small NETD option,

- low IR light level with medium resolution for high performance laser gated imaging detector. These advanced functions will offer a lot of flexibility to the IR systems, and will allow systems to be more compact, to reduce their cost while improving their performances. Thanks to this new generation of cooled IR detectors, IR systems will be more versatile in different environments.

3 AVALANCHE PHOTODIODES FOR LOW IR LIGHT LEVEL DETECTION

3.1 Avalanche multiplication principle

For several applications involving low flux detection there is a strong interest to get amplification into the pixel at the photodiode level. Avalanche multiplication is a way to get this amplification and recent works, from DRS for example [23], demonstrated for n on p MWIR HgCdTe photodiode high avalanche gain together without any excess of noise. These exceptional characteristics of HgCdTe APDs are due to an exclusive impact ionization of the electrons, this is why theses devices have been called electrons initiated avalanche photodiodes (e-APDs). The fact that this effect is extremely efficient for electron multiplication has for consequence that only n on p device architectures can be used in backside illumination (we need minority carrier’s photo generated to be electrons). Several groups have reported multiplication gains of 100-1000 for low values of inverse bias around V=10V, associated with a quasi deterministic multiplication yielding a conserved signal to noise ratio, SNR. The high gain at low bias and the low noise factor makes the HgCdTe APDs particularly well suited for integration in the next generation FPAs.

3.2 Avalanche photodiode performance results

Using a special design of the junction we analyzed the possibility to get avalanche multiplication in our n on p detectors. The results obtained on the MWIR photodiode 77K under standard conditions of F/2, 300K BB are presented in Figure 1. A very high avalanche gain of 5300 could be obtained on such detectors biased with only 12.5 Volts without modification of the spectral and spatial response of detector. The noise spectral density versus voltage is fully consistent with no excess noise and a F factor very close to 1 up to 10 Volts (figure 2) [24], [25], [26]. These results which are the best results obtained today in Europe and worldwide have been achieved thanks to the quality of the French MCT material and technology. The demonstration was performed in 2006 by CEA-Leti [24][25][26]. Figure 1 shows the gain and the noise factor of avalanche photodiodes with respect to the bias.

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Histogramme N

Cumule

Histogramme N

4.5-

2.5

5.5-15.5 -9.5 -8.5 -7.8 -8.8 -5.8 -4.8 -3.8 -2.8 -1.8 8.8

Bias (3)

1.0

10.0

100.0

1000.0

10000.0

-14.0 -13.0 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0

Bias (V)

Gai

n

Figure 1: Gain as versus bias voltage of a MW HgCdTe APD and Noise factor as a function of bias Ratio

One of the outstanding properties of HgCdTe APD is the low noise factor, F. The noise factor is the measure of SNR degradation during amplification. For example, a shot noise limited input signal amplified by a gain M with noise factor F will have an output current noise density:

FMqIi inoutnoise2

_ 2=

F is determined by the randomness of the multiplication process. Considering a standard deviation σM for one

multiplication event with a mean gain M, the noise factor is given by: 2

2

1M

F Mσ+=

The close to one noise factor observed for HgCdTe APD is an indication of close to deterministic multiplication process, with σM<M, due to a multiplication which is strongly dominated by the electron multiplication and the shape of the energy dependent ionization coefficients. In III-V APD (for example InGaAs), where the both holes and electrons are multiplied, the noise factor is always high, F>>2. The passive imaging performance of the APDs have been deeply analyzed at CEA-Leti as part of DEFIR studies. The e-APD test vehicles are based on Liquid Phase Epitaxy (LPE) and Molecular Beam Epitaxy (MBE) MW epilayers and were characterized using a large area multi gain read-out integrated circuit (ROIC).

The average values dispersion and operability of the responsivity and the NETD for the LPE arrays are summarized in table 1. The LPE MW e-APD FPAs have an average responsivity of is R=9.6pA/K, which corresponds to a quantum efficiency of about QE=75%. As no anti-reflecting coating was deposited on the backside of the APD-array, the internal quantum efficiency should be close to 100%. This corresponds to an ultra low dispersion of the responsivity. The relative dispersion, uncorrected for the fluctuations in CTIA gain, is σ/<R>=4.3% (3% in the 15µm pitch FPA). The operability at 1.5x <R> were high for the 15µm pitch and the 30µm pitch arrays, 99.95 and 99.96, respectively.

The NETD histograms for the 15µm and 30µm pitch LPE e-APDs are compared in figure 2. The average NETD is 12mK with a relative dispersion of about 10% for both devices. The operability of the equivalent TV/4 15µm pitch devices at 2x <NETD> is 99.94% for both the LPE e-APDs and 99.92% for the 15µm pitch MBE e-APDs.

Figure 2: NETD histogram (left scale) and cumulated operability (%) (right scale) for the a)15µm and b)30µm pixel sizes: LPE MW HgCdTe e-APD FPAs, characterized at f/4, T=77K

The level of performance is comparable with the one obtained on standard MW FPAs production unit.

a) LPE 15µm b) LPE 30 µm

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LPE, λc=5.3µm, T=77K

15µm pitch 30 µm pitch

< > σ/<> Op. < > σ/<> Op.

Responsivity 9.6pA/K 3% 99.95 9.5pA/k 4.3% 99.96

NETD 12.0mK 10% 99.94 12.3mK 10% 99.94

Table 1: Passive imaging performance of LPE HgCdTe e-APDs FPAs measured at f/4, T=77K

The gain curves for 80 LPE and MBE MW e-APDs are reported as a function of bias in the figure 3. The cumulated gain distributions for average gains close to 100 are illustrated in figure 4. The gain in the LPE e-APDs is found to be higher than in the MBE e-APDs. This can be correlated to a smaller band-gap in the LPE e-APDs, illustrated by the higher cut off wavelength in the LPE layer, λc-LPE=5.3µm, compared to λc-MBE=4.9µm. For both types of APDs we observe a narrow distribution of the gain. The relative gain dispersion is very low for both type of components, σM/M= 2% for the MBE e-APDs and σM/M<1% for the LPE e-APDs, up to average gains of <M>=200. This result shows that the CEA-Leti e-APD technology introduces only small short range fluctuations in gain and that it is compatible with the manufacturing of large are FPAs in which the e-APD gain does not degrade the homogeneity of the response of the FPA.

In the LPE e-APDs we observe an exponential gain up to a reverse bias of 7V. At higher reverse bias, there is a change in slope which is induced by an increased dark current generation, due to tunneling effects in the junction. The MBE e-APDs are characterized by an exponential gain for reverse biases higher than 8V, indicating that the tunneling effects are lower in this type of APDs. The reduced tunneling current in the MBE e-APDs is consistent with the higher band gap and lower gain observed in this device. We note that the variation in gain and tunneling current in these devices can also be induced by a variation in the width of the depletion layer, which has been shown to yield a variation in gain and tunneling currents [24].

The cumulated distributions of the excess noise factor measured at average gains of <M>=30 and <M>=90 for a LPE device is reported in figure 5. The zero bias photon current was about Icc=0.7nA, corresponding to 4 photo generated carriers per ns. We observe an average value of <F>=1.2 with an operability of 95% (F<2) for both values of gain. The observation of high values in excess noise factor (F>2) is correlated with a 1/f behavior and is believed to be related to extrinsic noisy dark current generation which may be present on these test structures.

Figure 3: Gain as a function of bias in 80 MBE and LPE MW HgCdTe e-APDs

MBE

LPE

Mul

tiplic

atio

n ga

in M

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±iii

Figure 4: Cumulated distribution of the gain

for 80 MW HgCdTe e-APDs as a function of reverse bias

Figure 5: Cumulated distribution of the excess

noise for 80 MW HgCdTe e-APDs for gains of 30 (dashed line) and 90 (continuous line)

For the LPE sample, the average Ieq_in of 100fA at a gain of <M>=30, corresponding to 0.6 carriers per µs. The operability at Ieq_in<1pA was 95%. For the MBE (λc=4.8µm),the average value at a reverse bias between 6-7V is below 10fA and we observe values down to 1fA, corresponding to 6000e/s. These low values of Ieq_in show the potential of e-APDs to be used in ultra low flux application with integration times in the order 1ms, such as astronomy or biology.

The BW measured was close to independent of the gain, as expected for single type carrier multiplication APDs and a record high gain-bandwidth product have been measured in LPE APDs of GBW=1.1THz. Numerical simulations of a transition time limited MW HgCdTe indicates an optimal total response time of 25ps (BW= 20GHz), which is compatible with telecom applications.

The high gain and sensitivity performance was confirmed in a CTIA pixel design for 3D active imaging using HgCdTe e-APDs; the gain was M=200 at an reverse bias of 8V, the excess noise factor was lower than 1.5 and the background limited equivalent input current was dominated by the residual system photon flux of 2pA. For a well positioned gating of the TOF, enabling short integration times, the equivalent input charge noise was shown to be as low as 2 electrons rms on the reflected intensity image. These performances are associated with a resolution in TOF at the order of 2ns (30cm) and gives a short term perspective of high performance 3D active imaging FPAs.

3.3 E-APD applications

Based on these results, we have confirmed the ability of CEA-Leti e-APD to answer the needs of low IR light level applications including the laser gated imaging applications. In addition, the compliance with the small pixel pitches IR detectors is demonstrated as well as the use of LPE or MBE materials. Thus there are a large variety of FPAs that could take benefit of this extraordinary property of HgCdTe alloys, pushing them to the very high performances. Using this unique property, a new class of very low IR light level detectors can be offered and will have no equivalent or challengers using other technologies. The improvements offered by this new class of amplified IR detectors (e-APD) can be used in different ways at system level either to improve performance or to decrease system cost and constraints; for examples they can be used in the following applications: o Passive FPAs operating in the MWIR band (down to the visible): all weather condition by adjusting the optimum

gain to the device dynamic. This can also be used in SWIR and LWIR FPAs. o Passive low flux applications in several bands (SWIR to LWIR); this includes hyper spectral imaging, the

possibility to get amplification into the diode makes possible the used of read out circuit with and higher noise level. This includes the use of optics with very high f-number in order to reduce system size and cost.

o Active imaging in several infrared bands (2D or 3D imaging). This includes the 1.55µm imaging domain o Passive/Active FPAs (from SWIR to LWIR) o Multicolor Dual mode Passive /Passive or Passive/Active FPAs (from SWIR to LWIR).

4 DUAL BAND DETECTORS

For different applications, several architectures of devices and spectral bands are involved (SWIR/MWIR, MWIR/MWIR, MWIR/LWIR, LWIR/VLWIR bands): - FPA with spatial or quasi spatial coherence and simultaneous readout of the two bands - FPA with spatial coherence and sequential readout of the two bands

Cum

ulat

ed d

ist. MBE

LPE Excess noise factor F

Cum

ulat

ed d

ist.

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For dual bands FPAs operating in MWIR/LWIR, LWIR/VLWIR bands, detectors made with QWIP appears as short term attractive solution for medium performance, while HgCdTe will remain the best choice for the long term high performance. For dual bands FPAs operating in SWIR/MWIR, MWIR/MWIR bands, HgCdTe appears today as the best candidate for in competition with the InAsGaSb type II super lattice. For the fabrication of HgCdTe multicolor detectors, the key technology is the molecular beam epitaxy of HgCdTe on CdZnTe for LWIR bands and either on CdZnTe or germanium for SW/MWIR bands. Layers of alloys with chosen compositions can be successively grown by molecular beam epitaxy (MBE) to obtain multiple bands detection. MBE is a low temperature growth process that prevents interdiffusion of the multilayer HgCdTe structures. Such a stacked hetero structure may then be processed into arrays of diodes. CEA-LETI has developed two different architectures of FPA based on two different concepts. The first one is the classical npn back to back MCT photodiode MESA etched structure that operates within the MWIR in sequential mode with one connection per pixel. FPA with a complexity of TV/2 with pitches in the range of 25µm to 30µm are already available and more complex FPA expected soon. The second one called pseudo planar presents a totally different architecture based on the concept of two n on p diodes similar to the ones made in our standard planar process but on two different levels of three layers architecture. This architecture presents two major interests:

- Firstly it simply involves the fabrication of only planar diodes, without MESA structure (this one presents a major difficulty especially for LWIR photodiodes)

- Secondly fully temporal information is available for the two bands. In addition, the coupling of advanced functions, like APD or low NETD pixel design, will be possible with this structure opening the way of advanced IR detector structures. Thus the goal of the common R&D organization between CEA-Leti and Sofradir is to develop these new arrays keeping very low the manufacturing cost. Moreover the reduction of the time to market is a key driver for these developments. That is why we have chosen this pseudo planar architecture for the dual band development in order to be sure to get the highest technological yield and to use it very quickly at mass production level. At present, the developments in progress in France address low pixel pitch down to 20 µm and TV format. And to master this new generation it is important to start with the right quality of materials, to have a good control of the pixel pitch reduction regarding the photodiode process as well as the flip chip bonding process. These are the key points we have to validate for producing these new generation FPAs.

4.1 Pseudo planar dual band FPA architecture

Our preliminary work concerned pseudo planar structures had been grown with target cut-off wavelength in the MWIR and LWIR bands. Figure 6 shows a scheme of the material grown by molecular beam epitaxy [17], [18].

P

PN

NP

PN

NP

PN

NP

PN

N

MWIR

LWIR

NIR

N on P Ion implanted Photodiodes

Figure 6: Schematic cross section of a pseudo planar dual band pixel

Each pixel consists of two standard n on p photodiodes, located in two CdxHg1-xTe layers with different compositions. A common contact on the array side allows the biasing of the p doped substrate. On each pixel, two independent contacts for the two diodes allow to address simultaneously the two detectors. This architecture (see figure 7 A) thus provides both temporal and spatial coherence, which can be needed for some applications. Any composition combination may be achieved, depending on the application: MWIR/MWIR, MWIR/LWIR, or LWIR/LWIR. A demonstration has been made on MWIR/LWIR devices. With a 30 µm pitch, we have already fabricated 256x256 Focal Plane Arrays (FPA), and the hybridization yield is as good as for a similar size planar FPA.

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I— ___1.52.5 2.5 4.5 5.5 6.5 7.5 8.5 8.5 15.5 11.5 12.5 12.5

Wavelength (vim)

A normalized 77 K spectral response for a MWIR and a LWIR diode is plotted in figure 7B. The two diodes have good spectral signature with cut-off wavelength equal to 4.9µm and 10.0µm respectively. Spectral crosstalk is very low, in the range of a few percent depending on the considered wavelength.

Diodes: Up(lwir), down(mwir)Diodes: Up(lwir), down(mwir)

Figure 7: A/SEM pictures of pseudo-planar bispectral detectors arrays, with a 30µm pixel pitch and two indium bumps per pixel B/ Normalized 77K spectral response (cut-off are 4.9 µm and 10 µm)

Quantum efficiency measured in both bands is around 50% (with antireflection coating). It is limited by the absorption layer thickness for the LWIR diode. For the MWIR diode, as explained before, it is limited by the diode size, controlled by the implantation size and the carrier diffusion length. Both can be optimized in the future by adjusting these parameters.

4.2 MBE Crystal growth performance

With the dual band pixel architectures chosen in France, it is mandatory to use the MBE growth technique, because it allows growing complex heterostucture with very good quality layers. The main challenge is to control of surface defect density.

Figure 8: Multilayer epitaxy architecture for a dual band IR detector

Since multicolor detector requires the control by MBE of the growth of several layers of different compositions. The changes in cadmium composition favor non-ideal growth conditions resulting in an increase in the surface defect density (mostly Te precipitates). Changes in cadmium composition have to be balanced by changes in substrate temperature to ensure perfect growth conditions. The development conducted in the last years now allows obtaining reproducible good quality layers with low defect density, low dislocation density and good composition control. MBE growth takes place in a RIBER 32P chamber with standard effusion cells for Cd, Te, Hg and ZnTe. The multispectral MBE stack can be seen on the schematic drawing (figure 8). MCT growth is initiated on a (211)B CZT substrate heated to about 180°C. Because the growth is carried out at constant growth rate and mercury pressure, the substrate temperature is adjusted to account for the large changes in cadmium composition imposed by the two color detection. In that process, any deviation from the ideal temperature leads to a degradation in the material quality and an increase in the surface defect density. Despite the numerous layers, it is possible to control the growth temperature to avoid such undesirable effects. Indeed, by applying a different substrate temperature for each layer, we have demonstrated surface defect density very similar to those observed on monospectral structures.

(211)B CZT

B1

MCT nucleation layer

B2

Barrier

T1T2

T3T4

Cap layer

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In particular, the control of the substrate temperatures makes it possible to obtain defect density lower than 200/cm2 (monospectral typical density). The help of in situ spectroscopic ellipsometry appear to be of a major interest for the control of the growth of theses complex heterostructures. Because the multispectral structure is made of layers having very different cadmium composition, the lattice parameter is expected to change. Typically, absorbing layers have lattice constants very close to that of the substrate but barrier and cap layer lattice constant may differ significantly. These changes in lattice constant, associated with the growth of rather thick layers, usually leads to complete or partial plastic relaxation. This of course strongly affects the quality of the multispectral structure. Evidence for that is the increase in the full width at half maximum (FWHM) of the diffraction peaks and the increase in the dislocation number density. In order to avoid such effect, we add a very small percentage of zinc in the barrier and cap layers. By doing so, we can make the barrier and cap layer material identical to the absorbing material in terms of lattice dimension. Thanks to a good mastering of this process, we will produce lattice-matched structures. The High Resolution X-Ray Diffraction (HRXRD) peaks displayed in the figure 9 illustrate this point. From the red to the blue curve we have added more and more zinc into the barrier (BAR) and the cap layers. In the case of the red curve, the amount of zinc is not enough to balance for the change in lattice parameter so that the peaks associated with the barrier and the cap are far from the substrate which lies under the B1 and B2 peaks. By increasing the amount of zinc it is possible to make the cap layer peak move under the B1 peak (green and blue). The barrier peak also moves toward the absorbing layer peaks without reaching them. This suggests that the barrier zinc content might not yet be well optimized (Figure 9). Nevertheless, this process has enable us to produce almost perfectly matched structures resulting in HRXRD FWHM as low as 25 arcsecs and dislocation densities in the very low 10+5/cm2. Those results are comparable to the very best results obtained on monospectral structures demonstrating the control in the lattice of the multilayer stack.

Figure 9: HRXRD spectrum of a multilayer epitaxy for dual band detector

4.3 Pitch reduction down to 20 µm

The problem of pitch reduction is linked to the architecture of pseudo planar detectors that requires two indium bump connection per pixel on a two-level-diode architecture. On figure 10 is shown a scheme of the material grown by molecular beam epitaxy and of the pseudo-planar pixel design.

λ2 > λ1

CdH

gTe

N

BIIλ2

BIIλ1

λ2

λ1 NIR barrier

P

PN

N

λ2 > λ1

CdH

gTe

N

BIIλ2

BIIλ1

CdH

gTe

N

BIIλ2

BIIλ1

λ2

λ1 NIR barrier

P

PN

N

Figure 10: Schematic cross section of a pseudo planar dual band pixel

7 1 . 2 7 1 . 3 7 1 . 4 7 1 . 5 7 1 . 61 0 0

1 0 0 0

1 E 4

1 E 5

1 E 6 B 1 B 2

C a p la y e r

B A R

inte

nsity

(cou

nts)

2 θ ( ° )

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Each pixel consists of two standard n on p photodiodes, located in two CdxHg1-xTe layers with different compositions. One is at the bottom of a hole etched in the longest cut-off wavelength absorption layer, while the other is on the surface layer. A common contact on the array side allows the biasing of both lower and upper p-type layers (on the right side of figure 10). The barrier between upper and lower layers prevents electrical shortage between the diodes. On each pixel, two independent contacts for the two diodes allow to address simultaneously the two detectors on the left side of figure 10. This architecture thus provides both temporal and spatial coherence, which can be needed for some applications. Any composition combination may be achieved, depending on the application: SWIR/MWIR, MWIR/MWIR, MWIR/LWIR, or LWIR/LWIR. We have already developed both MWIR/MWIR and MWIR/LWIR devices. Decreasing the pseudo-planar pixel pitch demands that pixel design is adapted and several fabrication technology steps are improved. To maintain a good quantum efficiency for the upper diode, the hole diameter needs to be decreased. For the 24µm pitch pixel, it has been reduced to 8-10 µm. Down the hole, it is even smaller, at approximately 6µm, due to the side slope. Thanks to this slope, photons reflection on the hole side ensures a good optical fill factor, thus a good quantum efficiency. An example of hole etched in a pseudo-planar heterostructure is shown on figure 11.

Figure 11: SEM picture of a hole etched in a pseudo planar heterostructure

To ensure good pixel operability, distance between adjacent metallization pads is also reduced to 2-3µm, and criterion for subsequent photo mask alignment is 0.5µm or less. This demands to continuously improve each step of the fabrication process. We have particularly focused on some of them, which required specific developments:

- the etching process used to obtain holes in the upper absorption layer, with correct diameter, side slope and depth, on a whole TV format array. - the photolithography steps on the non-planar array surface obtained after the hole etching step. - the etching and deposition uniformity on large format array, to ensure low electrical performance dispersion. - the detector arrays hybridization on the CMOS read-out circuit.

The first issue has led to the study of several etching methods, which need to fulfill different requirements. In particular, the etching step must not create surface degradation, in terms of defects, roughness, stoechiometry change, or residues (polymers or metals). Moreover, there must be no electrical defect creation or doping type change. In the past, pure chemical etching was used when pixel size was above 40µm. Because of lateral erosion, it is no longer possible to use this method when pixel size is reduced, and developments of dry etching techniques are necessary. We currently study and use an inductively coupled plasma (ICP) reactors with processes based on CH4/H2/Ar gas mixture [27]. Our most recent results show that we obtain optimal results for a hybrid etching method, using both dry and wet etching. This method allows fulfilling all the requirements listed above for 20µm pixel size. Work is still underway to improve the pure dry etching process (in an ICP reactor), to be able to go down to pixel size even lower in the future. It is also important to control precisely the lower diode hole depth, because the P/N junction position down the hole is critical for diodes performances. We have improved this control by using laser interferometer that allows us to control and stop in real-time the etching process. An example of laser interferometer signal obtained during a hole etching step is shown in Figure 12. The difference in period and intensity of the signal is the signature of different etching rates in successive absorbing layers.

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F—I WD 9 Gd.L47KX

Figure 12: Laser interferometer signal (left) and SEM picture (right) of holes etched in a pseudo-planar

heterostructure

Concerning the photolithography steps, studies have been led both on the photo resist type and on the lithography step itself. Tests and selection of the most adapted photo resist has allowed us to control both resist thickness and planarization on non planar zones, like hole arrays. Pattern photolithography conditions (exposure, bake, development) are quite straightforward for the patterns located on the surface. With 24µm and 20µm pitch pixels and holes diameter about 8µm to 10µm, we manage to define patterns in a very precise way, with dimensions in the 3µm range, as shown for example in figure 13.

Figure 13: SEM picture of a photo resist pattern defined inside a hole. Photo resist opening diameter at the bottom

is approximately 3µm in this case

The passivation quality and uniformity is also critical for device quality. Examples of high passivation quality layers (controlled by SEM) in high aspect ratio patterns, for critical devices like MWIR/LWIR 30µm pitch focal plane arrays have already been shown [17]. This point is not critical when the pixel pitch is decreased, and good results obtained by electrical characterizations shown in the next sections show that this fabrication step is well controlled for 20µm pitch pixel size. The last important topic for dual-band detectors is the hybridization step. A lot of efforts in the last few years successfully conducted to the hybridization of mega-pixels arrays with a 15µm pitch [17]. This work allows us now to hybridize without difficulty our bispectral focal plane arrays. All the improvements made in our pseudo-planar fabrication process allow us to produce now large format arrays (TV format), like the ones shown in figure 14. Left is shown a SEM picture of 24µm pitch pixels during the fabrication process, and right is shown a SEM picture of a TV format 24µm pixel pitch array. Pseudo-planar diodes performances made with this process are detailed in the next below.

Figure 14: SEM pictures of 24µm pitch pseudo-planar pixels arrays.

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The spectral responses of the dual band detectors with a pitch of 24µm are presented in figure 15; the cut off wavelengths are respectively 4.4µm and 5.4µm. The cross talk between the two bands is very small, in the range of a few %. Similar behavior was obtained for the pitch of 20µm.

Figure 15: Spectral response of a dual band detector with a pitch of 24µm

I(V) and R(V) characteristics both for the 24µm pitch and the 20µm pitch are presented are excellent as can be shown in respectively in figures 16 and 17 at 78K (FOV: 30°, BB 300K).

R(Ohm)

V(V)

I(A)

V(V)

Figure 16: I(V) and R(V) characteristics of a dual band detector with a pitch of 24µm

R(Ohm)

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I(A)

V(V)

Figure 17: I(V) and R(V) characteristics of a dual band detector with a pitch of 20µm

Finally the histograms of current at 200mV and 140K are presented for the pitch of 24µm in figure 18. Therefore feasibility of pixel pitches as small as 20µm has already been demonstrated.

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.r1rrN TP

I I

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Figure 18: normalized histograms of current at a reverse bias of 200mV and T=140K of a dual band detector

4.4 ROIC architecture and performance

ROIC architecture are another key point for advanced IR detectors based on the fact that they have to include more and more complex function like e-APD read out, dual band pixels, low NETD detectors. As to the dual band detectors, we have designed an adapted ROIC as following:

- The array size is compliant with 640 * 512 bicolor pixels with a 24 µm pitch, - The sensitive diodes are connected with two indium bumps per pixel, - The dual band pixel implementation is shown in figure 19 hereafter.

Figure 19: dual band pixel

The readout circuit size is 17.72 * 15.06 mm² with a maximum storage capability of 3.5 Me for the first bandwidth and 10.5 Me for the second one. As described in figure 20, two separated input stages are designed in the same pixel in order to address the two different bandwidths. The circuit operates in a snapshot mode, and may be used following three different ways: Integrate While Read mode, Integrate Then Read mode or the new Double Time Integration mode which allows two different integration times during the same frame. The diodes are biased to the readout circuit in direct injection mode. The current is integrated in MOS capacitors and read out by a switched follower. The Switched follower output is sampled and multiplexed towards 4 outputs (two per spectral bands) at a maximum of 20Mpixels/s/output rate. The input stages have anti-blooming capability. The power consumption is lower than 80 mW for the complete circuit with a read out noise of 130µV.

Figure 20: dual band readout circuit

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The expected performances in each band of theses dual band FPAs are close to what is routinely obtained in single color detectors of the same cut of wavelength and same complexity and pitch: in term of NETD and operability thanks to the simple technological choices made.

5 CONCLUSION

Regarding dual band IR detectors, multilayer MCT grown by MBE exhibits outstanding characteristics and the full performance for pixel pitch size down to 20 µm have been demonstrated. All the necessary advanced functions are already demonstrated in France and the technological choices are mature. Finally TV format manufacturing is in progress based on the mature pseudo planar architecture and on an advanced ROIC design. In conclusion, new generation demonstrations are in progress in France using a specific approach which will lead to add advanced functions to dual band IRFPAs. These advanced functions are dealing with low NETD values and low IR light level ability. This approach will offer to system designer a lot of options to enhance new generation performance as well as to simplify their systems.

6 ACKNOWLEDGMENTS

The authors would like first to thank all the SOFRADIR and CEA-Léti-MINATEC teams dedicated to quality work and to challenging wins. The authors thank the French MoD for its support of CEA-Leti (LIR) and SOFRADIR technology including the third generation demonstrations.

7 REFERENCES

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US Workshop on the Physics and Chemistry of II VI materials (Boston USA, sept 2005) – Publication: J.Electron.Mater. 2006 vol 35, 6. [12] « From visible to infrared: a new approach » P.Chorier, P.Tribolet, G.Destefanis, – Communication: International SPIE meeting Infrared technology and application XXXII: (Orlando USA April 2006). Publication SPIE proceedings (vol 6206-01). [13] « From LWIR to VLWIR FPAs made with HgCdTe at Defir » O.Gravrand, E. Deborniol, G.Destefanis, A.Manissadjian, P.Tribolet, C.Pautet, P.Chorier – Communication : International SPIE meeting Sensors, Systems and Next generation Satellites XII (Stockholm Sweden September 2006). Publication SPIE proceedings (vol 6361-42). [14] « From LWIR to VLWIR FPAs made with HgCdTe n+n/p ion implantation technology » O. Gravrand, E. Deborniol, G. Destefanis – Communication: The 2006 US Workshop on the Physics and Chemistry of II VI materials (Newport USA, oct 2006) – Publication: to be published J.Electron.Mater. 2007. [15] «Characterization of high performances long wave and very long wave HgCdTe staring arrays» E. Deborniol, G. Destefanis, A. Manissadjian, P. Tribolet– Communication : International SPIE meeting : Remote sensing (Bruges Belgium sept 2005). SPIE proceedings (vol 5978-44). [16] «Long wave HgCdTe staring arrays at Sofradir: from 9µm to 13+µm cut-off for high performance applications» A. Manissadjian, P. Tribolet, G. Destefanis, E. Deborniol – Communication: International SPIE meeting Infrared technology and application XXXI: (Orlando USA April 2005). Publication SPIE proceedings (vol 5783). [17] « Status of HgCdTe bicolor and dual band infrared focal plane arrays at LETI » G.Destefanis, J.Baylet, P.Ballet, F.Rothan, O.Gravrand, J.Rothman, J.P.Chamonal, A.Million – Communication (Invited): The 2006 US Workshop on the Physics and Chemistry of II VI materials (Newport USA, oct 2006) – Publication: to be published J.Electron.Mater. 2007 vol 36(8). [18] « Bi-color and dual band infrared focal plane arrays at Defir » G.Destefanis, Ph. Ballet, J.Baylet, P.Castelein, O.Gravrand, J.Rothman, F.Rothan, G.Perrais, J.Chamonal, A.Million, P.Tribolet, B.Terrier, E.Sanson, P.Costa, L.Vial – Communication (Invited): International SPIE meeting Infrared technology and application XXXII: (Orlando USA April 2006). Publication SPIE proceedings (vol 6206-27). [19] «Demonstration of a 25µm pitch dual band HgCdTe infrared focal plane array with spatial coherence» P. Ballet, P. Castelein, J. Baylet, E. Laffosse, M. Fendler, F. Pottier, S. Gout, C. Vergnaud, S. Ballerand, O. Gravrand, JC. Desplanches, S. Martin, JP. Zanatta, JP. Chamonal, A. Million, G. Destefanis – Communication: International SPIE meeting Optics and Optoelectronics: (Bruges Belgium sept 2005). Publication: SPIE proceedings (vol 5978-44). [20] «Third generation and multicolor IRFPA developments: a unique approach based on Defir» P. Tribolet, G. Destefanis – Communication (invited): International SPIE meeting Infrared technology and application XXXI: (Orlando USA April 2005). Publication SPIE proceedings (vol 5783). [21] F. Ma, et al. Phys. Rev. Leti., 95, 176604 (2005). [22] R. Alabedra, et al., IEEE Trans. Electron Devices, ED-32, 1302 (1985) ; G. Levêque et al., Semicond. Sci. Technol. 8 1317 (1993). [23] J.D. Beck, C.-F.Wan, M.A. Kinch, J.E. Robinson, Proc. SPIE, 4454, 188 (2001); J.D. Beck, C.-F. Wan, M.A. Kinch, J.E. Robinson, P. Mitra, R. Scrithfield, F. Ma, J. Campbell, J. Electron. Mater. 35, 1166 (2006). [24] « Gain and Dark current characteristics of planar HgCdTe avalanche photodiodes » G.Perrais, O. Gravrand, J.Baylet, G. Destefanis and J.Rothman– Communication: The 2006 US Workshop on the Physics and Chemistry of II VI materials (Newport USA, oct 2006) – Publication: J.Electron.Mater. 2007vol 36(8). [25] « Demonstration of multifunction bicolor avalanche gain in HgCdTe FPA » G.Perrais; J.Rothman, G.Destefanis, J.P.Baylet, P.Castelein, J.Chamonal; P.Tribolet – Communication : International SPIE meeting Electro optical and infrared systems: technology and applications III (Stockholm Sweden September 2006). Publication SPIE proceedings (vol 6395-16). [26] Advance MCT technologies in France, Gérard Destefanis CEA LETI, Philippe Tribolet Sofradir, SPIE Orlando 2007 6542-12. [27] / E. Laffosse, J. Baylet, J.P. Chamonal, G. Destefanis, G. Cartry, C. Cardinaud, J. Electron. Mater. 34 (6), 740 (2005). [28] G. Destefanis, A. Astier, J. Baylet, P. Castelein, J.P. Chamonal, E. DeBorniol, O. Gravand, F. Marion, J.L. Martin, A. Million, P. Rambaud, F. Rothan, J.P. Zanatta, J. Electron. Mater. 32 (7), 592 (2003). [29] G. Destefanis,, J. Baylet, P. Ballet, P. Castelein, F. Rothan, O. Gravrand, J. Rothman, J.P. Chamonal, A. Million, J. Electron. Mater. 36 (8), 1031 (2007).

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