2.1.

Device Fabrication and Release

Double-sided polished $300\mu \mathrm{m}$ thick Si and $500\mu \mathrm{m}$ thick FS wafers were placed on UV-curable dicing tape and diced into Si ($360\times 1000{\mu \mathrm{m}}^2$) and fused-silica glass ($815\times 2315{\mu \mathrm{m}}^2$) pieces using a Disco DAD 2H/6T dicer with cutting speed of $5\mathrm{mm}/\mathrm{s}$ and spindle speed of 30,000 rpm. These devices were released by exposing the back side of the wafer/tape with $250\mathrm{mJ}/{\mathrm{cm}}^2$ dose of UV at an exposure density of $10\mathrm{mW}/{\mathrm{cm}}^2$ using a SUSS Microtec MA6 mask aligner. The step-by-step process of releasing and transferring devices is shown in Fig. 2. To demonstrate printing of actual Si and FS lenses, 10× off-the-shelf lenses of each type supplied from Axetris (Part No. FCA250Si, silicon, $1\times 4$ array, $250\mu \mathrm{m}$ array pitch, $240\mu \mathrm{m}$ lens diameter; Part No. FCA250FS, $1\times 8$ array, $250\mu \mathrm{m}$ array pitch, and $240\mu \mathrm{m}$ lens diameter) are used.^38,39 These lenses are placed on UV-curable dicing tape with 6-in. dicing frame.

Fig. 2

Schematic of step-by-step $\mu \mathrm{TP}$ process using UV-release method.

Silicon and fused-silica coupons of various sizes and thicknesses (300 to $1000\mu \mathrm{m}$) are printed on a 4-in. silicon target wafer coated with $2\mu \mathrm{m}$ InterVia as shown in Fig. 3.

Fig. 3

(a) Image of Si and FS coupons printed on $2\mu \mathrm{m}$ thick InterVia coated target wafer, (b) X-celeprint machine set-up and (c), (d) higher magnified images showing printed Si and FS micro-lenses.

2.2.

Device Printing

The PDMS stamp post area is critical to pick up these devices, post as small as 70% of coupon size are used to successfully pick-up and print device as printing depends on the contact area between two.⁹ This is further discussed in Sec. 3.5. Three single-side polished Si wafers (2 with alignment marks and 1 without) are coated with $2\mu \mathrm{m}$ thick InterVia (adhesive) to be used as target wafers. InterVia is spin coated followed by 5 min of baking at 90°C. After this process is complete, the InterVia is exposed to UV light for 2 min, then baked for a further 3 min at 100°C. Next, the InterVia is hard-cured in an oven for 3 h at 175°C. As a result of curing, the InterVia bond becomes stronger, and the devices are seen to adhere strongly to the new target.

2.3.

Shear Test

10 samples of the pseudo-Si and FS micro-lens arrays are used for shear testing analysis. This test is performed before and after InterVia curing using a Royce shear tester model 552 as per MIL-STD-883 standard. For normalization, shear strength is calculated in megapascal (MPa) to take into account different contact surface areas between different lenses. For comparison of strength, four samples of $4\times 4{\mathrm{mm}}^2$ FS dies were bonded to a silicon substrate via a $2\mu \mathrm{m}$ thick layer of UV-curable Dymax-OP29 epoxy. A UV lamp was then used for 2 min to completely cure the epoxy. The shear strength of the bonded sample using InterVia was compared with Dymax-OP29 and optical epoxies used in the existing literature.

2.4.

Microstructure of Bonded Pseudo Lenses

Two micro-transfer printed (Si on target Si wafer and FS on target Si wafer) and two epoxy bonded pseudo lenses are molded in epoxy to hardener ratio of 2:15 and kept for 8 h to get hard mold. The grinding of these molded samples is done from silicon carbide (SiC) abrasive papers of 400, 600, 800, 1200, 2500, and 3000 grits. Next, the polishing of these samples is done on SiC polishing cloth with $1\mu \mathrm{m}$ alumina suspension. To check the uniformity of the bonding layer, SEM images of the left, center, and right corners of the polished sample are taken by FEI Quanta 650 SEM. Also, the thickness of InterVia is measured.

2.5.

Microstructure and Alignment Accuracy of Actual Lenses

The microstructure of 20 lenses before and after printing is observed using VHX2000 Keyence 3D-Microscope at 500× and 1000× magnification. Horizontal and vertical misalignments and respective rotations are measured by printing $1000\mu \mathrm{m}$ thick lenses on the grating coupler (GC) of a PIC. Manually at higher magnification, the center of micro-lens was recognized during the pattern registration process, and this registration was kept the same for subsequent printing. After printing, the measurement of misalignment was captured by measuring the distance from the edge of the micro-lens to the alignment mark reference defined on the target and comparing them with designed distances without micro-lens.

2.6.

Surface Roughness

The change in surface roughness of five lenses before and after $\mu \mathrm{TP}$ is determined using Bruker AFM. The test is performed by scanning $1\times 1{\mu \mathrm{m}}^2$ area of the lens top curve surface. The calculation of root mean square roughness (Sq) and leveling is done in Gwyddion software.

3.1.

Shear Strength

The average shear strength of $\mu \mathrm{TP}$ $360\times 1000{\mu \mathrm{m}}^2$ pseudo-Si and $815\times 2315{\mu \mathrm{m}}^2$ FS lenses before and after InterVia curing is shown in Fig. 4.

Fig. 4

Shear strength plots for (a) $360\times 1000{\mu \mathrm{m}}^2$ pseudo-Si micro-lens array and (b) $815\times 2315{\mu \mathrm{m}}^2$ pseudo FS micro-lens array.

After InterVia curing, the average shear strength of printed pseudo-Si lenses is $19.76\pm 5.5\mathrm{MPa}$ which is $\sim 10\times$ average shear strength before curing ($2.9\pm 0.14\mathrm{MPa}$). In the case of pseudo-FS lenses, after curing the average strength is $18.86\pm 4.9\mathrm{MPa}$ which is $\sim 10\times$ shear strength obtained before curing. Shear strength values obtained from pseudo-Si and FS lenses in the current study are compared with existing optical epoxies (Table 2).

Table 2

Comparison of shear strength of InterVia with existing studies.

As presented in Table 2, the shear strength of pseudo-Si lenses printed using InterVia is higher than most of the epoxies and comparable to Boble LV740 adhesive and BCB used in other studies. A comparison was also done between OP-29 Dymax optical epoxy and $\mu \mathrm{TP}$ samples using InterVia. The results in Table 2 demonstrate that the shear strength of micro-transfer printed Si on target Si (19.76 MPa) and FS on target Si (18.86 MPa) using InterVia is higher than FS on Si (6.84 MPa) prepared by optical epoxy OP-29. This makes InterVia a highly strong bonding material as compared to OP-29 epoxy.

3.2.

Microstructure of Bonded Samples

SEM images of $\mu \mathrm{TP}$ Si on a Si target substrate are presented in Fig. 4. Pseudo-Si lenses are successfully bonded on the Si target substrate with no delamination observed at the Si-InterVia-Si interfaces [Fig. 5(a)]. The average thickness of InterVia is $2.54\pm 0.03\mu \mathrm{m}$. Similarly, pseudo-FS lenses are successfully printed on a target Si substrate with no delamination observed at FS-InterVia-Si interfaces [Fig. 5(b)]. The thickness of InterVia is $2.04\pm 0.03\mu \mathrm{m}$. Differences in thicknesses of Si-InterVia-Si and FS-InterVia-Si interfaces are due to thickness and area of coupon. For example, the thickness and area of Si coupons were $300\mu \mathrm{m}$ and $360\times 1000{\mu \mathrm{m}}^2$ whereas the thickness and area of fused-silica coupons were $500\mu \mathrm{m}$ and $815\times 2315{\mu \mathrm{m}}^2$, respectively. This could lead to different pressure distributions during the $\mu \mathrm{TP}$ process resulting in differences in thicknesses. InterVia is uniformly spread throughout the device with a thickness variation between the left, center, and right ends of 30 to 40 nm.

Fig. 5

SEM images of (a) silicon and (b) fused-silica micro-lenses $\mu \mathrm{TP}$ on silicon wafer showing InterVia on interface.

SEM images of the FS die bonded on Si substrate using Dymax epoxy are shown in Fig. 6. No delamination was found between the silicon-epoxy-glass interface and the gap between chips was $0.960\pm 0.040\mu \mathrm{m}$.

Fig. 6

SEM image of conventionally bonded dummy fused-silica die on silicon substrate showing $0.96\mu \mathrm{m}$ thick epoxy.

3.3.

Microstructure and Alignment Accuracy

The microstructure of the micro-lens arrays before and after printing is presented in Fig. 7. The surface of each lens in an array was analyzed. The diameter and height of lens, and pitch remained same after printing as of before printing. Also, no imprint or mark of stamp post and foreign particles on top surface of a lens was found. Therefore, the PDMS stamp contact with the top surface of the lens array during pick-up and printing did not change the surface and shape of the micro-lenses.

Fig. 7

3D-microscopic images of (a) $380\mu \mathrm{m}$ thick micro-lens array before printing, (b) single micro-lens before printing at higher magnification, (c) micro-lens array after printing, and (d) single micro-lens after printing at higher magnification.

The horizontal and vertical misalignments of the center of the five micro-lenses array with respect to the origin considered on the target wafer are shown in Fig. 8.

Fig. 8

Microscopic measurement of alignment area on (a), (c) target and (b), (d) micro-lens array printed on target.

The horizontal distance of the lens origin to origin in the target area (based on GDS design) was $542.9\mu \mathrm{m}$ whereas the measured distance after printing was $543.2\mu \mathrm{m}$; therefore, the horizontal misalignment was 300 nm. Similarly, the expected vertical distance was $176.2\mu \mathrm{m}$; however, the actual distance obtained after the printing process was $174.9\mu \mathrm{m}$ which was off by $1.3\mu \mathrm{m}$. Rotational misalignments in horizontal and vertical directions were 0.0028 and 0.0024 deg, respectively (Table 3).

Table 3

Misalignment calculation of μTP micro-lenses on grating coupler.

Alignment accuracy is very important for efficient optical coupling in photonic packaging as a few microns of misalignment result in a huge coupling loss.³⁸

3.4.

Surface Roughness

Sq value of the top curve surface of micro-lenses before and after printing was measured and AFM images of $1\times 1{\mu \mathrm{m}}^2$ surface area are shown in Fig. 9.

Fig. 9

AFM images of lens surface (a) before printing and (b) after printing.

The average values of Sq for five samples before and after printing were $1.750\pm 0.019\mathrm{nm}$ and $1.688\pm 0.025\mathrm{nm}$. This shows that surface roughness of micro-lenses is not affected by (a) stamp post during pick-up and printing and (b) process parameters.

3.5.

Discussion

This work presents the proof of concept for $\mu \mathrm{TP}$ thick optical components, especially lenses. These lenses can be used to expand and collimate the light beam, which can increase light coupling alignment tolerances significantly. $\mu \mathrm{TP}$ of optical components in photonics packaging will enable wafer-scale integration with high alignment accuracy. The higher shear strength of printed samples using InterVia could be related to the constituents involved in the preparation of polymer InterVia. Optical epoxies and InterVia are made of solvent, epoxy resin, coupling agent, and adhesives.^47–50 Based on the datasheet provided by suppliers, bisphenol A-(epichlorohydrin) epoxy resin (number average molecular $\text{weight}\le 700$) was used in InterVia.⁴⁸ Whereas OP-29 Dymax was prepared with 2-hydroxyethyl methacrylate (molecular $\text{weight}=130\mathrm{g}/\mathrm{mol}$) epoxy resin.⁴⁹ The strength of a polymer depends on its molecular weight, cross-linking, and crystallinity. Increasing the molecular weight enables more cross-linking/binding sites thereby giving higher strength to a polymer.⁴⁷ The higher molecular weight of epoxy resin used in InterVia can be the reason behind the higher shear strength of $\mu \mathrm{TP}$ samples using InterVia as compared to OP-29 optical epoxy bonded samples. InterVia thickness is a critical parameter, and it is seen that Si coupons as thick as $300\mu \mathrm{m}$ can be successfully printed on as thin as 50 nm InterVia layer. The average shear strength of 10 such printed coupons after curing was $5.85\pm 2.1\mathrm{MPa}$. The microstructure of printed Si on Si target wafer is shown in Fig. 10.

Fig. 10

SEM image of Si $\mu \mathrm{TP}$ on 50 nm InterVia coated Si target.

Beyond $300\mu \mathrm{m}$ thickness of coupons, printing was not happening on 50 nm InterVia as coupons were shearing on target, but they did not leave stamp post as adhesion of coupons with stamp post was higher than 50 nm InterVia coated target wafer.^1,9 Failure to print on 50 nm thick InterVia could also be possibly due to the rougher back surface of that particular lot of coupons. Coupons with a thickness range of 300 to $1000\mu \mathrm{m}$ can be printed on $\ge 2\mu \mathrm{m}$ thick InterVia. After curing, the spin coating process gave the desired InterVia thickness of between 2 and $2.5\mu \mathrm{m}$ where the expected thickness before curing was $2\mu \mathrm{m}$ as shown in Figs. 4 and 5. In the conventional way of die bonding using epoxy dispensing processes, the expected thickness was $2\mu \mathrm{m}$; however, the realized thickness measured afterward was 0.9 to $1\mu \mathrm{m}$ (Fig. 6). Due to challenges in the conventional way of die bonding process, the epoxy thickness was difficult to control. Whereas InterVia thickness due to the spin coating process was controlled to a few nanometers. This shows that spin coating and $\mu \mathrm{TP}$ processes are more stable and uniform than conventional die bonding and epoxy dispensing techniques.

It is important to discuss stamp post area selection for picking up thick optical components as stamp post area optimization is critical. From various experiments on different coupon sizes ($460\times 1100{\mu \mathrm{m}}^2$, $360\times 1000{\mu \mathrm{m}}^2$, $815\times 2315{\mu \mathrm{m}}^2$, $3000\times 3000{\mu \mathrm{m}}^2$, $700\times 4000{\mu \mathrm{m}}^2$, $800\times 3000{\mu \mathrm{m}}^2$, $800\times 1500{\mu \mathrm{m}}^2$), it is found that area of stamp post should be in the range of 30% to 70% of coupon area. For $2\mu \mathrm{m}$ thick InterVia, stamp posts smaller than 30% were not able to pick up coupons whereas stamp post areas higher than 70% were decreasing print yield as the pick-up and printing process rely on the contact area between coupon and stamp.^1,9 In summary, the post area $\sim 70\%$ of the coupon area is most desirable for printing with high yield.

In the $\mu \mathrm{TP}$ process, a PDMS stamp is put in contact with the lens surface and process parameters, such as shear distance, speed, and retraction, may change the microstructure and roughness of lenses. This change in roughness will affect the coupling of light in and out of the PIC.⁵⁰ The highly rough surface of optical devices reduces light coupling due to scattering.^51–53 This has been proven in a study where the effect of surface roughness of fiber surface on light coupling efficiency was demonstrated.⁵³ The results showed the amount of light coupled increased by double when average surface roughness declined from 45 to 2 nm. In our study, the surface roughness of the micro-lens array is within the specs provided by the supplier.³⁹ Also, the $\mu \mathrm{TP}$ process did not introduce any additional roughness.