Advanced Surface Finishing for Photonics, LED and Precision Optics: Polishing for Enhanced Optical Performance

Optical components play a significant role in modern technology, including telecommunication, medical imaging, scientific research, and manufacturing processes. To achieve optimal optical performance, optical components require high-quality surface finishing. Surface finishing is a critical step in the manufacturing of optical components. The appropriate surface finishing technique must be selected based on the type of material and the required optical specifications. Kemet provides exceptional surface finishing solutions for a wide range of optical components and materials, ensuring high precision and accuracy in their processes.

The surface finishing techniques used in optical component manufacturing are dependant on the type of material and the required optical specifications. The technique of rotating or scanning optical components in a liquid is a frequently employed method for surface finishing. The following surface finishing techniques are used in optical component manufacturing:

Polishing pads are used to achieve a smooth surface finish on optical components. To achieve a surface roughness level of up to 0.3 nm RMS value, ultra-polishing pads are utilised. Contact polishing and quasi-polishing pads are also used to achieve the required surface finish. In contact polishing, the optical component is pressed against the polishing pad to achieve a uniform surface finish. Quasi-polishing is a similar process, but it uses a specially designed polishing pad to achieve the desired surface finish.

Slurry jet polishing is a surface finishing technique used to achieve an improved surface finish on optical components. In this technique, a high-velocity slurry jet is used to polish the optical component. The slurry jet contains abrasive particles that impact the optical component at high velocity, resulting in a smooth surface finish.

Precision lapping is a process used to produce flat and parallel surfaces on optical components. In this process, a series of abrasive particles are used to remove material from the optical component. The process is repeated until the desired flatness and parallelism are achieved. Diamond machining is a process used to achieve high precision and accuracy in the surface finishing of optical components. Diamond tools are used to remove material from the optical component, resulting in a smooth surface finish. The process is particularly useful for hard and brittle materials such as sapphire, quartz, and ceramics.

Cleaning optoelectronic components is a crucial step in the manufacturing process. It involves removing dirt, dust, and other contaminants that may affect the optical properties of the material. The material must be cleaned efficiently to obtain optimal visuals and image creation.

Kemet provides exceptional surface finishing procedures for a wide range of optical components and materials. They provide high precision and accuracy in their processes, achieving optimal surface quality for the desired outcome. Their processes include precision lapping, polishing, and diamond machining. They also offer advanced cleaning processes for optoelectronic components.

Photonics and Optical Components Surface Finishing

Lapping and polishing are critical processes in the LED industry for achieving the desired level of precision and quality in the production of various LED components. Kemet is a leading provider of lapping and polishing products designed specifically for use in the LED industry.

Kemet Dual Face Grinding, Lapping & Polishing Machines feature a 4-way motion and variable speed drive, which can execute complex routines while producing precise and repeatable results on a wide variety of materials. These machines come with options to build with HMI control, Automatic Thickness Control, and Load Cell for precise pneumatic pressure and parts dimensional controls during lapping and polishing operations. They are highly versatile and can be used for a range of applications, including: Sapphire Wafers, Silicon Wafers, Precision Optics, Filter Glass, Sapphire Watch Glass, Quartz Crystals Disk Drive Components

Kemet XJ Machines are heavy-duty single-sided precision lappers and CMP polishers applicable to a wide variety of processes for mass production. These machines are highly efficient and are used for a variety of applications, including: LED substrates, Silicon wafers, Magnetic heads, Hard disk drive components, Facing and Grooving Units

Kemet Facing and Grooving Units are used for the efficient lapping and polishing of flat surfaces, and for cutting or grooving circular parts. These units can be customised to meet the specific needs of different LED applications. Kemet Wax Bonders are used for bonding wafers, substrates, and other components with precision and accuracy. They are an essential tool for the LED industry and can help achieve the desired level of quality and precision in the bonding process.

Zygo Measurement: Surface Roughness and Tropel Measurement: Total Thickness Variation are two of the most important aspects of the lapping and polishing process in the LED industry. Kemet provides these measurement services to its customers, ensuring that their products meet the required quality standards.

Kemet offers training and technical support services to its customers. These services are provided in Kemet's fully equipped workshop and cover a range of topics related to lapping and polishing technology, including process development, troubleshooting, and maintenance. Kemet's Far East presence in Singapore and the Asia Pacific region allows the company to serve the latest manufacturing activities in research and development. Kemet regularly assists customers by developing new products and processes, utilising the latest precision flat lapping technology, and diamond abrasive technology. Customers' samples are evaluated free-of-charge, and comprehensive reports are issued covering all aspects of the processes, inspections, and the likely consumables cost involved.

Beta-barium oxide

Beryllium Copper

Cadmium sulfide

Calcium fluoride

Fused Silica

Gallium arsenide

Germanium

Lithium niobate

Lithium triborate

Magnesium fluoride

Quartz

Sapphire

Silicon

Silicon carbide

YAG

Zinc Selenide

Beta Barium Borate

Beta Barium Borate is a compound that exhibits non-linear optical behaviour and maintains transparency within the range of approximately 190 to 1300 nm when a crystal layer of a few millimetres is used. This compound is extensively used for generating quantum-linked photons and producing harmonic generations that are utilised in optical oscillators and optoelectronic devices. It also offers excellent resistance to ultraviolet radiation. By modifying its optical properties, a wide range of optical profiles can be obtained, which are widely utilised in the development of optical processing devices.

Beryllium Copper

Beryllium Copper is a copper-based alloy known for its non-magnetic and non-sparking properties. This alloy is commonly customised for various applications, including oilfields, landing gear of aeroplanes, mould making, and robotic welding. Its high strength and electrical conductivity make it valuable in the electronics industry. In addition, it is being used in optoelectronic devices to provide better contact properties, particularly in optical communication devices. Beryllium Copper is utilised in repeater housings to enhance signal strength over long distances. It also offers photonic characteristics, which are utilised in photoelectric devices for various applications. Surface Finish of Beryllium Copper using Lapping and Polishing

Surface Finish of Beryllium Copper using Lapping and Polishing

In this case study, we aimed to achieve a blemish-free mirror finish on various Beryllium Copper samples without specific flatness requirements. The lapping and polishing process involved three stages, each with specific equipment and parameters.

For the first stage, a Kemet 15” diamond lapping/polishing machine with a Kemet XP lapping plate and 3µ Type K STD diamond slurry was used. The component was subjected to a weight of 4 kg for 15 minutes, resulting in a good reflective finish, but with some visible scratches.

In the second stage, the component was pre-polished on an ASFL-AW silk cloth using 3µ Type K STD diamond slurry for 5 minutes with a weight of 4 kg. This step prepared the component for final polishing, resulting in a 90% blemish-free finish.

The final stage involved polishing the component to achieve a blemish-free mirror finish. The component was run on a KemCol 15 - Chemical Mechanical Polishing Machine with a CHEM cloth and COL-K colloidal silica for 5 minutes with a weight of 4 kg.

The recommended equipment for running one component includes a diamond lapping machine, a colloidal silicate lapping machine, and other necessary accessories. This three-stage process can result in a blemish-free mirror finish on Beryllium Copper samples, with each stage utilising specific machines, plates, cloths, and abrasive slurries.

Before Polishing Beryllium Copper

After Polishing Beryllium Copper

Cadmium Sulfide

Cadmium sulfide is a yellow-coloured inorganic solid formed by combining two crystal structures, greenockite and hawleyite. It is widely used as a light sensor in photo resistors or light-dependant resistors, as well as in optical switches to detect light signals when electrical signals are received. CdS is also used in biomedical lasers to enhance the photoconductivity behaviour of the laser by combining it with polymers. With its semiconductor properties and non-magnetic nature, Cadmium sulfide is an ideal material for efficient solar cells that convert light into electrical energy. Therefore, it is an essential compound for optoelectronic devices requiring higher efficiency and energy harvesting.

Calcium Fluoride

Calcium fluoride is an inorganic white compound widely used to manufacture optical components like lenses and windows. It has broad applications in thermal imaging systems, spectroscopic techniques, telescopes, and lasers. Due to its homogeneous behaviour and high refractive index, it is used in photolithography for fused lenses, making optical devices stable and efficient.

Fused Silica

Fused silica is an amorphous form of silicon dioxide used extensively in optical instruments like lenses, mirrors, metrology components, and parts of optoelectronic devices. Obtaining optimal reflective, refractive, and transmittance properties requires excellent surface finish. To achieve this, abrasive powders like cerium oxide particles are commonly used. Kemet has developed an effective polishing procedure that yields a highly flat and pure fused silica surface.

Gallium Arsenide

Gallium arsenide is a semiconductor material used to produce integrated circuits, field-effect transistors, and linear devices like oscillators and amplifiers. It is widely employed in optical communication and control systems due to its superior electrical and optical properties. Rear mirrors and lenses are made from this compound due to its cost-effectiveness and better properties. It is also used in optoelectronic and microelectronic devices to enhance their electrical and optical properties.

Germanium

Germanium is a lustrous elemental semiconductor similar in appearance to silicon. It is primarily used in optical applications such as cameras, microscopes, fibre optics, and infrared optics. Germanium's excellent light and laser qualities make it useful in wide-angle lenses, windows, prisms, and other optical components for optoelectronic devices. Germanium is also a natural beam splitter (50%) without coatings and used as a substrate material to build optical filters. Photodiodes and transistors are made from germanium, forming the basis of microelectronics. Surface Finishing Germanium Parts using Lapping and Polishing

Surface Finishing Germanium Parts using Lapping and Polishing

Germanium parts surface finishing material test using lapping and polishing processes. The process involved two stages using specific machines, lap plates, abrasives, and additional pressure. The first stage used a Kemet 15" diamond lapping/polishing machine with a cast iron lap plate and Kemox 0800S abrasive type/grade. The second stage used the same machine with an ASFL polishing cloth lap plate and Kemet Liquid diamond Type K 1 Micron standard abrasive type/grade. The process successfully achieved the required surface finish and achieved better flatness and Rz than required. Although the Ra achieved was slightly higher than the required specification, it was still within an acceptable range.

Before Lapping & Polishing Germanium

After Lapping & Polishing Germanium

Lithium Niobate

In its single crystal form, Lithium Niobate has a trigonal crystal structure and is prized for its optoelectronic properties. It finds numerous applications in mobile phones, optical waveguides, piezoelectric sensors, linear and non-linear optics, and optical modulators. This compound exhibits exceptional optoelectronic properties due to its electromechanical coupling phenomenon, making it a highly efficient modulator for both electrical and optical signals.

Lithium Triborate

Lithium Triborate is a non-linear optical crystal that offers excellent mechanical, optical, and chemical properties, making it an ideal choice for optoelectronic devices. Its high damage threshold makes it durable for microelectronic and optoelectronic devices, and it is also used in lasers, low-refractive index lenses, and frequency doubling and tripling of high-power peaks of sapphire and dye lasers.

Magnesium Fluoride

A transparent inorganic white crystalline salt, Magnesium Fluoride has a wide range of applications in optical development, space telescopes, and excimer laser applications. It is known for its mechanical properties, including durability, strength, and thermal shock resistance, along with its excellent optical properties that make it suitable for anti-reflection and multilayer coatings, polarisers, and optical protective coatings.

Quartz

Also known as silica, Quartz is composed of SiO4 tetrahedra and boasts high optical transmission value and low thermal expansion coefficient, allowing it to withstand very high operating temperatures. Its unique combination of optical and mechanical properties makes it an ideal choice for manufacturing optoelectronic device components like windows, lenses, and external bodies of optical components. Its chemical compatibility feature increases the lifespan of optical devices, and its stability in a wide range of ultraviolet to infrared radiation makes it widely used in optoelectronics.

Surface Finishing Quartz using Lapping and Polishing

In this material test, two quartz samples, a glass tube with an outer diameter of approximately 62mm, and a glass rod with an outer diameter of approximately 41mm were lapped and polished to achieve a flatness of better than 1 µm and an Ra of better than 1 nm. The process involved two stages, lapping and polishing, with specific abrasive types and grades, along with the use of weights and non-slip film to hold the glass in place during the process. Prior to lapping and polishing, the quartz sections were precisely cut using a Micracut 201 precision saw with a diamond cutoff wheel and slow feed rate. After the process, the samples achieved a flatness of 1-2 light bands, less than 1 micron, and an Ra value better than 1 nm.

Before Polishing Quartz

After Polishing Quartz

PR3 Plate Quartz Polishing Test

A test was performed to assess the effectiveness of a surface finishing process on a quartz workpiece using a PR3 plate. The in-situ method was utilised for polishing, and a cerium oxide slurry of size 1~2 µm with 5% weight and a flow rate of 5ml/min was used. The process involved two steps: Process 1 with 80g/cm2 pressure for 30 minutes and Process 2 with 30g/cm2 pressure for 20 minutes. The achieved flatness result was 0.130µm (λ/4.88). Although achieving the desired surface finish on quartz components can be difficult, a combination of diamond lapping and polishing processes can deliver the required flatness and Ra values, making it suitable for various industrial applications.

Sapphire

Sapphire is an aluminium oxide material that has remarkable toughness, next only to diamond. Its transmission spectrum range from 150 to 6000nm makes it suitable for various optoelectronic applications. Optical grade sapphire is widely used in devices and electronics with an optical axis, particularly for sensitive applications such as satellite communication and control systems. Despite its high cost, sapphire's refractive index of 1.76 to 1.77 makes it an excellent material for precise optical components. Thinned sapphire offers better optical properties near the transmission limits, and Kemet provides high-quality surface finishing processes for sapphire.

Silicon

Silicon is a hard and brittle solid that works as a semiconductor with a crystal structure similar to that of a diamond. Due to its semiconductor properties, it is used as a transistor in electrical devices. Silicon wafers of high quality are used in computers, mobile phone sims, and integrated circuits. Silicon offers excellent electrical conductivity and control, making it ideal for modern electronics. Silicon sensors and fibre optics are used for their high-temperature performance, high bandwidth, and compatibility.

Silicon Carbide (SiC)

SiC is a synthesised semiconductor made from crystalline silicon and carbon. It is primarily used in grinding wheels, cutting tools, and sandpaper. Because of its low weight, it finds application in scan mirrors, reflective imaging systems, semiconductor wafer handling, and mounts. SiC-mounted optic structures are used in optical devices. Its applications extend to space-based telescopes. This compound is utilised to produce precision micro-optical devices and nano-photonic devices. Silicon carbide's excellent mechanical properties are utilised to produce optical resonators, junction field transistors, and resistors. Surface Finish Silicon Carbide using Lapping and Polishing Techniques

Surface Finish Silicon Carbide using Lapping and Polishing Techniques

Objective: The aim of this test was to achieve optimal surface finishing on 65mmø silicon carbide blanks before pitch polishing, using a three-stage process.

Test Requirements: Silicon carbide blanks were used, and the process involved different machines, lap plates, abrasive types/grades, and additional pressure at each stage.

Process:

Stage 1: Used a Kemet 15 diamond lapping/polishing machine with a Kemet iron lap plate with spiral grooves. The abrasive used was Kemet liquid diamond type K 14 micron standard, and the additional pressure was 3.5 kgs for one part. The dispenser setting was 2-second spray every 40 seconds, and the process time was 5-10 minutes for the front surface.

Stage 2: Used a Kemet 15 diamond lapping machine with facing unit and a Kemet copper lap plate with spiral grooves. The abrasive used was 6-KDS1488 (water-based) diamond slurries, and the additional pressure was 4 kgs. The dispenser setting was 2-second spray every 40 seconds, and the process time was 10-15 minutes.

Stage 3: Used a Kemet 15 diamond lapping machine with facing unit and a pure tin lap plate. The abrasive used was 0.75-KDS1438 (water-based) diamond slurries, and the additional pressure was 4 kgs. The dispenser setting was 2-second spray every 40 seconds, and the process time was 1.5-2 hours.

Results:

Stage 1: The first stage resulted in the removal of 10-20 µm of material in 5-10 minutes. The achieved surface finish ranged from 0.08~0.084µm.

Stage 2: The second stage provided a semi-reflective surface with a surface finish of Ra 0.056~0.060µm.

Stage 3: The third stage resulted in a mirror surface finish of Ra 0.007µm.

To achieve the desired surface finish and flatness, pitch polishing should be used. The flatness was measured on a Zygo and was 0.341 µm (Peak to Valley) after the final polishing of silicon carbide. The results show that the implemented surface finishing process was successful in achieving the desired surface finish and flatness for silicon carbide blanks.

After Stage 1 Polishing Silicon Carbide

Final Flatness After Polishing Silicon Carbide

Yttrium Aluminium Garnet (YAG)

YAG is a crystal compound that plays a crucial role in medical laser applications. Synthetic YAG is widely used in solid-state lasers due to its ability to control pulse ranges in microseconds, making it the most efficient material for laser production. It has a transmission range of 0.21 to 5.5 μm and a reflection loss of 16.7%. It is also used in optoelectronic and semiconductor devices for its exceptional optical control properties.

Zinc Selenide

ZnSe is an intrinsic semiconductor with a light-yellow crystal structure and a bandgap of 2.70 eV at 25°C. It has a wide transmission range of 0.6 to 21.0 μm, making it an ideal material for optical applications. ZnSe is commonly used in light emitting diodes and lasers, and when doped with chromium, it emits blue light. Its low absorption percentage and visible transmission make it popular for applications such as lenses, output couplers, lasers, windows, and beam expanders.

PR3 Polishing Plate For Optics

The Kemet PR3 Polishing Plate is a resin plate that is thermally stable and provides an alternative to polyurethane pads when polishing precision optics. The plate is compatible with diamond, cerium oxide, and aluminium oxide slurries and is available as complete plates or discs for easy installation. Using the Kemet PR3 Polishing Plate provides several advantages. Firstly, it has been designed to achieve flatness below 0.08 microns, ensuring excellent flatness during polishing. Additionally, it offers improved edge exclusion, maintaining consistency in surface finish across the entire polished area. The plate can produce surface finishes as low as 1 nanometre, which is essential in achieving high-quality results. It is also easy to machine and can be shaped to meet the specific requirements of the optical component being polished. The Kemet PR3 Polishing Plate is an excellent alternative to polyurethane pads for polishing precision optics, offering superior performance and quality.