In a previous post viewtopic.php?t=36503, I described my motivation and design thoughts for making a chrome on fused quartz SHG slit. One design parameter I did not go into was the choice of optical density (OD) for the chrome mask. The way the semiconductor masks are made is a large number of substrate blanks are prepared with a chrome layer, an antireflective coating and then a photoresist layer. A customer then specifies a pattern to be lithographically etched in the chrome. The manufacturer typically only has a certain stock of blanks with different substrates (i.e. glass or quartz), chrome thicknesses and anti-reflective coatings.
For a five-inch fused quartz substrate, I was offered a choice of OD3 or OD5 chrome. The absorption versus wavelength for the two types of chrome are shown below.
As the charts show, the chrome layer transmits significantly more light at longer wavelengths. This is not so much of an issue at the UV wavelengths normally used in semiconductor manufacturing (the i, h and g refer to mercury emission lines commonly used as a light source). But for the visible spectrum, and in particularly at 650nm, near hydrogen alpha, the OD5 chrome seemed like the better choice. OD3 at H-alpha would have an actual optical density of only around 2.5 (about 0.3% transmission) while OD5 at H-alpha would have an actual optical density of around 3.5 (about 0.03% transmission). The higher contrast ratio seemed important so I went with the OD5 chrome which the manufacturer had in stock.
Mounting the fused quartz chips: Having used Thorlabs slits previously, my SHG equipment is set up for using 1-inch optical tube components. I therefore acquired some copper disks 0.9mm thick, 25.22 outside diameter, with a 12.76mm central hole. These were made by the process of fine blanking, which results in a very flat disk at a fairly low cost. The bare copper has sufficiently good thermal and mechanical properties to withstand any incident solar radiation. I mounted the 15mm fused quartz squares onto the 1-inch copper disks using very small drops of cyanoacrylate glue at the four corners. The following photos show a quartz chip glued onto a copper disk, then held in a short Thorlabs 1-inch optical tube.
The quartz chips are mounted with the chrome layer away from the copper disk. Sunlight from the telescope illuminates the circular hole in the copper disk, passes through the 2.25mm-thick quartz substrate, and is masked by the chrome layer on the rear surface. At this point, it is important to note that both sides of the quartz chip look very dark (in fact, they are a very dark blue). I did not realise this when I made my order, but OD5 chrome is typically made with TWO antireflection oxide coatings, as shown in the diagram below.
The dark oxide coating on TOP of the chrome is necessary in semiconductor manufacturing to prevent light from bouncing back and forth between the mask and the silicon substrate being patterned -- the two are put in very close proximity. In the SHG case, the top chrome layer is also somewhat useful to minimise reflections.
The dark oxide BELOW the chrome layer is designed to minimise transmission in the visible but in the SHG case, leads to a large amount of solar radiation being absorbed, rather than reflected. I had actually expected the chrome/quartz interface to be highly reflective. This necessitated some extensive thermal tests, as described below, to determine whether this chrome structure was suitable for solar work.
First, however, some calculations for incident solar power and solar power density. It can be shown that total power (P) incident on the chip is approximately:
where D is the telescope aperture in mm and the power is in Watts. For a 100mm refracting telescope, the incident solar power is approximately 7.85 Watts -- not an extremely high heat load.
The power density (J) at the chip can be shown to be approximately:
where F is the focal ratio (focal length/aperture) and J is in Watts per cm squared. For a telescope with F = 8, the power density is approximately 15.6 W/cm2. This power density is moderately high but is well within the tolerance for both the chrome layer and the quartz substrate.
Testing the robustness of the quartz chips in the real world: we mounted one of the chips (in its optical tube) at the focal point of a 300mm f5.6 camera lens. This gives an incident power of 2.25W but a rather high power density of 32 W/cm2 (due to the fast f-ratio). The experimental setup is show below.
Looking through the back, one can see the focused image of the Sun partially transmitted through the OD5 chrome layer (the camera did not focus well on the chip). Note that Baader Solar Safety Film is also rated at ND 5.0 and the image does look similar.
Using an equatorial mount, the solar image was maintained over the middle of the slit for several hours around midday during a cloudless period in London. Afterward, there was no sign of any damage to either the quartz substrate or to the chrome mask. I therefore conclude the blue oxide / chrome / blue oxide mask is likely suitable for this application. One possible advantage of this mask structure is there is not much reflection off the quartz chip back towards the telescope optics, which could cause unwanted reflections, as well as being potentially dangerous to spectators.
With hindsight, the OD3 chrome coating might have been a better choice, but it is hard to tell at this stage. The manufacturer has told me that a specialised OD5 chrome coating with blue oxide on only the air interface could be made, but it would be a special order and therefore would be approximately double the price compared to their in-stock blanks.
Since the chrome slit survived the initial thermal load test (as well as some shock testing by banging the optical tube on a table), we will next progress towards full spectroheliograph imaging in the near future.
Thermal testing of 9mm-wide fused quartz SHG slit
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Re: Thermal testing of 9mm-wide fused quartz SHG slit
Woaw!!! Amazing!! Looking forward to see 1st results!!
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Re: Thermal testing of 9mm-wide fused quartz SHG slit
Very interesting
regards Rainer
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Re: Thermal testing of 9mm-wide fused quartz SHG slit
I had these made for me by a mask design shop. So in that sense, I did not do the photolithography myself (it is a rather specialised business to get sub-micron accuracy). I did have to make a lot of design decisions though (no doubt, some of which will probably turn out not to have been optimal).