2.2.1

Array heritage

A prototype of 1 million pixels was developed in 2002-2004 to confirm UMC 0.35µm excellent radiometric performances [2] in term of Spectral Detection Efficiency (SDE) and Modulation Transfer Function (MTF), see Fig. 2-4 and 2-5.

Fig. 2-4:

COBRA 1M device): 8 inch wafer (left); COBRA1M in package (top right); COBRA1M pixels (bottom right).

Fig. 2-5:

COBRA 1M performances

A new prototype COBRA1MP with various ionic implantations was tested in 2005 to maximize output voltage swing [3], offering improved charge handling capacity necessary for photonic noise dominated application as Geostationary Ocean Colour Imaging

2.2.2

Packaging heritage

Experience gained in the frame of LOLA mission [4] with the development of a 750x750 airborne APS [5] was used to define the COBRA2M package, using the same concept of Interstitial Pin Grid Array (IPGA) connected to a PCB Flex with multiple flexible parts, see Fig. 2.6, anticipating a Focal Plane Array with the detector back glued on a SiC part to offer excellent thermal control.

Fig. 2-6:

LOLA APS750FAST packaging

2.3.1

Design phase

From the radiometric performances extrapolated from COBRA1M and the increased performances of the readout circuit in term of output voltage swing, tested on COBRA1MP, accurate modelling of the COBRA2M performances was possible, especially in term of elementary integration time and the number of cumulated images necessary to meet specified SNR of 1000 for each of the eight spectral bands.

For the band of maximum flux and maximum Spectral Detection Efficiency, a minimum elementary integration time of 100msec was necessary to collect 100ke-. The frame period for this value is 100msec and was provided by four outputs with a pixel rate of 5.3 MPixels/sec.

Row and column decoders, already developed in the frame of COBRA1M, were implemented, see Fig. 2-7.

Fig. 2-7:

COBRA2M Detector Architecture

2.3.2

Demonstrator Model (DM) manufacturing and test phase

A small number of devices were packaged by e2v and tested by Astrium.

Readout circuit performances were confirmed with respect to COBRA 1M_P test results, see Fig. 2-8, with a output voltage swing greater than 1.1V for a 3.3V power supply and a Readout noise in darkness of 500µV rms due to a√(2kT/C) hard reset noise and a estimated 200µV rms noise for sampling circuit.

Fig. 2-8:

readout circuit and radiometric performances of output 1 at 550nm

MTF performances measured on COBRA1M were confirmed, using a specific Astrium test bench, see Fig. 2-9

Fig. 2-9:

COBRA2M on MTF test bench

At the end of the DM phase, a COBRA2M device was soldered to a PCB Flex and connected to the Front End Electronic (FEE) to test compatibility, see Fig. 2-10.

Fig. 2-10:

Detector-Front End Electronic coupling on COBRA thermal controlled test bench

Electro-optical tests were repeated to validate FEE behaviour.

2.3.3

Flight Model (FM) manufacturing and qualification phase

DM test results presented at the Critical Design Review (CDR) authorized the FM manufacturing and qualification phase. A FM test bench was implemented in an Astrium clean room, see Fig. 2.11 to screen the 32 devices packaged and delivered by e2v and select Qualification models, Flight model and Flight spares, see Fig. 2.12. The qualification test results demonstrated only Dark Current increases with Total Ionizing Dose (5krad) and protons irradiations (2 10¹⁰ p+/cm² 60MeV) as depicted in figure 2.13.

Fig. 2-11:

COBRA2M FM test bench in clean room with class 100 flux and device regulated at 17°C.

Fig. 2-12:

location of Qualification and Flight Models on the wafer processed by e2v.

Fig. 2-13:

electro-optical parameters before after proton irradiation (2 10¹⁰ p+/cm² 60MeV).

2.2.4

COBRA2M development lessons learnt

A learning curve is always necessary when using a new foundry and that some prototypes are necessary before producing flight models. Developing an efficient fully automated test bench greatly assists the conduction of testing during an FM production phase, securing both schedule and quality of test results, especially for the repeatability of test before and after qualification tests. The selected Flight Model was delivered in June 2007, (two years after project kick off), to be soldered on the Flight PCB Flex and then glued onto the Focal Plane Array Sic part, before to be mounted on the Instrument telescope.

3.2.1

Array heritage

A multi linear array prototype COBRA NxK providing two lines with 10m resolution and nine lines with 20m resolution associated to two outputs was developed and tested in 2006 and beginning of 2007, see Fig. 3-3.

Fig. 3-3:

COBRA NxK prototype

This prototype validates the Correlated Double Sampling readout circuit with the expected readout noise reduction, due to photodiode reset noise suppression.

Specific test vehicles (COBRA CVF) were developed and tested to validate conversion gain adjustment.

3.2.2

Test bench and method heritage

The detector development took also advantage of know-how gained in the frame of the COBRA2M development, in term of test bench architecture, definition and implementation, see Fig. 3-4.

Fig. 3-4:

S2 VNIR test bench architecture, using COBRA2M elements

The same test methods and processing were mastered, providing high accuracy in test results, see Fig. 3-5.

Fig. 3-5:

typical SDE, CVF and RON estimation with non linear estimation method [7]

3.2.3

Black coating heritage

As for COBRA2M, e2v works on the S2 VNIR back end manufacturing. Due to VNIR reflectivity requirements, a post process step is needed in order to strongly reduce the die global reflectivity. A black coating will be deposit on the non photosensitive area of the die to meet reflectivity requirements. This process has been still validated on COBRA NxK wafers, although black coated COBRA NxK devices have been packaged by e2v, see Fig. 3-6. Fig. 3-7 illustrates reflectivity measurements. Very low reflectance is achieved.

Fig. 3-6:

COBRA NxK black coated device

Fig. 3-7:

black coating reflectivity

3.3.1

End of Design and validation phase

Today the design and validation phase is nearing completion, concluded by a Preliminary Design Review (PDR)

Fig. 3-8 presents the retained architecture:

Fig. 3-8 :

S2 VNIR architecture

Fig. 3-9 shows a breadboard of the S2 VNIR device using COBRA NxK die to demonstrate required photosensitive flatness better than 10µm.

Fig. 3-9:

breadboard of the S2 VNIR detector using COBRA NxK die.

3.3.2

EM and FM phases

Procurements of different parts: die wafers, package parts, see Fig. 3-10 and manufacturing of jigs for assembly and tests, will be initiated after PDR release. Adaptation of test bench software and test bench acceptance are in progress, as definition of manufacturing and testing flows is established to meet schedule and quality requirements. EM phase ends with CDR in April 2009.

Fig. 3-10:

3D detector view; detector is equipped with a temporary window.

FM qualification and delivery is foreseen for January 2010.