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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Microfabrication Laboratory (Cleanroom)

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Fig. (1) A wafer with devices is being loaded into our reactive-ion etcher. In order to keep the lab space clean the main lab rooms are only accessible via air lock and the personnel is required to dress-up in overall, hairnet and gloves. 

Introduction

The microfabrication laboratory, a cleanroom with air lock and independent air conditioning and filtering system, is a cornerstone to the institute's infrastructure. The lab is run by the detector group's cleanroom personnel and is specialised in the fabrication of superconducting devices for our radioastronomy application. It is fully equipped with the thin-film and microstructuring capabilities needed to fabricate superconducting detector devices and silicon micromechanical structures, very similar to what is used in the semiconductor industry.
 
The cleanroom lab space consists of five rooms with four of them being connected with common air lock for entry:
  1. deposition lab: tools for deposition of thin films
  2. etcher lab: reactive-ion etcher for structuring of thin films
  3. litho lab 1: optical lithography with mask aligner, inspection microscope and wet chemical processing, yellow lighting
  4. litho lab 2: wet chemical processing and inspection microscope, yellow lighting
  5. ebeam lab: e-beam lithography tool and device mounting facilities
In addition, there are auxilliary rooms for dicing and lapping of wafers (i.e. "dirty processes") as well as storage of consumables and spares.
 
The lab was founded in 1989 and the first 230 GHz SIS junctions were already successfully fabricated in early 1990. The laboratory features a suite of lithographic tools to define device structures down to the nanometer range by means of electron-beam lithography as well as deposition equipment for the materials required for our circuits, e.g. superconductors, normal metals and dielectrics and an inductively coupled plasma (ICP) etcher for structuring of thin film and MEMS structures. These tools are completed by two lithography rooms with wet benches for liquid chemistry processing, resist spinners and hot plates and yellow lighting for handling of photosensitive materials. For quick inspection several high high power (magnifications up to 1000x, with differential interference contrast) optical microscopes from Zeiss are available.
 
With the support of an engineer and a technician and under supervision and assistence by senior scientists, many PhD students have earned their doctoral degree with fabrication of devices as part of their thesis work.
 
 

ICP Plasma Etcher (Oxford Instruments Plasmalab System 100)

Fig. (2)  The Oxford Instruments ICP etcher can anisoptropically etch the thin films of our circuits and MEMS structure silicon. It is equipped with an end-point control system by means of a dual-beam laser beam.
 
The Oxford Instruments Deep Reactive Ion Etcher (DRIE) uses independent inductively-coupled and RIE plasma sources for etching (removing) of our thin film materials. This state-of-the-art tool features cryogenic and Bosch Silicon etching capabilites that are heavily used for structuring of ultra-thin substrates with SOI technology and for silicon micromachining (MEMS) of e.g. optical components such as the AR structured Si vacuum windows used in our THz receivers. The MEMS capabilities are crucial for the waveguide devices we develop and allow the micrometer precise definition of arbitrarily shaped (by means of photolithographic processes) Si membranes. For the so-called Bosch process the etcher features a dual-beam interferometer for precise in-situ measurement and online end-point detection of etch depth. In single beam mode reflectrometry is used for determination of end-point, e.g. during etch of SIS junctions or HEB microbridges.
 
 

Electron Beam Lithography System (Raith PIONEER)

Fig. (3)  The Raith PIONEER electron beam lithography system is used for the smallest features on our devices.
 
Our state-of-the-art Raith PIONEER electron beam lithography system, in use since September 2013 and equipped with a field emission source and a laser interferometer stage, is used for fabricating devices with dimensions reproducible better than 50 nm. With the laser interferometer high resolution stage writing fields, e.g. 100 µm x 100 µm, can be stitched with better than 50 nm repeatability. Compared to our old home-grown e-beam system, which was decommissioned ever since this tool went online, the vastly expanded capabilities of this e-beam tool allow us stitch together the RF circuits of the THz mixers, e.g. for the 4.7 THz HEB mixer, under fully automated software control. Only its significantly improved imaging (writing) optics allowed a precise control of the lithography processes required for fabrication of our 4.7 THz HEB mixers. Its capability is more than sufficient for future higher frequency work.
 
The much higher lithographic resolution achievable of this system, when compared to optical lithography (see below), comes with the disadvantage of longer processing times. Already in the design stage of a new detector circuit we take this into  account and with typical result of employing a mix and match approach using e-beam for the critical structures (THz RF circuit, SIS junction, tuning circuit, HEB microbrigde and contacts) and the much faster mask aligner lithography for everything else.
 
 

Optical lithography (Karl Süss MJB3)

Fig. (4)  A Karl Süss MJB3 mask aligner with UV300 option is used for photolithographic definition of large scale structures. Fig. (5)  One of our Zeiss Axioscope 1000x inspection microscope with imaging metrology system.
 
One Karl Süss MJB3 contact mask aligner is used for photolithographic definition of larger, wafer-scale structures such as wiring to each device. This tool has been around since 1989 and is the workhorse for quick lithographic processing not only of device structures but also IF circuits etc.. Due to the UV300 option (Hg light source, 320 nm line) the mask aligner can reliably define structures down to 0.8 µm line width. Optical photolithography is much faster than the more powerful e-beam lithography.
 
During circuit design a trade-off always has to be made between the two lithography options, e.g. resolution vs. speed. Typically our circuits are defined by a mix and match technique, i.e. using e-beam lithography for the critical areas on the wafer and optical lithography for everything else.
 
 

Deposition Tools

Sputtering

Fig. (6)  Old sputtering machine for deposition of superconducting and normal metals (CJT). Fig. (7)  Multi-chamber sputtering system for deposition of superconducting metals and ICP plasma source for AlN tunnel barriers (4-tec).
 
We have two sputtering tools, from CJT and 4-tec, for deposition of superconductor (Nb, NbTiN, NbN) and normal metal (Au, Al, Ti) thin films. Both feature four 3" sputter guns with DC or RF capability and are load-locked with additional RIE cathode. On both systems the sputter chambers are pumped by a large cryo pump in order to enable high flow rates. The machines are used for SIS trilayer deposition as well as the HEB fabrication processes. The CJT is our trusty old trilayer system for 2" wafers in operation since 1989 and can go into the mid 1E-9 mbar range. It is mainly used for the standard Nb/AlOx/Nb trilayer processes as well as the Au normal metal layers for our THz circuits and HEB contact pads.
 
The 4-tec (installed 1999) due to a 1000°C heater station is primarily used for the NbN HEB processing, and, additionally for nitridation of AlN barrieres using an in-situ ICP source. Its vacuum is in the 1E-8 mbar range and it can handle wafers up to 4". The 4-tec has now been decomissioned and its capability will be fully replaced by the new UHV sputterer from AJA Inc. (see below).
 
In addition, we have a sputterer that is setup for deposition of SiO2 (Si, SiN) by means of a 3" RF sputter gun. The SiO2 is mainly used as dielectric for our planar circuits. With a cryo pump the vacuum gets down fast into the 1E-7 mbar range without the need for a load-lock.
 

Evaporators

Fig. (8)  Bell-type thermal evaporator (self-made). Fig. (9)  RF sputtering machine for dielectrics and e-beam evaporator (CJT).
One self-made bell-type thermal evaporator is used for deposition of various materials, incl. SiO for circuit dielectric as well as the normal metals Au and Cu. It feature a plasma clean capability and it end vacuum is in the 1E-6 mbar range. The RF sputterer mentioned above also features an e-beam evaporator source for material development.
 

New UHV Sputtering Tool

We have a new UHV dual chamber load-locked multi-deposition system from AJA Inc. arriving within the next few weeks (Jan 2017). The system features a sputter chamber with five 4" and, additionally, to two 1.5" confocal sputter sources, and a second chamber for nitridation with an ICP source. The tool includes several substrate station options incl. 1000°C heating as well as RF for substrate biasing. This tool will not only replace the 4-tec sputterer for the HEB NbN and AlN SIS trilayer processes but also be used for materials development. Due to its siginficantly larger diameter sputter guns this system will allow processing of 4" wafers with good thin film uniformities, which will be required for future larger scale planar circuits such as MKID arrays and integrated 2SB/balanced SIS mixer devices. The system is fully bakeable and will go into the low 1E-9 mbar range.
 
 

Lithography Labs

Fig. (10) One of two wet benches for chemical processes required during fabrication. such as development of resist or organic solvents for lift-off processes. Fig. (11) One of two resist spinner and hot plate setups available.
 
Our cleanroom is equipped with two lithography labs, litho 1 and 2, that use yellow lighting in order to allow processing with photosensitive materials such as photoresist. Litho 1 is equipped with the mask aligner as well as a process oven for baking of e.g. thick photoresists. Both labs feature high resolution optical microscopes (Zeiss Axioscope, 1000x magnification) with differential interference contrast and imaging systems for metrology. Both labs feature wet benches with their own fume extraction for processing with liquid chemicals such as organic solvents, photoresist related chemicals or acids. For resist processing each lab is equipped with a spinner and two hot plates with precise temperature control (+/- 0.1°C).
 
 

Device mounting and contacting

Fig. (12)  Micrometer precision device mounting setup using a Smarpod™ hexapod translation and rotation stage under a ZEISS Axioscope (self-made). Fig. (13)  Ultrasonic wedge bonder used for Al or Au wire and tab bonds for beamleads (Hybond 522).
 
For mounting of our detector devices with the micrometer precision that THz waveguide circuits require we have developed our specialised setup based on a hexapod stage (SmarPod™) with 6-degrees of freedom. A single device is held by a vacuum tweezer and carefully aligned with the waveguide structures using this a remote control unit. Naturally a high power microscope is required for this handling. For reproducible mounting procedures of many devices, i.e. for the focal plane arrays that we populate, this nano-robotic tool has proven to be invaluable.
 
A wirebonder (Hybond 522) is used for contacting the devices and circuits. Tradionally wedge bonded 25 µm diameter aluminium or gold wires are used, currently mostly only used for connecting IF circuits.The membrane type mixer devices that we developed in the recent years to not need wires any more for contacting, as we provide the devices with beamleads during microfabrication. The beamleads not only suspend and palce the devices in the waveguide environment but also serve as DC and RF contact. For this we swap out the usual wedge with a tab bond tool.
 
 

Additional tools

Wafer processing

For "dirty processes" we have an auxiliary room outside of the cleanroom area, which holds to dicing saw and lapping machine. A wafer dicing machine (Tempress) is used to cut out devices of eg. 1800 µm x 50 µm x 30 µm from the quartz or silicon wafer on which the devices are fabricated. Before we had the DRIE capability only rectangular shaped devices therefore could be cut. For all membrane-beamlead type devices that run through the DRIE process we do not need the dicing saw anymore and only use it to dice section from wafers for DC characterisation of several devices a time.
 
A crystal polishing machine (Logitech PM4) available for precision lapping of substrates or wafers down to to about 30 µm thickness with micrometer precision.
 

Thin film profilometer

A thin film profilometer (Tencor Alphastep) is available in order to measure film thickness down to 5 nm. It is used for quick process control of deposited thin films and e.g. for establishing current sputter rates.
 

Chemical planarization process

In the cleanroom have a dedicated chemical-mechanical polishing (CMP) setup for planarisation of dielectric layers e.g. for SiO2. Similar to what the industry required CMP for their multi-layer circuits we use this CMP setup in the past for planarisation and contacting structures that are isolated in a dielectric e.g. the SIS junction top electrode. CMP allows very small and high quality junction areas to be defined.
 

Electroplating Setup

We use a simple electroplating setup for growing of our thick (2-3 µm) beamlead structures that all new heterodyne devices are based on. The setup is placed onto one of the wet benches when required.