The IBL sensor material

A quantity to measure the increase of radiation damage at the Pixel Detector is the leakage current of the sensors. Figure 6.1 shows the rising leakage current for the B-Layer and the Layer-1 after an integrated luminosity of 350 pb⁻¹, 1.3 fb⁻¹ and 2.3 fb⁻¹. The current especially rises higher for the B-Layer due to the closer distance to the interaction point.

For the IBL more radiation hard sensors have been developed. These two different sensor types have to fulfil the requirements of the IBL: 3D and planar sensors which have to resist a fluence of 2×10¹⁵ n_eq/cm² and fit into the marginal space foreseen for the IBL. Both are made of silicon material. The choice of having two different sensors for IBL has been made after a longer development phase for increased radiation hard sensors compared to the current Pixel Module sensor. 3D and the improved planar sensor with a slim edge show individual advantages regarding high irradiation or operation experience and thus both of them have been chosen for IBL and are explained in the following.

6.4 The ATLAS IBL Module

Figure 6.1: The increase of the leakage current for all modules in the B-Layer (top) and in Layer-1 (bottom) after an integrated luminosity of 350 pb⁻¹, 1.3 fb⁻¹ and 2.3 fb⁻¹. It can be seen that the current increases higher for the B-Layer as it is closer to the interaction point [38].

The planar silicon sensor

The planar sensor is based on the current Pixel Detector sensor. It is an n-in-n sensor made of diffusion-oxygenated float-zone silicon. The pixel size is 50×250µm² and thus the granularity is increased compared to the old sensor layout.

The thickness of the sensor was reduced down to 200µm compared to the FE-I3 module sensor. Thus, a higher electric field at the same bias voltage and a higher charge collection after irradiation are achieved [82]. This is an advantage as the bias voltage is limited to 1000 V due to the insulation of the cabling [83]. Besides the sensor, the FE-I4 chip has a reduced thickness, too. This comes along with a bowing of the material and is especially unfavourable during

6 The Insertable B-Layer Upgrade

the chip-flip process while bump bonding. To reduce the bowing, a carrier board is needed as support structure which complicates the production process [81].

Due to the marginal space foreseen for the IBL there is no overlap in z-direction between the modules like in the current layers of the Pixel Detector. Hence, insensitive detector material at the edges of the modules has to be avoided. Developments have been undertaken to receive smaller inactive edges than present in the former sensor design. The resulting “slim-edge”

sensors have a reduced inactive edge of 225µm by moving the guard rings partially underneath the pixels. Therefore, these outermost pixels have a length of 500 µm and are longer than the common pixels.

The temperature of the sensor is ∼ −15^◦C to avoid the thermal runaway after irradiation and is a requirement for the successful operation of the planar sensors.

An advantage of this type of sensor is the already gained experience in production and de-tector operation. Moreover, it is delivered by many vendors. The production process is simpler compared to the one for 3D sensors. As a result, the yield is higher and costs can be kept down.

The 3D silicon sensor

The 3D sensor type has a p-doped bulk with alternating n⁺- and p⁺-doped columns which are etched perpendicular to the sensor surface inside the bulk, see Figure 6.2 on the right. Thus, the electric field is parallel to the sensor surface and the depletion zone is generated laterally between

m.i.p. m.i.p.

Figure 6.2: Difference of charge collection inside a planar (left) and 3D (right) sensor induced by a traversing m.i.p. In the planar case the drift distance of the charges depends on the bulk thickness ∆. In the case of 3D this distance is only dependant on the distance between the electrodes L but not on the bulk thickness ∆. The drift path thus decreases from 200−300µm in the planar case to ∼50 µm in the 3D case [84].

the electrodes. With this method, electron-hole pairs induced by a m.i.p. have a shorter drift distance towards the electrodes. This reduces charge trapping and increases the charge collection time from several tens of ns to a few ns [84]. The reduced charge trapping is especially important after the sensor is irradiated. The major effect of the short distance between the electrodes is the low electric field needed to generate the depletion zone. 3D sensors only need a depletion voltage of<10 V whereas the planar sensors need several tens of volts.

If a particle traverses the sensor along the electrodes no electron-hole pairs can be generated and thus the particle cannot be measured. In contrast at higher incident angles generated

6.4 The ATLAS IBL Module

electron-hole pairs drift to several electrodes. This charge sharing increases the spatial resolution.

To take advantage of this effect the 3D modules are slightly tilted by an angle of 10 to 15^◦. Two different versions of the 3D sensors are used. The “Full 3D” sensor consists of columns which are etched from one side and completely traverse the bulk. Here, an extra coating is needed to isolate the n⁺-columns from the bias voltage on the back side of the sensor. Similarly, it is done for the p⁺-columns. The second version is called “double-sided 3D” sensor. In this case the etching is done from both sides and stops immediately before the surface is reached.

Hence, n⁺-columns are etched from top of the sensor surface and p⁺-columns from the backside.

The bulk itself serves as isolation towards the opposed potential.

Both 3D sensors use a slim edge to prevent short circuits inside the bulk due to crystal-defects at the edge. This edge is realised by p⁺-doped “guard fence” columns which steadily decrease the electric field towards the edge of the sensor [74]. The width of the inactive edge is 200µm [84].

As there is no thermal runaway due to the low reverse bias voltage, sensor cooling is not as important as in the case of planar sensors. However, it prevents the movement of defects inside the material.

Although the 3D technology has many advantages, the production of the etched electrodes is difficult and a relatively new method compared to the reliable processing of planar sensors.

Hence, a high production yield and a good uniformity has to be demonstrated to use them for the IBL [74].

The sensor partitioning on one IBL stave is not settled yet. It is either planned to have 75%

planar and 25% 3D sensors or 100% planar sensors in case the fabrication of 3D modules is not fast enough. The 3D modules are placed in the forward region. In the central region planar modules are placed.