Micro-combs: a novel generation of optical sources

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade [1–5]. OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hänsch and J. Hall for their breakthrough in OFC science [5]. They demonstrated that a coherent OFC source with a large spectrum – covering at least one octave – can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities [6] are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources [7–13] but also of optical sensors and compact light modulators [6,14]

Effect of Chromium-Gold and Titanium-Titanium Nitride-Platinum-Gold Metallization on Wire/Ribbon Bondability

Gold metallization on wafer substrates for wire/ribbon bond applications require good bond strength to the substrate without weakening the wire/ribbon. This paper compares the ribbon bondability of Cr/Au and Ti/TiN/Pt/Au metallization systems. Both chromium and titanium are used to promote adhesion between substrates and sputtered gold films. Both can diffuse the gold surface after annealing and degrade the wire/ribbon bondability. Restoring bondability by ceric ammonium nitrate (CAN) etch was investigated. Experiments were conducted to investigate the effect of Cr/Au and Ti/TiN/Pt/Au, annealing, and CAN etch processes on 25.4 times; 254 μm (1 × 10 mil) ribbon bonding. All bonds were evaluated by noting pull strengths and examining specific failure modes. The results show that there is no significant difference in bondability between Cr/Au and Ti/TiN/Pt/Au before the annealing process. At this point excellent bond strength can be achieved. However, wire/ribbon bondability of Cr/Au degraded after the wafers are annealed. The experimental results show that a CAN etch can remove Cr oxide. Improvement of wire/ribbon bondability of Cr/Au depends on the CAN etch time. The annealing process does not have significant effect on bondability of Ti/TiN/Pt/Au metallization. Auger electron spectroscopy was used to investigate what caused the difference in bondability between the two metallization

Effect of Chromium–Gold and Titanium–Titanium Nitride–Platinum–Gold Metallization on Wire/Ribbon Bondability

Gold metallization on wafer substrates for wire/ribbon bond applications requires good bond strength to the substrate without weakening the wire/ribbon. This paper compares the ribbon bondability of Cr-Au and Ti-TiN-Pt-Au metallization systems for an optoelectronic application. Both Chromium and Titanium are used to promote adhesion between semiconductor substrates and sputtered gold films. However, both will be oxidized if they diffuse to the gold surface and result in the degradation of the wire/ribbon bondability. Restoring bondability by ceric ammonium nitrate (CAN) etch was investigated. Experiments were conducted to investigate the effect of Cr-Au and Ti-TiN-Pt-Au, annealing, and CAN etch processes, on 25.4times254 mum (1 times 10 mil) ribbon bonding. All bonds were evaluated by noting pull strengths and examining specific failure modes. The results show that there is no significant difference in bondability between Cr-Au and Ti-TiN-Pt-Au before the annealing process. At this point, excellent bond strength can be achieved. However, wire/ribbon bondability of Cr-Au degraded after the wafers were annealed. The experimental results also show that a CAN etch can remove Cr oxide, and that the improvement in wire/ribbon bondability of Cr-Au depends on the CAN etch time. It is further demonstrated that the same annealing process does not have a significant effect on the bondability of Ti-TiN-Pt-Au metallization on the same type substrate materials. Auger electron spectroscopy was used to investigate the causes of the difference in bondability between these two metallizations