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X-ray pixel detectors (5)

Brevi interventi destinati ad illustrare le proprietà ed il funzionamento dei rivelatori a pixel con un approfondimento particolare rivolto alle tecnologie realizzative.

Bump bonding flip chip technology require different process steps:

1) On Detector and/or amplifier chips

– Under Bump Metallisation (UBM)

– Bumping

2) On detector or on amplifier chip, depending on the application

– Flip chip

– Reflow, anneal o adhesive bonding

– Underfilling

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- Under Bump Metallisation (UBM) -

To ensure a low and stable contact resistance at the bump-bond pad interface, the aluminium bond pads must be re-metallised to eliminate non-conductive aluminium oxide.

This remetallisation is the reason for depositing an under bump metallisation (UBM). In addition, the UBM must fulfil the following requirements:

· Adhere to the aluminium and passivation layer of the chip

· Hermetically seal to the chip passivation to protect the IC’s metals from the environment

· Act as a diffusion barrier between the solder bump metals and the IC final metal (an important function for eutectic solder)

· Offer a wettable surface for optimum solder reflow

· Induce the least amount of stress so that long-term reliability is not adversely affected

Today’s standard process steps for UBM (Figure 4) include a native oxide etch prior to sputtering thin layers, such as Cr-Cr: Cu-Cu-(Au); Ti-Ti:W(N)-Cu or Al-Ni: V-Cu, onto the entire wafer surface.

Process steps require to fabricate UBM can be summarised in:

Approach 1

– Metal layers deposition (Sputter or Evaporated)

– Normal photolithography

– Metal etching

Approach 2

– Spin on 1-5 µm photoresist

– Metal layers deposition (Sputter or Evaporated)

– Lift off

Metal layers utilised :

– 1st: Diffusion barrier and adherence

– 2nd: Soldering

– 3rd: Passivation for 2nd layer

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As an example, in the figure, you can se results obtained for Ti/Pt/Au (UBM)+ Sn/Au After reflow at 300 °C. Diffusion barrier is not good enough and therefore bump structure can diffuse into the substrates during reflow process.
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On the contrary, if you introduce Al/Ti/Pt/Au (UBM) + Sn/Au and utilise same reflow process, can obtain results completely different. In fact, as show in the figure, bump structure remain well defined, without any diffusion into the substrates.
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