Fully populated lens arrays master diamond turned with DPI®

To demonstrate the performances of its patented DPI® technology for diamond turning of lens arrays, Wielandts UPMT has designed a fully populated master based on typical industry requirements in terms of lens sizes, number of lenses, master dimensions and tolerances for imaging applications.
The master is a 200 mm diameter disc made of aluminum alloy plated with a 500 µm nickel-phosphorous (NiP) layer. The structure was cut into the NiP because this alloy delivers a very high optical quality (low surface roughness, few surface defects) and a higher hardness than aluminum, which makes cleaning and handling of the master easier and less risky.
The lens surface was defined as an asphere with a gull-wing profile as shown hereunder. The optical lens profile with both concave and convex curvature is particularly challenging to originate, especially because it is highly intolerant to tool offset and tool radius errors during diamond turning, which can lead to high curvature error and form irregularity. The selected lens profile is thus considered a good choice for bench-marking the mastering technology.

What is DPI® ?
DPI® stands for “Dynamic Part Indexing”. This technology has been developed and patented by Wielandts UPMT as an innovative positioning equipment to automatically shift a part on the spindle of an ultra-precision lathe, allowing efficient on-axis diamond turning of large lens arrays. This equipment overcomes the difficulties of accurate positioning and spindle balancing.

Figure 1: Isometric view of one MLA with 4 alignment lenses.

Figure 2: Gullwing profile of the aspherical lenses.

The lenses’ sag varies between 142 µm at its maximum and 91 µm at the apex.
The packing of lenses inside the MLAs is of hexagonal type. The center-to-center distance between each lens is 2.595 mm. Because the center-to-center distance is smaller than the lenses’ maximum diameter (3 mm), a natural intersection is formed between adjoining lenses. This natural intersection is extremely sharp (less than 1 µm rounding), which leads to a very high effective fill-factor.
Each MLA contains 23 identical lenses. 4 additional alignment lenses have also been diamond turned around each MLA. These lenses or features are often requested to serve as alignment features during the replication, dicing and assembly processes.
29 MLAs were machined over the surface of the 200 mm diameter master. The MLAs are arranged in orthogonal rows and columns with a regular spacing. The master also contains a set of two additional alignment marks, which can serve as fiducials for a mask aligner in UV imprint replication for example.

Figure 3: MLA master isometric top view.

The master was diamond turned using the DPI® technology in a State-of-the-Art diamond turning lathe.
Depending on the master/lens design, DPI® leads to significantly faster origination compared to any other available diamond machining technique (like e.g. diamond micro-milling or fast-tool servo turning). On top of these speed gains, DPI® offers drastically improved design freedom in terms of surface slope angle, sags, possibility to make diffractive features, achievable roughness and form error, etc.
In most cases, other diamond machining techniques are not even considered because they lead to impossible machining times (multiple weeks to several months). Thanks to DPI®, partially or fully populated masters with few or no step & repeat can be considered.

Figure 4: Pictures of machined fully populated master.

The surface roughness was measured by laser scanning confocal microscopy. The roughness was evaluated along a line perpendicular to the cutting direction of the diamond tool (critical cross-section, where the surface roughness is the highest). According to the ISO 4288, the high pass cutoff length for this roughness must be set to 80 µm.
The measured surface roughness is between 2.8 and 5.2 nm Ra.

The lens surface form error was measured by tactile profilometry. The master was aligned under the instrument using its linear and rotary motorized stages and by probing the alignment fiducials. Once correctly aligned under the instrument, long profiles were acquired across the lens arrays.
Seven different MLAs were measured across their long side comprising five lenses. The measured and fitted profile for one MLA is shown hereunder. For each lens taken separately, the design or nominal profile was fitted (position and tilt only, not optimization of the radius of curvature, conical constant or other aspherical coefficients) by optimizing the residual RMS form error.
The surface form error varies from 19 nm to 34 nm RMS on 7 different MLAs.

	Average PV (nm)	Average RMS (nm)
MLA 1	94	20
MLA 2	82	19
MLA 3	104	25
MLA 4	139	34
MLA 5	111	26
MLA 6	121	28
MLA 7	127	30

Table 1: Surface form error evaluated along 35 lenses in 7 MLAs.

Figure 6: Measured and fitted aspherical profiles for one MLA.

Figure 5: Roughness measurement results.

In conclusion, a fully populated master has been manufactured using the DPI® technology. This master contains 29 lens arrays, each one composed of 23 identical aspherical lenses (667 lenses in total). A very sharp natural intersection is created by intersecting lenses, which leads to a 100% fill-factor for each MLA. While DPI® is constrained by certain design limitations (e.g. minimum aperture and minimum sag >10 µm), it has been demonstrated that it is a very effective technology to manufacture (micro) lens array masters beyond any existing solution. By using this technology, fully populated masters can be produced with multiple lenses or lens arrays covering the total surface of a master. By adding any type of alignment marks and fiducials, DPI® can be considered as a complete breakthrough mastering solution for optics replication processes. Read more about the DPI® technology.