optical fiber array silicon V-groove; anisotropic etching; measurements of the fiber surface

One dimension optical fiber array has been extensively applied in optical communication, optical imaging and detection systems recently · The manufacturing method of one dimension optical fiber array and the principle of etching Si-V grooves are introduced. The size of Si-V grooves and the space bet ween the m are calculated according to the radius of the fiber, and a formula on the minimum depth of the Si-V grooves is given, which related with the fiber radius and the opening space of the grooves. As a typical example, the silicon V groove array is micro machined with anisotropic etching process, then the fibers are arranged and adhered in corresponding to the Si -V grooves. The roughness of polished fiber surface measured with at o mic force microscope (A F M )is in nano meter magnitude and the position error is 3～5 micron, which shows these works will lay the foundation for further study on two dimension fiber array.

In recent years, one-dimensional fiber arrays have received extensive attention as an important optical device.

For example, in the field of optical communication, due to the strict requirements on the alignment accuracy of the optical fiber and the chip coupling in the optical device, a large number of optical fiber arrays FA (Fiber Array) are used to realize the precise connection of optical components.

One-dimensional optical fiber arrays are also used in optical fiber imaging devices. Compared with traditional optical imaging systems, optical fiber imaging devices have the characteristics of flexible imaging, large freedom of use, easy realization of slender structures, and light weight, and are widely used. In medicine, industry, scientific research, military and many other fields. In addition, in the detection of planets outside the solar system, Jian Ge, Dan Mc Davitt, etc. applied one-dimensional fiber arrays to terrestrial planetary detectors to effectively eliminate the influence of residual star leakage. Therefore, it is of great significance to study the fabrication methods and reliability of one-dimensional fiber arrays for different structural requirements in various fields.

At present, the methods of developing one-dimensional optical fiber arrays at home and abroad mainly include drilling method, optical channel close-packing method and V-groove method.

The drilling method is to make an array of positioning holes on a substrate with a certain thickness, insert the optical fiber, and then inject glue to solidify and grind it. The fiber spacing can be determined by needs, and the displacement error is small, but it is not suitable for densely packed fiber arrays, and the angular deviation is large.

The optical channel close-packed method is to arrange and fix the optical fibers tightly in the groove with high flatness. This method has good scalability, but cannot adjust the spacing of fiber channels arbitrarily, is only suitable for making densely arranged fiber arrays, and has a large cumulative error.

The V-groove method is to engrave a V-groove on a high-flatness substrate, and arrange and fix the fibers in the V-groove. If single-crystal silicon is used as the substrate, the fabricated V-groove has the advantages of precise structure and good consistency, and is suitable for both discrete and densely arranged fiber arrays.

In practical applications, the characteristics of FA play a very important role in the reliability of optical devices.

The structural parameters of FA mainly include fiber displacement error, angular deviation, fiber array end face roughness and longitudinal position error.

Optical parameters mainly include insertion loss and return loss. In order to study the fabrication of one-dimensional fiber arrays that can be applied to various fields, various fiber types, different line lengths, separated or densely arranged, and methods to improve reliability.

Taking the single-mode fiber array as an example, this paper expounds the development method of the one-dimensional fiber array and the corrosion mechanism of the silicon V-groove in the V-groove method. The dimension design of the V-groove and the minimum groove depth formula are given.

Silicon V-grooves were fabricated by anisotropic etching technology, optical fibers were arranged and bonded in the V-grooves, a one-dimensional optical fiber array was fabricated and the structural parameters were tested, and the factors affecting the reliability of FA were analyzed.

One-dimensional fiber arrays were fabricated by the V-groove method.

Using (100) single crystal silicon wafer as the substrate, the V-groove was fabricated by anisotropic wet etching technology. In the anisotropic etching of single crystal silicon, the etching rate of the (111) crystal plane is the slowest, so the exposed groove side is the (111) plane. On the (100) plane along the (110) crystal direction lithography lines, the etched structure is a V-shaped groove, and the angle between the (111) side of the groove and the (100) plane on the upper surface of the substrate is 54.74°.

The hydroxides of alkaline metals such as KOH, NaOH, CeOH can be used as anisotropic etchants for silicon. The basic reaction is:

In the process of anisotropic etching, the shape of the trench first presents a trapezoidal groove with (111) as the side and (100) as the bottom.

As shown in the shape of groove B in Fig. 1, as the depth continues to increase, the bottom surface of (100) continues to shrink. When etched to an appropriate depth, the bottom surface shrinks into a straight line, forming a V-shaped groove structure, such as groove A in Figure 1. The shape of the groove is determined by the groove depth and the size of the opening. At the same time, the selection of the masking film and the compensation of the lateral undercut are very important to achieve the design accuracy of the V-groove.

Fig. 1 Schematic diagram of anisotropic etching in (100) silicon wafer

When the V-groove is used to fix the fiber, there must be two points in the fiber cross section that are in contact with the side wall of the groove to ensure the uniqueness of the radial position of the fiber.

Figure 2 is a schematic diagram of the positional relationship between the optical fiber and the V-groove. The fiber radius is r, the spacing between adjacent V-grooves is d, and the center-to-center distance between adjacent fibers is l.

Fig. 2 Schematic diagram of fiber array in V grooves

In order to design the lithography version of the silicon V-groove, for the three cases where the center of the optical fiber section is on the surface of the silicon wafer, above and below, Under the premise of given l and r, the opening width of V-shaped groove and the distance between adjacent V-shaped grooves are calculated.

Figure 3 shows the V-groove opening width EF (or E’F’, E”F”), depth CB (or C’B, C”B”), and the fiber radius r, and the distance between the center of the fiber section and the silicon wafer The relationship between the surface vertical distance OC (or OC′, OC″).

In Figure 3, OA is r.

Fig. 3 Relationship of the V groove opening with fiber location

For Fig. 3(a), the center O of the optical fiber core is on the connection line of EF, that is, it coincides with the upper surface of the single crystal silicon wafer in height, and it can be obtained from the geometry:

For Figure 3(b), the center of the optical fiber core O is above the EF connection line, set OC′=h₁, there are:

For Figure 3(c), the center of the optical fiber core O is below the EF connection line, set C″O = h₂, there are:

In the actual production of the V-shaped groove, after determining the fiber radius and the opening width of the V-shaped groove, there is a minimum value x of the groove depth. When the groove depth is greater than or equal to x, the fiber position can be uniquely determined, as shown in Figure 4 (a); Conversely, if the groove depth is less than x, the fiber can move in the groove as shown in Figure 4(b).

Fig. 4 Relationship of the V-groove depth with fiber position

Calculations show that under the three positional relationships in Figure 3, the following formula can be used to find the minimum depth x:

The process shown in Figure 5 is to make silicon V-groove.

Fig. 5 Micro machining of V groove array

The specific steps are:
(a) Select the (100) silicon wafer with good parallelism and flatness after grinding and polishing for cleaning;
(b) Oxidation or nitridation to grow appropriate thicknesses of silicon dioxide and silicon nitride as a mask for silicon etching;
(c) Coating a photoresist with a thickness of about 1 μm, frying and baking;
(d) on the ultraviolet exposure machine, the line direction of the photoresist is adjusted to be parallel to the reference edge of the silicon wafer, and the photoresist pattern is formed by exposing and developing;
(e) Reactive ion etching was used to remove the silicon dioxide and silicon nitride films in the etched windows using the photoresist as a mask. In this step, the etching conditions need to be strictly controlled to avoid lateral undercut;
(f) removing the photoresist;
(g) etching silicon V-grooves with 70% KOH etchant at 70°C under a thin mask of silicon dioxide and silicon nitride;
(h) Finally, the silicon dioxide and silicon nitride films were removed to complete the fabrication of the V-shaped fiber groove array.

Figure 6 presents the SEM photographs of the fabricated silicon V-grooves.

Fig. 6 SEM of V-grooves etched on a silicon wafer

The optical fibers are arranged in a 10,000-class clean environment. The optical fibers were placed in parallel in the V-groove and bonded with curing glue.

Two kinds of adhesives, UV-curable adhesive and infrared epoxy adhesive, were used for bonding, and the relationship between adhesive type, concentration, curing conditions, adhesion and hardness was explored. The UV-curable adhesive solidifies very quickly, and does not cause problems such as long curing time and easy flow during curing. However, the adhesive strength of this kind of glue is slightly lower than that of infrared epoxy glue, which will reduce the reliability of the fiber array to a certain extent.

In addition, UV-curable adhesives require a UV-transparent substrate or cover, which is not suitable for the encapsulation of Si-fiber-Si structured fiber arrays. Norland UV curing adhesive and 353ND infrared adhesive have good adhesion, toughness and strength, and can be used for fiber fixation of high-precision one-dimensional fiber arrays.

Two fiber array polishing methods were used. One is to arrange the optical fibers in sequence after polishing, adjust the position, and bond and cure. The advantage of this method is that it is easy to observe during the fiber position adjustment process, but the longitudinal position accuracy of the fiber end face is not easy to guarantee.

Another method is to grind the fiber array as a whole after fixing the fiber, which can ensure the longitudinal position accuracy of the fiber end face. Figure 7 is an optical microscope photo of the one-dimensional single-mode fiber array polished by the second method, and the fiber diameter is 125 μm.

Fi g. 7 Front view of polished surface of one-dimension fiber array

Figure 8 is an enlarged SEM photo of the end face of the fiber array, and the period of the V-groove is 130 μm. According to the calculation of formula x, the minimum depth x is 44.7μm, and the actual depth of the V-shaped groove is 46μm.

Fig. 8 SEM of the surface in Fig. 7

A computer-aided CCD imaging measurement system was used. The CCD camera captures the image output by the image beam and displays it on the monitor screen, so as to measure the positional accuracy of the V-groove array and the fiber array, as shown in Figure 9.

Fig. 9 Measuring error of 2-D optical fiber bundle array

The measured fiber core height position error is less than 2μm, and the distance error between adjacent optical fibers is less than 1μm. The surface roughness of the optical fiber array measured by atomic force microscope is less than 3 nm, and the longitudinal position error of the optical fiber end face is detected by Z YGO digital interferometer is less than 1 μm.

The factors that affect the position accuracy of the fiber array mainly include the accuracy of the silicon groove, the dimensional accuracy of the fiber, and the thickness and uniformity of the adhesive.

Since the precision of the single crystal silicon V-groove fabrication technology is in the sub-micron level, the position error is mainly determined by the latter factors. In addition, the influence of the process environment on the accuracy cannot be ignored. As shown in Figure 10, impurities in the groove can cause errors in the height and spacing of the fiber.

Fig. 10 Position error resulting from impurity in grooves

The silicon V-groove method for fabricating one-dimensional fiber arrays has higher precision than other methods.

According to the structural characteristics of silicon single crystal, anisotropic wet etching was performed on (100) silicon wafer with KOH etching solution by anisotropic etching technology to fabricate silicon V-groove array.

The calculation formula of V-groove opening and minimum corrosion depth is given. In the bonding process of optical fiber arrays, the performance of UV and infrared adhesives were compared. Norland UV curing adhesive and 353ND infrared adhesive have good adhesion, toughness and strength, and can be used as high-precision one-dimensional fiber arrays. fiber optic fixing adhesive.

A one-dimensional single-mode fiber array was fabricated by the method of fiber alignment and polishing. The test result is that the core height position error is less than 2μm, and the spacing error between adjacent fibers is less than 1μm.

The surface roughness of the fiber array is less than 3nm, and the longitudinal position error of the end face of the fiber is less than 1μm. The position error is mainly determined by factors such as the dimensional accuracy of the optical fiber, the thickness of the adhesive, and the uniformity.

In addition, the influence of the process environment on the accuracy is also important.