Dark current measurements of diodes on two SLID test wafers

Transcription

1 Dark current measurements of diodes on two SLID test wafers Information collected by M. Beimforde for the MPP Inner Detector Upgrade Working Group August 16, 2007 Abstract In this short report the dark current measurements of test diodes on two dierently processed wafers are presented. They were carried out to quantify the impact of the processing steps needed for the SLID interconnection technique on sensors used for particle detectors. The measurements show that the dark currents stay approximately constant and hence, indicate the absence of a diusion of copper into the silicon. 1 Introduction For the high luminosity upgrade of the LHC, i.e. the Super LHC, new semiconductor detectors have to be developed to withstand the increased radiation close to the interaction point. In our approach [1] for a radiation hard upgrade of the ATLAS pixel detector, the SLID (solid liquid inter diusion ) interconnection technique, developed at the IZM [2], is envisaged to be used in combination with thin sensors and readout chips as well as inter chip vias (ICV). To eventually employ these new techniques, rst tests towards the SLID interconnection have been carried out with two wafers accommodating thin diodes for dark current measurements. Since the SLID process involves the deposition of copper onto the wafers as well as a temperature treatment it is crucial to verify that no copper diuses into the silicon which would manifests itself by generating large dark currents. The dark currents measured before, between and after the dierent process steps of the two wafers are compared in this report. 2 Processing the wafers The two test wafers are equipped with a number of test structures, including boxes containing four diodes each, Fig. 1. There are `open' and `closed' diodes, without and with a BCB (benzocyclobutene) passivation over the diode area, respectively. Each diode has

2 2.1 Wafer 1 2 Figure 1: A diode box with dierent diodes. an area of 10 mm 2 and is 50 µm thick. Including the 450 µm handle wafer which is wafer bonded [3] to support the thin diodes, the total thickness is 500 µm. For the SLID interconnection dierent layers of metals need to be deposited on the contacts of the structures that are being connected, Fig. 2(a). The Cu and Sn are used to form a conducting alloy with a high melting point whereas the TiW is used as a diusion barrier between the silicon and the copper. The actual soldering is carried out by applying a pressure of 5 bar to the aligned contacts and exposing the SLID sandwich to a temperature of about 300 C, Fig. 2(b). In the presented case only the metal layers were deposited and the wafers were heated without being connected. 2.1 Wafer 1 For wafer 1 two process steps were carried out at the IZM. In a rst step a 100 nm TiW barrier and 200 nm of Cu were sputtered onto the wafer. Afterwards 1 µm of copper was galvanically grown on top of this. Thus, wafer 1 corresponds to the bottom part of Fig. 2(a). The second process step was the SLID temperature treatment, i.e. the heating up to 320 C in a formation gas for 15 minutes. Before, inbetween, and after the process steps the dark currents were measured at the HLL [4] or the MPP [5], see Sec Wafer 2 For wafer 2 one more process step was needed. Comparable to wafer 1, the rst step was the sputtering of the 100 nm TiW barrier and 200 nm of Cu as well as the galvanic deposition of 1 µm of copper. Due to incorrect metal depositions, the second step was to etch o the SLID layers and to redo them. This time, 5 µm of Cu and 1 µm of Sn were

3 3 (a) SLID layers before soldering (b) SLID layers after soldering Figure 2: The SLID process. galvanically grown on the sputtered layers. Thus, wafer 2 corresponds to the top part of Fig. 2(a). Afterwards, as the third step, the temperature treatment was carried out as for wafer 1. Therefore for wafer 2 four measurements were carried out before, inbetween, and after the three mentioned process steps. 3 Measurements The initial measurements, before any SLID process steps, were carried out at the HLL for both wafers. After the metal depositions the measurements were carried out at the MPP. 3.1 The setup The setup at the MPP consists of a probe station in a dark box, a Keithley Picoamperemeter 6517 A, and a PC to control it. For the investigated diodes IV-curves were measured with reverse bias voltages ranging from 0 to 100 V or breakthrough. The plateau of the IV-curves was tted between 30 and 75 V with a straight line and the t's value at 50 V was used as a reference to compare various diodes. The measurements were carried out at a temperature of 22 C to about within 1 C. 3.2 Results In Fig. 3 the dark currents before and after the metal depositions on both wafers are compared. For wafer 1, one of the 23 measured diodes was excluded from this analysis because of a very unstable measurement before SLID processing. For wafer 2, 18 diodes were compared, also shown in Fig. 3. While the dark current stays almost constant for wafer 1, a minor increase is seen for wafer 2. Nevertheless, this suggests that no signicant amounts of copper have diused into the silicon.

4 3.2 Results 4 Figure 3: Dark currents before and after metal deposits. Fit: y = p 0 x. Wafer 1: p 0 = 0.97 ± 0.01, wafer 2: p 0 = 1.33 ± As stated in Sec. 2.2 the SLID layers of wafer 2 had to be etched o and redeposited. A comparison of the dark currents of 24 diodes before and after this procedure is shown in Fig. 4. Again the currents change only slightly and, thus, show that even the etching and redeposition did not globally harm the test structures. After the SLID temperature treatment the dark currents are lower than before, Fig. 5. A possible explanation for this might be that the temperature treatment was likely carried out in hydrogen rich formation gas. The protons of this gas could diuse into the diodes and remove dangling bonds between the Si and SiO 2 that act as a source of leakage current. For wafer 2, four of 27 diodes were excluded from the comparing plots after the SLID temperature treatment because of: 1. an early breakthrough at 30 V 2. a very unstable measurement 3. a very high leakage current 4. a very high current without bias On all four diodes optical checks showed either a damaged contact, Fig. 6(b), or metal spills on the central contact or the guard ring, Fig. 6(c) and 6(d). These defects were already observed before the temperature treatment was carried out.

5 3.2 Results 5 Figure 4: Dark currents for wafer 2 before and after redepositing the SLID layers. (a) Wafer 1 (b) Wafer 2 Figure 5: Dark currents before and after the SLID temperature treatment. Fit: y = p 0 x.

6 6 (a) Damaged contact, wafer 1 (b) Damaged contact, wafer 2 (c) Pileups on contact, wafer 2 (d) Pileups on guard ring, wafer 2 Figure 6: Examples of damaged diodes. 4 Optical tests Besides the dark current measurements also optical inspections of the test diodes were made. They showed that the SLID process steps can also be a risk to the silicon structures. On both wafers a damaged contact was observed, Fig. 6(a) and 6(b), and on some of the diodes of wafer 2 metal pileups or spillings were seen, Fig. 6(c) and 6(d). While for the diode in Fig. 6(a) the dark current is quite low, the wafer 2 diode in Fig. 6(b) was excluded in Fig. 5(b) due to very high and uctuating currents. In total from the 27 diodes measured on wafer 2 after the temperature treatment 7 showed some visual damage. Four of these diodes had to be excluded from the analysis while none of the undamaged diodes showed a very unexpected behavior. Another eect of the temperature treatment of wafer 2 is visible on the guard ring in Fig. 6(b). Here and on almost all other diode guard rings dark spots of dierent sizes can be observed. However they seem not to inuence the measurements.

7 7 5 Summary As the measurements show, the dark currents of most diodes on both wafers do not dramatically change despite the dierent SLID process steps. Especially a diusion of copper into the silicon bulk is unlikely for most of the diodes since this would have lead to dark currents in the nano- to microampere range. A total of 7 diodes of wafer 2 showed visual damages or irregularities after the SLID layers were redeposited. Four of these 7 diodes showed very irregular behavior after the temperature treatment and, thus, were excluded in the comparison. The chance that these 4 irregular measurements are just coincidentally seen for the 7 damaged diodes is less than 0.2%. Overall this analysis shows that no signicant deteriation of the sensor properties due to the SLID process is expected, provided that irregularities seen on several diodes are avoided. References [1] RD50 - Radiation hard semiconductor devices for very high luminosity colliders. URL [2] Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration. URL izm.fhg.de/izm/index.html. [3] Q. Y. Tong and U. Goesele. Semiconductor Wafer Bonding. New York: Wiley, [4] MPI Halbleiterlabor. URL [5] Max-Planck-Institut für Physik (Werner-Heisenberg-Institut). URL mppmu.mpg.de.