Lens array with submicron position accuracy

UPMT’s DPI® and CLAP™ proprietary technologies further push the limitations of micro optics mastering by achieving submicron position accuracy over the complete area of a large master, introducing new solutions for small series to large volume replication.

DPI® : a unique solution for optical lens mastering

Multiple approaches have been developed for the manufacturing of lens array molds and masters, like lithography-based processes, laser direct writing, or machining methods (by chip removal), like diamond milling or slow slide/fast tool diamond turning. However, these manufacturing techniques are all facing geometrical limitations: limited sags, 100% fill factor no achievable, insufficient shape accuracy, etc.

It has been demonstrated that UPMT’s proprietary technology called DPI® overcomes many of these limitations. By developing this innovative positioning tooling to automatically shift the substrate, efficient on-axis diamond turning of large lens arrays is made possible while taking advantage of the ultra precision of diamond turning. Based on this manufacturing technology, arrays of lenses can be machined even with high slopes (>80°), important sags (up to mm’s) or 100% fill factor (look at the design freedom offered by the DPI® technology).

Figure 1: DPI® – Microscopy.

Figure 2: DPI® – Profilometry measurement.

Limitations due to unpredictable external conditions

While the DPI® system is compensated for all repeatable errors (manufacturing, assembly errors, deformations, etc), it still has to face unpredictable environmental conditions. For most applications, their impact is considered as acceptable. However, high precision optical applications like AR/VR, automotive and consumer goods imaging and sensing are continuously requiring more extreme tolerances that can be impacted by very slight variations of the environmental conditions.

This is especially the case when the machining time takes more than a few hours. In such case, thermal variations of the environment will lead to dimensional changes of the machine and components that will principally result in degraded position (X/Y/Z) and shape accuracy (e.g. RoC) of the lenses. Up to now, sub-micron position accuracy required by cutting-edge applications could not be achieved by any diamond machining technology, not even by DPI®. To solve this problem, UPMT has successfully developed additional solutions in order to upgrade the capabilities of DPI®.

CLAP™: the ultimate XYZ accuracy on machined lens array masterS

To achieve the required positioning accuracy (X/Y/Z), stability and reliability, a new process as been developed as an add-on to the DPI® machining. This new process called CLAP™ (for Closed Loop Automatic Positioning) dynamically controls and compensates for the position errors of the substrate and of the cutting tool. This compensation is running during the complete manufacturing time such that extreme accuracy can be achieved over the complete master without being degraded by varying environmental conditions.

To demonstrate the capabilities of the new CLAP™ alignment process combined with DPI®, a test master was machined and measured. 4 identical MLAs containing 196 lenses each were diamond turned on a 200mm diameter NiP substrate. The selected lens profile was an asphere with a sag of about 80µm.

Figure 3: CLAP – Test master.

Figure 4: CLAP – Test master lens profile.

Measurements results

XY Position error

Z Position error

The measurement results show that the CLAP™ system achieves sub-micron position accuracy (XY) over all the lenses of all the machined lens arrays, with a typical average position error of 0.3µm (!).

The position of each MLA with respect to each other was also measured. As shown in the table here under, the achieved position error from MLA to MLA is below 1µm.

	X Position (mm)	Y Position (mm)	Position error (mm)
MLA 1	0	0	0
MLA 2	100.0004	-0.0005	0.00064
MLA 3	99.9995	99.9999	0.00051
MLA 4	-0.0001	-100.0002	0.00022

The errors observed on the sag of the lenses are also directly linked to unstable environmental conditions affecting the entire machining process. As shown on the diagram, while using the CLAP™ technology the residual errors measured are substantially lowered with a maximum sag variation of +/- 0.5µm.

In conclusion, despite the duration of the machining process and the large size of the master, the measurements performed on this demonstrator show that the DPI® technology coupled with the new CLAP™ alignment system delivers outstanding XYZ accuracy. While maintaining the unrivaled form and roughness quality achieved by diamond turning, these new developments allow to machine lens array masters or fully populated masters with incomparable lens to lens accuracy, for example allowing more products to be replicated simultaneously while avoiding (or even compensating for) errors introduced by other consecutive processes.