Isolation methods

The isolation methods are only needed in sensors with

n

-type implants. The irradiation of the detectors at high fluences has different negative effects on the sensors (these effects will be explained in detail in section 3.3). One of these effects is the creation of a layer of electrons or holes (depending on the sensor type) at the surface.

This can cause high electric field regions in sensors that use

n

implants and lead to a breakdown of the sensor. To avoid this, different isolation methods can be used. In this kind of silicon sensors p-stop and p-spray methods are the most commonly used.

Figure 3.11: Schematic transversal view of an⁺p silicon sensor. The bulk isp-type silicon and the electrodes aren⁺implants. As in figure3.8SiO₂is used to protect the silicon of the wafer and an aluminium layer is used to connect the strips to the readout electronics. With this configuration electrons are registered by the readout.

•

P-stop isolation: This technique introduces a high dose of

p

⁺ boron implant surrounding the strips [65]. Figure3.12shows an sketch of this method.

Figure 3.12: P-stop isolation technique for adjacentn⁺ implants. The maximum field regions are located at the lateralpn-junctions. High dose ofp⁺boron implant is used.

A typical dose of boron ions guarantee a good isolation (

10

¹⁴boron ions/

cm

²).

The potential of the

p

-stop depends on the implant geometry, the backplane bias and the effective doping concentration of the substrate. The potential difference between

n

⁺ strips and

p

-stops increases with the radiation fluence, leading to an increase in the electric field. Therefore, the breakdown voltage of sensors with

p

-stop isolation decreases with irradiation.

3.2 Silicon Detectors Technology 79

•

P-spray isolation: This technique uses a low dose of

p

⁺ boron implant that covers the whole surface [66]. The point of maximal electrical field is at the lateral

pn

-junction between the isolation boron implant and the n+ strips (as in the

p

-stop case). Figure3.13shows an sketch of this method.

Figure 3.13: P-spray isolation technique for adjacentn⁺implants. The maximum field regions are located at the lateralpn-junctions. Low dose ofp⁺boron implant is used.

With the increase of the oxide charge to its saturation value the shallow p-spray layer moves into the depleted region and the electric field decreases. When the boron implant matches the saturation value of the oxide charge the lowest electric field is reached.

For HL-LHC strip sensors

n − on − p

are selected as the best option considering different advantages above mentioned:

•

Due to electrons collection in the readout electrodes less trapping of charge carriers are produced and the signal collection is higher for the short collection times at HL-LHC. This leads to a higher charge collection efficiency.

•

The growth of the depletion region goes from the implants to the backplane allowing the detector to operate partially depleted. This is a benefit compared to the sensors that need full depletion to operate since with irradiation the full depletion voltage increases and can be higher than the breakdown voltage.

The

pn

junction is always located between the implants and the silicon bulk and therefore there’s no type inversion.

• N − on − p

sensors can be fabricated using a single-side lithography process, making them more cost-effective than

n − on − n

sensors, which require a double-side process. For large strip detectors these costs have to be decreased significantly.

The microstrips which correspond to the implants on top of the silicon bulk surface are typically

10 − 20 µm

^{wide and}

1 − 3 µm

deep (see figure 3.11). The bulk of the detector usually has a doping concentration of

10

¹²

atoms/cm

³. This should be compared to the intrinsic carrier concentration which is of the order of

10

¹⁰

cm

⁻³.

Each of the implanted strips is bonded to the front-end readout electronics, which amplifies the signal produced by ionizing radiation. In addition, other elements that can be also seen in figure3.11are necessary to form a proper silicon detector for the upgrade of ATLAS detector.

•

An oxide layer (approximately

1 − 4 µm

thick) lies on top of the implanted strips, known as the AC oxide, which prevents the leakage current flowing directly to the readout electronics.

•

The signal from each of the strips is

AC

coupled to a metal (aluminium) strip lying directly above the strip implants, and the charge is read out through this ohmic contact.

•

A DC path is required between the back and front contacts to bias all the strips.

This path is realized via a common bias line and placed on the strip side of the device. It is an implant running across all strips and connected to each strip via a polysilicon bias resistor and returned to the backplane. The DC path will carry the leakage current of the device, dominated by thermally generated carriers in the bulk.

•

To maintain isolation between the implants p-stop technology is used.

•

A low resistance ohmic contact to the back of the device is used to apply the high voltage to the sensor. It is obtained through a doped implant (of the same type of the bulk) with a layer of metal in direct contact covering the entire backside of the device. This doped implant is used to prevent the depletion region reaching the metallisation.

3.2 Silicon Detectors Technology 81

•

^{In figure3.11}one can also distinguish the guard ring structure which is independent of the type of sensor. This ring prevents the sensor from a possible electrical breakdown minimising the leakage current at the detector edges. Due to the complex mechanical cutting procedure of the sensor edges they will be conductive and at the backplane potential, which is the bias voltage. Due to the lateral extension of the depletion, when the space charge reaches the cutting edge the strong crystal damage which is present there acts as a very effective generation center and causes a dramatic increase of the leakage current. The purpose of the guard ring (or multiguard rings) is to stablish a smooth voltage drop toward the cutting edge and to assure that the outermost ring is on the backplane potential. No space charge region can then stablish outside the outermost ring.

Most of these features can be observed in the photograph of a

n

-on-

p

silicon microstrip detector which is represented in figure3.14.

Figure 3.14: Microscope view of a silicon microstrip detector. There are pointed the strips, the bias resistance, the bias line and the guard rings.

In document Silicon Strip Detectors for the ATLAS End-Cap Tracker at the HL-LHC (Page 89-93)