1. The Development of the Fabrication Process of Low Mass circuits Rui de Oliveira TS-DEM

2. Alice Pixel Aluminium Bus Alice tracker detector description Material budget Staircase shaping Aluminium vacuum deposition Creating micro vias Patterning aluminium Aluminium plating and bonding Testing Conclusions

3. Image:INFN(Padova) ALICE PIXEL DETECTOR DESCRIPTION

4. ladder Pixel_chip Pilot & Optical link Extender carrier bus TOP VIEW OF ALICE PIXEL DETECTOR ONE SECTOR Image:INFN(Padova)

5. 1 2 READOUT CHIP PIXEL DETECTOR Aluminium Polyimide CARBON FIBER SUPPORT 1 GND 50µ 2 VDD 50µ 3 SIG 1 10µ 4 SIG 2 10µ 5 SIG 3+ SMD pad 10µ Glue COOLING TUBE 3 4 5 SMD COMPONENTS PIXEL BUS CROSS SECTION

6. Aluminium bus Size : 160 mm x 16 mm Working support Thick Cu (300 μm) Chemically removed after finalisation of circuit

7. Carbon mechanical support cell Cooling pipe groove

8. Material budget MaterialRadiation length [cm] Density [gr/cc] Resistivity [uohms*cm] Aluminium 8.9 2.7 Copper 1.4 9.0 1.7 Beryllium 35.3 1.9 3.3 Gold 0.3 19.3 2.4 Glass epoxy 19.4 Polyimide 29.0 Copper is close to 6.5 times more transparent than aluminium And aluminium has only 1.6 times the resistivity of copper Polyimide is 1.5 times better than glass epoxy.

9. - Thinning of read out chips (150 μm) - Thinning of silicon detector (200 μm) - Two Bus Power layers and three signal layers in aluminium - Thin dielectrics between the bus layers - Copper instead of aluminium in the three signal layers will give 10% extra mass and degrade the detector performance by at least 20%! - Carbon composite substrate Actions to reduce the Material budget

10. Staircase shaping PIXEL BUS MCM Signals1 VDD GND Pixel readout chip Pixel detector Signals2 Signals3 GND VDD Signals1 Signals3 Signals2 Staircase shaped side

11. Aluminium Vacuum deposition 15 μm aluminium deposition with sputtering assisted with a DC magnetron Pre-cleaning with oxygen and argon plasma 0.1μm Cr adhesion layer Problems Thick aluminium  High temp process  degassing during deposition High degassing  nodules in deposition  bad chemical etching aluminium thickness uniformity Improvements Put the degassing under control Improve the surface treatment before deposition (sandblast) Target position during deposition (uniformity)

12. Al on Al flat deposition Al on polyimide Backed 12h@180deg Prismatic & columnar deposition Al on Polyimide Backed 12h@140deg Al on polyimide Backed 12h@160deg Nodules: 3μm big nodules (10μm)

13. Creating micro vias Problems -Thin layer of polyimide and glue set up a process control on this etching -Hole shape In order to create contacts between signal layers micro vias should be made in the dielectric insulators. This is done with chemical etching.

14.  vias – Process  Chemical Via Al Cu Dielectric Glue Cooper Etching Dielectric Etching Anisotropic Glue Etching Metallization Copper etching Al deposition

15. Define polyimide etching time We developed a way to mesure exactly the polyimide etching time polyimide 12 μm protective resin Ohmmeter dipped zoneundipped copper

16. aluminium tracks layer n-1 Glue Deposited aluminium Polyimide aluminium tracks layer n-1 Glue Deposited aluminium Polyimide cavity Filled cavity

17. Aluminium etching In order to etch precise aluminium tracks on thick deposited aluminium and glued foil layers we have tried: - Different types of resist (liquid, solid) - Different ways of resist deposition (spraying, dipping, laminating, spinning) - Different chemicals to etch the aluminium (phosphoric acid, ferric perchloride, KoH, NaOH etc…) Some examples

18. Bad solutions to etch Vacuum deposited aluminium Direct phosphoric acid etching - micro delamination - bad edges quality Direct ferric percloride etching - solid resist - over etchings and parts not etched Phosphoric etching with no resist baking - resist delamination before the end of the process - residues of aluminium

19. Phosphoric acid etching - solid resist (140deg baking) - 45 min etching - nitric acid pre-cleaning (before resist lamination) Ferric percloride etching - solid resist -15 sec etching - nitric acid desoxydation during process Good solutions for aluminium etching 10 μm vacuum deposited aluminium 15 μm aluminium foil glued

20. Vertical lines Analogue power Analogue ground Bonding and plating 400µ digital power Digital ground Pixel readout chip Pixel detector

21. Bonding Close up view LadderBondingbus Plating defect Al +Zincate + Ni + Au +Special pre-treatment

22. Testing Continuity test Normal PCB Fine pitch 140μm Standard pitch Isolation test Isolation test with small pitch Al Bus Protected area vias Flying probe tester

23. Conclusions Up to now we have produced 20 mixed buses and recently 3 working full aluminium versions. ALICE needs 200 in total. During the last two years we tried to find companies to build this circuit. We did not find anyone interested and/or having the technical capabilities. We expect more interest in this technology as the significance of the physics measurements will be at least 20% higher because of this development. From the beginning we have split the development in two tracks: a/ mixed bus with aluminium and copper b/ full aluminium The reason was to have quickly working samples to verify assembly and integration procedures. It reduced risks and offered a fallback solution.