Embedded pitch adapters: A high-yield interconnection solution for strip sensors

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Abstract

A proposal to fabricate large area strip sensors with integrated, or embedded, pitch adapters is presented for the End-cap part of the Inner Tracker in the ATLAS experiment. To implement the embedded pitch adapters, a second metal layer is used in the sensor fabrication, for signal routing to the ASICs. Sensors with different embedded pitch adapters have been fabricated in order to optimize the design and technology. Inter-strip capacitance, noise, pick-up, cross-talk, signal efficiency, and fabrication yield have been taken into account in their design and fabrication. Inter-strip capacitance tests taking into account all channel neighbors reveal the important differences between the various designs considered. These tests have been correlated with noise figures obtained in full assembled modules, showing that the tests performed on the bare sensors are a valid tool to estimate the final noise in the full module. The full modules have been subjected to test beam experiments in order to evaluate the incidence of cross-talk, pick-up, and signal loss. The detailed analysis shows no indication of cross-talk or pick-up as no additional hits can be observed in any channel not being hit by the beam above 170 mV threshold, and the signal in those channels is always below 1% of the signal recorded in the channel being hit, above 100 mV threshold. First results on irradiated mini-sensors with embedded pitch adapters do not show any change in the interstrip capacitance measurements with only the first neighbors connected.

Section snippets

Introduction and motivation

The interconnection of sensors and readout electronics is a subject of critical impact in the module design for High Energy Physics experiments, such as ATLAS. The sensors are made progressively larger, and the readout ASICs smaller, and both contain increasingly more channels. On the other hand, the pitch between bonding pads is usually very different between sensors and ASICs, which leads to unrealizable wire-bonding angles between them. With all this, the actual design of the electrical

Design of the embedded pitch adapters

In view of the results just mentioned about noise variability across the detector due to the different inter-strip capacitance in each channel for the initial design, 4 new layouts were designed in order to attempt reducing this variability and the rest of effects mentioned above. Fig. 2 shows the 5 layouts that have been tried.

  • (a)

    Basic: This is the first layout made for the initial batches and tests. The second-metal tracks keep the same angle (in each quadrant), i.e. they are parallel to each

Technology and fabrication

One full batch of TOP petalet sensors have been fabricated in the clean room of the Centro Nacional de Microelectronica (IMB-CNM, CSIC), Barcelona, Spain. High-resistivity, p-type, 300 µm thick wafers with 4 in. diameter have been used as substrates. The “standard” first-metal tracks, which are the top plate of the coupling capacitors, are made with a 0.5 µm thick sputtered Aluminum layer. The inter-metal oxide layer is deposited by a low-temperature Plasma Enhanced Chemical Vapor Deposition

Inter-strip capacitance Tests

As explained above, the use of an additional second-metal track on every channel in order to make the connectivity and pitch adaptation to the ASIC, can produce an increase in the inter-strip capacitance of the channel. This will result in an increase of the noise per channel. Additionally, the inter-strip capacitance will vary from channel to channel as the second-metal track is in principle different for every channel, and so will do the noise, increasing the noise variability. In order to

Beam test

The full module assembled with two TOP petalet sensors from one of the wafers fabricated with 1 µm inter-metal oxide thickness have been subjected to test beam experiments at the Diamond Light Source synchrotron in the UK. The 15 keV beam has a rate of up to 2 kHz and it is microfocused to a spot size of 3.27 µm×1.65 µm (FWHM) which guarantees to hit only one strip at a time. There is an xyz-stage with micron precision positioning to move the beam across the target. As for the setup, the module was

Conclusion

A new proposal is presented to improve the throughput and reliability of the interconnection between sensors and electronics in the Inner Tracker on the ATLAS Upgrade experiment for the HL-LHC. A second metal layer is used to make the pitch adaptation integrated in the sensor. New layouts for these Embedded Pitch Adapters are tested, together with new design and technological improvements such as the reduction of second-metal track width, and the increase in the inter-metal oxide thickness. The

Acknowledgment

The irradiations were performed at Cyclotron and Radioisotope Center (CYRIC), Tohoku University, with Y. Sakemi, M. Ito, and T. Wakui. The test beam was performed in the Diamond Light Source Synchrotron in the UK. The research was supported and financed in part by the Ministry of Education, Youth and Sports of the Czech Republic (Grant no. LG13009), the German Federal Ministry of Education and Research, and the Helmholtz Association, the European Social Fund and by the Ministry of Science,

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