XIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of
writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the
polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently
exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrally-spatially-
resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics.
Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective
area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega
launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest
Observer competitive program and it is organized as a consortium across Europe with main contributions from
Italy, Germany, Spain, United Kingdom, Poland, Sweden.
As the mask technology matures, critical printing features and sub-resolution assist features
(SRAF) shrink below 100 nm, forcing critical cleaning processes to face significant challenges.
These challenges include use of new materials, oxidation, chemical contamination sensitivity,
proportionally decreasing printable defect size, and a requirement for a damage-free clean.
CO2 cryogenic aerosol cleaning has the potential to offer a wide process window for meeting
these new challenges, if residue adder issues and damage can be eliminated. Some key
differentiations of CO2 cryogenic aerosol cleaning are the non-oxidizing and non-etching
properties compared to conventional chemical wet clean processes with or without megasonics.
In prior work, the feasibility of CO2 cryogenic aerosol in post AFM repair photomask cleaning
was demonstrated. In this paper, recent advancements of CO2 cryogenic aerosol cleaning
technology are presented, focusing on the traditional problem areas of particle adders,
electrostatic discharge (ESD), and mask damage mitigation.
Key aspects of successful CO2 cryogenic aerosol cleaning include the spray nozzle design, CO2
liquid purity, and system design. The design of the nozzle directly controls the size, density,
and velocity of the CO2 snow particles. Methodology and measurements of the solid CO2
particle size and velocity distributions will be presented, and their responses to various control
parameters will be discussed. Adder control can be achieved only through use of highly
purified CO2 and careful materials selection. Recent advances in CO2 purity will be discussed
and data shown. The mask cleaning efficiency by CO2 cryogenic aerosol and damage control
is essentially an optimization of the momentum of the solid CO2 particles and elimination of
adders. The previous damage threshold of 150 nm SRAF structures has been reduced to 70nm
and data will be shown indicating 60 nm is possible in the near future. Data on CO2
tribocharge mitigation, the main cause of ESD, will also be presented and application to current
technology nodes discussed.
This paper describes a new surface cleaning approach for photomask cleaning. This non-conventional cleaning method uses momentum transfer between aerosolized, frozen, CO2 particles and the contaminants for effective removal from surface. The purity of CO2 used is an important component in the determination of cleaning efficiency. The authors will present two methods developed to analyze hydrocarbon contamination in CO2. These analytical methods were used to compare different grades of CO2 including Ultra High Pure (UHP) grade developed for sub-micron particle removal from photomasks. Using the UHP grade CO2, it was shown that greater than 99% particle removal efficiency is possible from silicon wafer surfaces, with higher removal efficiency of sub-micron particles compared to larger size range. This particular characteristic of particle removal by cryogenic aerosol method is theoretically derived in an earlier paper. In this paper results of CO2 cryogenic aerosol cleaning with respect to electrostatic discharges on two different binary masks are presented. The paper also shows the removal of 99.9% of the progressive defects such as haze of 0.5 to 1.0 μm size. Cleaning characterization of attenuating phase shift masks with MoSiON films indicate 0.04% change in transmission and 0.37% change in phase angle after 16 cleaning cycles, suggesting that cryogenic cleaning has minimal effect on transmission and phase of att-PSM.
This paper describes the mechanism and cleaning results of a dry cleaning technology using CO2 cryogenic aerosols. The cleaning mechanism relies on momentum transfer from the aerosol particles to overcome the force of adhesion of the contaminant particles on the surface. Particle removal is possible without degradation or etching of underlying film or the need for drying with IPA as in wet cleaning. A theoretical model of particle removal based on momentum transfer is described, predicting higher removal efficiency for sub-micron particles compared to larger particles. Experimental results with Si3N4 particles on silicon wafers show that removal of sub-micron particles is 10% higher than larger particles up to 30 μm, as predicted by the model. The paper also shows experimental results of various types of contaminant particle removal in photomask cleaning. Results of post mechanical repair cleaning of photomasks show effective removal of the quartz particles without damage to the adjacent chrome lines. Inorganic contaminants such as ammonium sulphate, commonly known as "haze", is removed by cryogenic aerosol cleaning with 99% efficiency as seen using optical inspection tool. The effect of cleaning on the phase and transmission of the mask is measured with multiple cleaning. The results show that over 16 cleaning cycles, the change in transmission is 0.04% an the change in phase is 0.37°. Thus a non-invasive cleaning for sub-micron particles from photomasks is possible with CO2 cryogenic aerosols.
CO2 snow cleaning technology has demonstrated the ability to clean a variety of contamination types in a non damaging, environmentally friendly manner. CO2 snow cleaning methods have been used to clean both particles and light organics and currently can be found in a variety of high tech industries to include, semiconductor (Si and GaAs), flat panel display, disk head and media manufacturing, and fiber optics. This paper offers an introduction to basic CO2 properties and cleaning methods in general and photomask cleaning in particular. Focus will be placed on the CO2 snow cleaning method to include: the snow making process, process parameters, process issues, contaminant types, CO2 sources and specification, and safety. A new CO2 snow system to clean current and next generation photomask substrates has been developed and is currently being tested. Machine specifics, process parameters, process issues, and cleaning data, will be discussed in detail.
The location of particulates in and about a telescope assembly after jet spray cleaning of a contaminated primary mirror surface under vacuum conditions is determined. The Pathfinder experiment described here is designed to track both the location and level of residual particulate contamination in a simulated telescope assembly after jet spray. Silicon witness plates at various positions in the telescope mockup are used to track remaining contamination after jet spray cleaning of a severely contaminated primary mirror. Scatter measurement at a wavelength of 0.5145 microns is used to determine the contamination level of the silicon witness plates before and after jet spray cleaning. Analysis of scatter and visual particle counting data demonstrated that 99.99 percent of particles are removed. Residual particles levels inside the tube are determined to be on the order of background levels.
Space optical systems operating at cryogenic temperatures may have stringent contamination levels due to high off-axis rejection requirements for the optics. The extreme difficulty in launching and maintaining clean cryogenic optics on-orbit is alleviated by incorporation on-orbit cleaners. This paper describes a jet spray method for removing contaminants from space optics operating down to 30 K and the apparatus. Results of experiments performed on four substrate materials using various cleaning snows and jet spray arrangements will be discussed. Cleaning efficiencies of the jet spray were measured using an in situ bidirectional reflectance distribution function (BRDF) scatterometer built into the space simulation chamber.