1.

Introduction

Presently, augmented reality (AR) is increasingly prevalent, being used to showcase products by companies in a range of industries. For example, automotive companies such as Volkswagen featured AR-based head-up-displays in their vehicles. Consumer electronics companies such as TCL, Vuzix, Sony, HTC, and Canon have also demonstrated their commitment to AR glasses by announcing new products in their AR/virtual reality (VR) portfolio. This growing market interest has only solidified our resolve as a consortium of companies to continue exploring the potential of high-volume and low-cost manufacturing of waveguides based on surface relief gratings (SRGs). Furthermore, it should be noted that it is important to consider the entire process flow from design to final waveguides to achieve the best performing waveguides.

In this paper, based on the proceedings of SPIE 2022 and 2023,^1,2 we focus primarily on quality. We aim to demonstrate the replication fidelity and image quality data across the large Gen5-size format ($1100\mathrm{mm}\times 1300\mathrm{mm}$), as well as reproducibility over time. In addition, we are able to improve in quantity by producing 270 waveguides per imprint cycle on a Gen5-size substrate by tiling 9 individual $300\mathrm{mm}\times 300\mathrm{mm}$ size, 1.9 refractive index glasses. Together, quality and quantity are required to meet the potential mass-market demand for cost-effective AR waveguides.

2.

Waveguide Design

The design of the waveguide of the demonstrator was performed by LightTrans International. As a starting point, a well-known and understood basic layout was chosen: a so-called 1D-1D pupil expansion, which typically consists of three different grating regions [incoupler, exit pupil expander (EPE), and outcoupler, see Fig. 1]. As the name already implies, this particular approach is based on the separation of the directions of the pupil expansion into two different grating regions. While a uniformly distributed expansion of the light of the desired field of view (FoV) and the efficient transport of the light from incoupler to the demanded eye box are the main challenges of every waveguide design, in this approach the actual pupil expander grating is designed to multiply the pupil just in the $x$-direction and the outcoupler just in the $y$-direction (see Fig. 1). This separation of the pupil expansion allows for the utilization of 1D-periodic (so-called lamellar) grating structures for all mentioned grating regions, which enables a simpler design process as well as feasible and cost-efficient manufacturing. While this separation is the main advantage of this approach, it comes at the price of a limited maximum FoV. A larger angular spectrum of the light will require an increased extension of the expanding grating region, resulting in higher demands on the grating characteristics to achieve an acceptable uniformity.

Fig. 1

Top view of the lateral layout of the designed waveguide. The orientations of the grating lines are indicated in green.

After choosing a general design layout, its parameters (i.e., extension and shapes of the grating regions, as well as the required distances) can be calculated by geometrical considerations regarding the desired extent and position of the resulting eyebox. For this particular design task, the physical-optics software “VirtualLab Fusion” was used, which provides versatile tools for the design and analysis of such layouts and other complex waveguide systems. In the following step, the required grating periods can be determined according to the used wavelength, the refractive index of the substrate, and the desired FoV. In this work, we show an example design for 533 nm (with a Gaussian spectrum and a bandwidth of 60 nm) in combination with the desired FoV of $35°\times 18°$. Taking into account the refractive index of the intended glass substrate at 533 nm ($n=1.9$, thickness: 1 mm), the grating periods were designed to 415 nm for incoupler and outcoupler, and 293.45 nm for the expansion grating. The latter value already includes the rotation of the grating lines by 45° in the plane of the substrate surface. To determine the required lateral extent of the outcoupler, the desired size of the eye box ($15\mathrm{mm}\times 8\mathrm{mm}$) and the eye-relief must be taken into account (see Fig. 1).

After setting proper extensions of all grating areas, the design of the specific grating structures was performed. For the incoupler, a blazed grating structure was chosen to enable a higher incoupling efficiency of the intended diffraction order ($T+1$), due to the asymmetry of the structure. Furthermore, gratings with a binary shape were chosen as expansion and outcouple gratings, offering a good compromise between optical performance and feasible fabrication. As for the grating material, a high-refractive index resin was applied ($n=1.88$ at 533 nm), which almost matches the index of the substrate and assures appropriately performing gratings.

To achieve a suitable optical performance, which usually means a good lateral and angular uniformity in combination with an adequate efficiency of the whole device, the diffraction efficiency in EPE and outcoupler must be controlled via a lateral variation of the grating parameters. For this purpose, smooth modulations of grating height and ridge width (respectively fill factor) were introduced for EPE and outcoupler. Regarding the desired direction of pupil expansion for each grating region, the modulation was configured horizontally ($x$-direction) for the EPE and vertically ($y$-direction) for the outcoupler (see Fig. 2) with a linear slope. This linear and continuous modulation allows for a distinct reduction of the free parameters in the optimization while not limiting the optical performance considerably.

Fig. 2

Depiction of the modulation of grating parameters in the EPE and outcoupler with results obtained through parametric optimization.

In the next step, a parametric optimization was applied to determine the optimal set of parameters of the grating structures regarding the desired merit function. The following parameters were varied during this step: blaze angle of incoupler, height, and ridge width for EPE and outcoupler. As for the merit function, the lateral uniformity and efficiency were evaluated for 5 modes inside the desired FoV [central mode and one of each FoV quadrant $(\pm 11°,\pm 5°)]$. While the uniformity error should be as small as possible, the efficiencies of the different modes of the FoV are desired to be equal to provide a proper angular uniformity as well. The design and optimization steps were performed with the VirtualLab Fusion software, which enables a full-vectorial and accurate analysis of such complex waveguide systems by propagating electromagnetic waves. Moreover, the diffraction efficiencies of the gratings are calculated by rigorous coupled wave analysis, which also includes polarization effects during the light propagation inside the device. Due to the distinct sensitivity with respect to the polarization state of light for gratings in this range of structure size, the rigorous and local consideration for each individual interaction allows for very accurate modeling and design of the whole device.

The result of such optimizations usually exhibits a conflict between high efficiency and good uniformity, hence a design with good uniformity (mean uniformity error: 50.2%) and acceptable efficiency (mean efficiency per FoV mode: 0.8%) for the 5 FoV modes was chosen. The optimized grating parameters are shown in Table 1.

Table 1

Overview of optimized grating parameters in the different regions of the designed waveguide.

The simulation results of the optimized system are shown in Fig. 3. The designed system exhibits a lateral uniformity error of 20.2% and an efficiency of 0.86% for the central direction of the FoV [see Fig. 3(a)]. Furthermore, the angle-dependent efficiency of the designed waveguide is assessed using a checkerboard pattern, where 1 rectangle has an angular extension of 5° by 6° [see Fig. 3(b)]. In the designed FoV, which is the area of interest, a mean efficiency of 0.62% with an angular uniformity error of 82% is obtained. According to the chosen design approach of a 1D-1D pupil expander, these are adequate values. A higher performance would be achievable by exchanging the gratings in incoupler and outcoupler: slanted grating profiles would offer more flexibility in the design but are more complex and expensive to manufacture.

Fig. 3

(a) Irradiance distribution in the eyebox for the central mode of the FoV (0.86% efficiency, 20.2% lateral uniformity error) obtained by physical optics modeling in a real color view. (b) Angle-dependent distribution of the efficiency of the designed waveguide in the whole FoV (area of interest) and beyond, evaluated by a checkerboard pattern in angular coordinates (0.62% mean efficiency, 82.0% angular uniformity error).

3.

Materials and Methods

3.1.

Waveguide Master

The SRG waveguide master is made by NIL Technology in silicon by electron beam lithography (EBL) and dry etching. This approach is chosen for the master fabrication to ensure the highest possible quality of the SRGs. Figure 4(a) shows the final AR sub-master made in a UV replication process utilizing UV cured polymer on glass. As detailed in Fig. 4, a design with a binary pupil expander and output gratings and a blazed input grating was chosen to facilitate replication. The EBL is performed by a Jeol JBX-9500FSZ Gaussian-shaped 100 kV EBL tool. For the blazed input grating, a unique proprietary NIL Technology process is applied to ensure the highest possible quality of the blazed surfaces. The blazed grating period is defined by EBL, and the blazed grating is formed by dry etching. The binary pupil expander and multi-depth output gratings are defined by EBL and etched by multiple inductively coupled plasma etching cycles with additional lithography steps in between.

Fig. 4

(a) Final AR sub-master consisting of cured polymer on glass. (b) Top: (1) sub-master blazed input grating, (2) fill factor modulated pupil expander grating, and (3) depth and fill factor modulated output grating. Bottom: atomic force microscopy (AFM) scans of output grating showing both fill factor modulation from 17% (top) to 56% (bottom) and depth modulation from 72 (top) to 92 nm (bottom). (c) AFM scan of blazed input grating showing the sharp profile with 29° blaze angle, and 80° to horizontal anti-blaze angle.

The use of EBL to define all SRGs ensures a high accuracy of the grating periods and lateral dimensions while the use of dry etching to form the gratings in silicon ensures a high structure fidelity and precision of etch depths. After pattern transfer, the master is cleaned, and a first-generation sub-master is generated from the master by UV replication into a UV curable material on glass for subsequent recombination and fabrication of the waveguides.

The design parameters in the presented SRG waveguide are well within the fabrication rules of the silicon master as well as the replication process. NIL Technology is also able to produce more complex SRGs, such as slanted gratings and more advanced binary gratings. All these types of gratings can be combined on the master with total design freedom on relative placement and rotation of the individual gratings.

3.4.

Large Area Nanoimprinting

To ensure mass manufacturing at high accuracy, roll-to-plate (R2P) NIL technology was used at Morphotonics. In the R2P nanoimprinting system, a transparent flexible stamp containing the desired inversed texture is mounted around rollers; these rollers guide the flexible stamp. The UV curable resin is then dispensed onto a substrate using jet dispensing, and subsequently pressed under the flexible stamp and rollers. The resin is cured using a UV light source (365 nm), and the flexible stamp is delaminated from the substrate. In this way, the pattern is imprinted on the substrate. The advantage of this imprint technology is its scalability to larger substrate sizes, beyond wafer-scale, while maintaining good replication fidelity (preserved texture shape) and dimensional stability (no track pitch variation). Multiple rigid wafers can be imprinted in one pass, each containing a multiple-up of products adjacent to each other. Hereby, mass volume production is enabled.

The process starts with the scaling of the sub-master, containing one waveguiding eye-piece, to a scaled-up sub-master. With the Morphotonics proprietary upscaling process, an array of waveguiding eye-pieces is made. In this demo effort, the squared upscaled sub-master contains 6 rows of each having 5 eyepieces for a total of 30 eyepieces, as shown in Fig. 5. Height variations between the different waveguiding products as well as the height and width of the seam in between the different waveguiding products have to be tightly controlled. Height variations result in a varying imprint pressure close to the seam. With the controlled Morphotonics’ upscaling process, the area next to the seam with deviations in imprint quality is minimized to a few millimeter. This small “bleeding” area is outside the active area and is removed in the singulation of the eye-pieces.

Fig. 5

Imprint from the 30-up sub-master.

The flexible stamp used in the large-area R2P imprint steps can contain multiple scaled-up sub-master areas, imprinting on multiple wafers placed on a carrier. In this work, the replication is made on a square $300\mathrm{mm}\times 300\mathrm{mm}$ wafer. With the use of the Morphotonics Portis NIL600 imprint module, handling up to $600\mathrm{mm}\times 800\mathrm{mm}$ substrates, 4 wafers can be imprinted in 1 imprint cycle. The Morphotonics Portis NIL1100 can imprint up to a size of $1100\mathrm{mm}\times 1300\mathrm{mm}$, containing 9 wafers on a carrier.

In the replication process, there are several key parameters such as the re-usability of the flexible stamp, the dimensional stability, the replication fidelity into resins with high refractive index, and the layer thickness uniformity.

The re-usability of the flexible stamp is important to ensure a reproducible process at lowest costs. Each imprint having the same imprint quality limits the quality control step significantly. The Morphotonics flexible stamps have a re-usability of more than 500 times, proven in volume production with different customers. In addition, the stamps must ensure dimension stability. The track pitch of the different optical elements must remain constant within an imprint as well among different imprints. The flexible stamp is not allowed to be stretched due to the applied forces or thermal or humidity changes. To ensure this dimensional stability, Morphotonics uses a flexible stamp, that is, a composite foil with excellent physical properties, such as a thermal expansion coefficient of $5\mathrm{ppm}/°\mathrm{C}$, a hygroscopic expansion $<1\mathrm{ppm}/\%\mathrm{RH}$, and a Young’s modulus $>20\mathrm{GPa}$. Here, we call this a high dimension stability flexible stamp.

4.

Results and Discussion

Waveguide imprints were made with R2P technology using the upscaled sub-master, high-refractive index glass and resin. Two types of run of imprints were performed, which here we call homogeneity and reproducibility runs. In a homogeneity run, the goal is to show that quality is kept over a large area imprint. In a Gen5 area, we show that it is possible to imprint on 9 square wafers ($300\mathrm{mm}\times 300\mathrm{mm}$), with 30 waveguides each, on 1 imprint cycle. This corresponds to 270 waveguides imprinted in 1 machine cycle (see Fig. 6).

Fig. 6

Replication of the 30-up scaled-up sub-master on 9 single wafers producing 270 waveguides in one imprint pass.

Furthermore, in a reproducibility run, the goal is to ensure that using the same flexible stamp, the quality is kept over multiple imprints. Here, more than 100 runs were performed using a flexible stamp made from a 30-up upscaled sub-master (Fig. 5).

We measured waveguides from the two different groups of samples, the homogeneity (H1 to H3) and reproducibility (R1 to R6) test waveguides (see Fig. 7). Their quality was assessed by structural and functional tests.

Fig. 7

(a) Homogeneity (H1 to H3) and (b) reproducibility (R1 to R6) test waveguides.

To assess the replication fidelity of the samples, we measured with a Littrow diffractometer the input, pupil expander, and output gratings for the nine different samples. In total, 45 points on the outcoupler grating, 47 points on the expander grating, and 5 points on the incoupler grating were measured to get an optimal sample rate, but also managing strict time constraints. Examples of one outcoupler period measurements have been shown in Fig. 8.

Fig. 8

Example of outcoupler grating period uniformity for one sample.

The average period with standard deviation and standard deviation of the relative orientation for each grating are shown in Fig. 9. From these results, we can conclude that the uniformity for all the gratings is excellent. For example, the designed grating period for the outcoupler was 415 nm and 90° for the orientation. We measured the average of the outcoupler grating period to be 414.99 nm with the standard deviation being $\pm 20\mathrm{pm}$ and the relative orientation standard deviation to be $\pm 5\mathrm{arcsec}$. This implies that the manufacturing process is very high quality, and the reproducibility is excellent for high-volume production.

Fig. 9

(a)–(c) Average period with standard deviation of each grating. The graph shows that the average period is similar from sample to sample. (d)–(f) Standard deviation of the relative orientation of the grating lines. Incoupler SD presents a low reliability due to small number of measurement points.

Uniform imprints, without variations in residual layer thickness, are required for the best contrast and lowest waveguide losses. The thickness of the residual layer is determined by the imprint process and resin characteristics. The lowest layer thickness variation is achieved with the lowest residual layer thickness. With the current solvent-free high-refractive index resin, a layer thickness of around $4.6\mu \mathrm{m}$ is obtained. Further development in solvent-free high-refractive index resin with lowered viscosity is needed to achieve minimal residual layer thicknesses.

The analysis results of the checkerboard contrast for both sample groups of the homogeneity (H1 to H3) and reproducibility (R1 to R6) test waveguides are shown in Fig. 10(c). The checkerboard contrast is calculated by analyzing the detected checkerboard image at the center of the eyebox of the waveguide. First, the dark current of the camera sensor is subtracted. By dividing the gray level of the center white square by the average gray level of the surrounding black squares, one can calculate the local checkerboard contrast of the illuminated waveguides. The very high inherent contrast value of the OptoProjector (180:1) guarantees a reliable contrast measurement for the designed waveguides. The insignificant fluctuation of the contrast values around their mean value for both groups of the test waveguides confirms the presence of homogeneity and reproducibility in the replication process.

Fig. 10

Regions of interest for analyzing the waveguide images based on the FoV of the test waveguides are shown as a red frame for both (a) checkerboard and (b) solid reticles. For both groups of the momogeneity (H1 to H3) and reproducibility (R1 to R6) waveguides, the analysis results of the checkerboard contrast and luminescence uniformity are presented in panels (c) and (d), respectively.

Typically, the largest challenge for diffractive waveguides is luminance uniformity. Any imperfection in the fabrication process can result in a remarkable quality drop in the luminance uniformity of the diffractive waveguide. Therefore, the assessment of this parameter can be considered a qualitative assessment for the fabrication process. In this study, the luminescence analysis is done based on IEC 63145 standard using 13 spots distributed across the waveguide FoV in an ANSI13 pattern.⁹ The luminance value for each spot with the same area is the average luminance. The projecting system should have a very high uniformity to validate the achieved qualitative analysis based on this approach. Our designed OptoProjector provided uniformity values higher than 96% at the FoV range of the waveguide. The analysis reports the luminescence nonuniformity in terms of a percentage. Then, the luminescence uniformity is calculated based on the acquired nonuniformity

The nonuniformity is calculated based on the acquired normalized luminescence mean value deviation of the selected 13 spots from their averaged maximum luminescence value

Figure 10(d) shows the evaluated uniformity of both groups of the test waveguides and compares the calculated values with their mean value. As is evident in this figure, the negligible fluctuations of this parameter from the mean value for the homogeneity waveguides (H1 to H3) confirm the expected fabrication uniformity of the diffractive waveguides. On the other hand, a very slight difference between the uniformity of R1 to R3 samples and their mean value validates the existing replication in the physical and optical properties of the fabricated waveguides.

Both the checkerboard contrast and luminescence uniformity results are supportive evidence for the presented Littrow measurement results and, consequently, for the accuracy, repeatability, and reproducibility of the proposed large-area waveguide fabrication method.

The results of the MTF measurements for all samples versus spatial frequency (cycles/mm) are shown in Fig. 11(a), and the MTF values for 40% contrast are shown in Fig. 11(b). In Fig. 11(a), MTF graphs show a good agreement between all samples of H1 to R4. In other words, not only do tiles in single imprints for H1 to H3 show a close correlation of MTF values but also the reproducibility of imprints is closely correlated. In R5 and R6, a slight decrease in MTF can be seen, possibly related to minor reproducibility issues after 100 imprints. In Fig. 11(b), the spatial frequency where MTF drops to 40% is shown for all imprints and tiles for a more detailed comparison.

Fig. 11

(a) MTF graphs for all samples versus spatial frequency, H1 to H3 are selected tiles from single imprint number 1, and R1 to R6 are imprints 2, 11, 50, 70, and 101. (b) MTF of 40% is shown for all imprints and tiles for a closer comparison.

The results in this paper arise from a combined effort of pioneering partners with limited time for optimization. The production chain has not been adjusted for deviations in the master process, sub-master up-scaling process or the different replication steps, nor have the materials used been fully optimized. For example, the texture height has only been corrected by approximation for the shrinkage of the resin. With further optimization, waveguides with even better performance could be realized.

5.

Conclusion

We have demonstrated in this paper that for a successful transition to high-volume manufacturing of AR waveguide optics, a display-oriented, high-quality focused, large-area manufacturing mindset is not only needed but is also already available. The array of high-index squared glass enables the increase in production volume of AR waveguide optics. Together with the complex design, the high-end mastering and the in-depth quality inspection capabilities available, the reproducible large-area nanoimprinted demo proves that the mass production route is feasible.

An end-to-end supply chain and cooperation between different disciplines is also key. As a consortium of pioneers, we were able to iterate based on a robust design, excellent mastering, using unprecedented materials and proven large-area nanoimprinting, topped with the unique metrology capabilities. We hope this exemplary work will inspire the industry to take the necessary steps to help fulfill the promise of smart glasses.