1) Production of thin Ultra-fast silicon detectors (UFSD)
The key technological development that has been proposed and produced in the first phase of the project is the production of thin low gain avalanche diodes (LGAD) that we called Ultra-Fast Silicon Detectors (UFSD). UFSDs are a new concept in silicon detector design, merging the best characteristics of standard silicon sensors with the main feature of Avalanche Photo Diodes (APD). The overarching idea is to design silicon detectors with signals that are large enough to assure excellent timing performances but to keep the gain as low as possible. The complicated technological step that needs to be mustered is the capability of keeping the gain value low and be able to segment it.
Charge multiplication in silicon sensors happens when the charge carriers are in electric fields of the order of E ∼ 300 kV/cm. Under this condition, the electrons (and to less extent the holes) acquire sufficient kinetic energy to generate additional e/h pairs. A field value of 300 kV/cm can be obtained by implanting an appropriate charge density that locally generates very high fields. We call this additional doping the "gain layer". The additional doping layer present at the n-p junction in the UFSD design, Figure 1, generates the high field necessary to achieve charge multiplication.
2) Development of the simulation program Weightfield2.
We have developed a full simulation program, Weightfield2 (WF2), with the specific aim of assessing the timing capability of silicon sensors with internal gain. The program (see Figure_2) has been validated by comparing its predictions for minimum ionizing particles and alpha particles with both measured signals and TCAD simulations, finding excellent agreement in both cases. Figure 3 shows on the left the comparison WF2-TCAD for the predicted current produced by a MIP in 300-micron silicon sensors with gain = 14 while on the right the comparison between WF2 and the impulse measured at a beam test with 120 GeV/c pions, using as readout a charge sensitive amplifier. The left side, therefore, shows excellent agreement in the simulation of the mechanisms involved in the current signal. In contrast, the right side shows how the program also correctly simulates the electronic response.
3) Design and production of a custom VLSI chip to readout UFSD sensors.
We designed two full custom amplifier-comparator readout chip for silicon detectors with internal gain designed for precise timing applications. Both ASICs have been developed in UMC 110 nm CMOS technology and aim to achieve a time resolution of ∼ 30 ps. Each channel is independent, and the signal processing chain is composed of (i) trans-impedance amplifier, (ii) single threshold discriminator, (iii) stretcher, and (iv) LVDS driver.
The ASIC is optimized for a sensor capacitance of 3-10 pF and an input charge between 3 and 30 fC. The total power budget per channel is less than 30 mW.
4) Measurements of time resolution of UFSD
We performed several beam tests at the CERN facility in Geneva with π-mesons with a momentum of 180 GeV/c and at FNAL at Fermilab with protons of 120 GeV/c. Many UFSD geometries have been tested successfully, demonstrating an excellent time resolution of about 30 ps.
5) Study of radiation damage to the gain layer
Radiation damage causes three primary effects in UFSD: (i) decrease of charge collection efficiency, (ii) increase of leakage current, and (iii) changes in doping concentration.
Effect (i) and (ii) are not influencing the performances of UFSD.
Changes in doping concentration: UFSD sensors have shown a decrease of gain values for fluences above 1e14neq/cm2, with a complete disappearance of the gain at 1e15neq/cm2. This effect has been understood as a gradual inactivation of acceptors due to radiation defects. Significant progress achieved in the past two years has been the understanding that the addition of impurities, especially that of carbon atoms, reduces radiation damage significantly. Now, carbon-infused UFSDs are the most radiation-hard UFSD sensors ever produced.
Results have been described in many conferences and invited talks. A detailed summary of the grant achievement has been written in a open-source book, "An introduction to Ultra-Fast Silicon Detector"
