Micromachining and Its Applications for Electronics

Abstract

Micromachining has emerged as a foundational technology in modern electronics, playing a crucial role in the creation of miniaturized and high-performance devices. Its significance is particularly pronounced in the domains of high-frequency RF devices and Terahertz communication, where the demand for precision, miniaturization, and performance enhancement is of utmost importance. The requirement of linear tolerances are of the order of 1–5 μm, surface toughness of 40–50 nm Ra and flatness of 0.3 μm.

1. Introduction

The development of micro and nanotechnology has taken place due to the rising demand for micro and miniature parts and systems. The process used to fabricate MEMS and other microelectronics products can be called MEMS micro-manufacturing or lithography-based micro and nanomanufacturing. The technologies like photolithography, chemical etching, plating, and LIGA are called micro-manufacturing technologies. Micro manufacturing is generally employed to realize parts or feature sizes ranging from tens or hundreds of micrometers. Although micro manufacturing may not be enough to produce the smallest feature size as would be possible using MEMS and NEMS. This technology is a bridging gap for macro and nanodomain of manufacturing. It has various advantages over lithography-based micro-manufacturing in terms of material choice, relative accuracy, and complexity of part geometry. Micromachining is a broad term that encompasses a variety of fabrication processes for the production of miniature parts and structures. Non-lithography-based micromachining processes, such as micro electrical discharge machining (micro EDM), micro-milling, laser cutting/patterning/drilling, micro-extrusion, micro embossing, micro stamping, and micro injection molding, are widely used in the electronics industry. These processes employ diverse working principles and exhibit unique characteristics in terms of achievable accuracy, surface finish, and production rate. However, they all share the ability to produce three-dimensional microparts from a variety of materials [1]. This chapter will focus on micro mechanical cutting processes, in which the geometry of the cutting tool is defined. The relative accuracy and feature size, which can be achieved in micro-manufacturing, is described by Chang et al. [2] identified he has established that micro and nano manufacturing technologies is one of the most significant emerging manufacturing processes to address the future challenges in high-value engineering. Micromachining is a fabrication process that can be used to create three-dimensional (3D) microparts from a variety of materials, including metals, alloys, polymers, composites, and ceramics. Micromachined parts can have dimensions as small as 10⁻³ to 10⁻⁵ meters (sub-micron accuracy). This is significantly higher than the accuracy achievable with MEMS-based processes, which typically have accuracies of 10⁻¹ to 10⁻³ meters [1].

A micro-cutting (micro-milling) has attracted growing attention from researchers and industry in the last 2–3 decades because mechanical cutting is a well-established area. Knowledge of macro cutting has been adapted to study the micro-cutting phenomenon. Two research approaches are being investigated; one is the minimization of the conventional cutting process, tooling, and equipment, with an emphasis on their scaling down effect. Micro cutting is kinematically similar to conventional cutting but fundamentally different from it in many aspects. Micro cutting refers to the mechanical micromachining (direct removal of metal) using geometrically defined cutter edge(s) on a conventional precision machine or micro machine. The selection of production method in microelectronics depends upon the number of replications and the degree of accuracy desired. Most of the production methods used nowadays are lithography based which are a combination of photolithography, etching and electrodeposition, and require costly setups and large volumes. The geometries obtained by these lithography-based processes are limited to 2 1/2D microstructures with high aspect ratios and accuracies. Mechanical and thermal methods provide a viable solution for higher material removal rate during electronics manufacturing process. Such processes allow manufacturing of complex 3D shapes with acceptable aspect ratios, Bissacco et al. [3]. CNCmicro milling offers to process a wide range of materials at low set up costs. A large knowledge database and infrastructure developed for macro milling can be utilized for stepping into micro zone. Mechanical micromachining, particularly micromilling, offers several advantages over other manufacturing processes for electronics manufacturing due to its versatility, accuracy, and surface quality. Micromilling can be used to machine a wide variety of materials, generate complex 3D geometries with minimal setups, and produce parts with high accuracy and surface quality. However, micromilling also faces a number of challenges, including the size effect, rapid tool wear, inherent burr formation, low tool stiffness, and limitations on the minimum feature size and surface roughness compared to lithographic techniques [1]. The machining of micro molds require very high level of geometrical tolerances and surface quality this makes the application of micro end milling very challenging Aramcharoen et al., and Vázquez et al. [4, 5]. The generated topography of the micro milled surface is mainly influenced by the tool, machining parameters and cutting strategy. Micromachining enables miniaturization and integration of high-frequency RF devices and Terahertz communication, where size reduction is critical for efficient signal propagation Sankar et al. [6]. It also achieves precision fabrication of High-Frequency devices such as antennas, filters, and resonators which ensures optimal electromagnetic performance. The tight tolerances and fine features achievable with micromilling contribute to minimizing signal losses and maintaining the required device characteristics Kunieda et al. [7]. It also enables to tailor the shape and dimensions of resonators, waveguides, and other elements enables precise tuning of their electromagnetic properties, leading to enhanced device performance Mekonnen [8]. Micro cutting is kinematically similar to conventional cutting, but fundamentally different in many aspects [2]. Micro cutting refers to mechanical micromachining (direct removal of metal) using geometrically defined cutting edge (s) carried out on a conventional precision machines or micro machines. Aramcharoen et al. [4], Jain [9] has achieved and described stringent tolerances using tool based micro cutting. Wu et al. [10], Paulo et al. [11], Aramcharoen et al. [4] have described the size effect in micro milling the uncut chip thickness (UCT) is of the scale of cutting-edge radius or the tool nose radius. Based on the size effect Jun et al. [12] explained the elastic-plastic stage, which occurs due to plowing, which most of the workpiece material accumulates and expands ahead of the cutting edge. Relatively large burr formation occurs as a result of inhomogeneous distribution of material flows causing high surface roughness that decreases the material removal rate. Liu et al. [13] and Weule et al. [14] have given the material removal mechanism. In micro end-milling, the thickness initially increases from zero to a maximum value approximately equal to feed per tooth. After that it reduces from feed per tooth to zero. Whereas in the case of side milling, the thickness reduces from maximum to zero. This stage may occur repeatedly for a single point cutting edge, or in most common scenarios, simultaneously for multiple cutting edges depending upon the employed tool geometries, and the cutting parameters. Malekian et al. [15] predicted the minimum uncut chip thickness MUCT for Aluminum alloy (Al6061). Chae et al. [16], Sun and Cheng [17] and Chen et al. [18] discussed the importance of tool geometry and its effect on micro milling. Klocke et al. [19] studied the microstructure of cutting tool and suggested alloying effect on strength. Cheng et al. [20] studied the effect of cutting tool and emphasized the tool nose radius, rake and relief angle. Chen et al., Boswell et al., Yang et al. [21, 22, 23] described the importance of minimum quantity lubrication (MQL) for different materials based on experimental studies. In micromilling, the cutting forces are significantly influenced by factors that are typically negligible in conventional milling, such as tool wear, tool run-out, and chatteras described by Gietzelt and Eichhorn [24]. Tool run- out is more in micro milling due the low rigidity of the tool and higher cutting speeds, which leads to deteriorated surface [17]. Therefore to maintain tighter tolerances, reduce cutting force, maintain constant chip load, and avoid premature tool failure in micro scale milling, the cutting paths and machining strategies should be optimized [25, 26].

According to Griffin et al. [27] in the sub-millimeter or THz frequency range, the skin depth of the oscillating electric field in a metal is typically very small. This means that the electric field is only able to penetrate a very short distance into the metal. If there are defects or non-uniformities on the surface of the metal, these can act as additional resistance to the electrons. This is because the electrons must flow around the defects or non-uniformities, which takes longer and uses more energy. As a result, surface roughness is one of the most important factors affecting the performance of metal waveguides at sub-millimeter or THz frequencies. Surface roughness can cause signal losses and distortions. This is especially important in small waveguides, where the skin depth is even smaller and the effects of surface roughness are more pronounced. As per Narayanan et al. [28], the split-block technique is a conventional machining process that can be used to fabricate receiver components for the submillimeter and low terahertz frequencies. It involves machining the circuit structures on two (or more) metal blocks and then mating the blocks together to form the complete component. The split-block technique offers a number of advantages over other fabrication methods. First, it is relatively straightforward to implement. Second, it allows for the easy placement of circuit components such as RF chokes, diodes, and coupling structures on the split blocks prior to assembly. Zhang et al. [29], suggested strategies for reducing vibrations during micro milling. Quintana and Ciurana [30] presented chatter minimization strategies for suppression of chatter to get better surface finish. Researchers [31, 32, 33, 34, 35] discussed burr formation in details along with their location of occurrence. Strategies for minimization of burr formation were also discussed. Machine tool construction for micro machining and error sources were discussed by Lamikiz et al. [36] and Hashmi et al. [37] especially the Abbe’s error and correction methods along with isolation from external vibration beds because of its very low thermal expansion coefficient of 6.5 μm/mK, 2.5 times less than the one of mineral casting. Additionally, granite with a density of 2.8 kg/dm³ is beneficial. According to Uriarte et al. [38] we are using small tools; the spindle must rotate at high revolutions (<30,000 rev/min) to achieve the adequate cutting speed for most materials. Apart from the speed, the spindle must be stiff (>25 N/μm) and must present a small run out (<1 μm) to ensure high precision in the cutting process.

2. Micromachining techniques in electronics applications

2.1 Overview of micromachining techniques

Mechanical micromachining techniques, such as micromilling, micro–Electrical Discharge Machining (EDM), and laser micromachining, are indispensable tools in the creation of miniaturized electronic components. These techniques offer precise material removal and intricate feature generation, making them essential for fabricating compact and high-performance devices. Micro milling involves the use of rotating cutting tools to remove material from a work piece, creating intricate geometries with high precision. This technique is instrumental in the creation of miniaturized electronic components due to its accuracy, repeatability, and versatility in handling various materials. Micro milling enables the production of micro-scale features such as grooves, channels, and complex 3D structures. These structures are critical in microelectronics, MEMS devices, and micro-optics. Micromilling’s role in miniaturized electronic components extends to applications like microfluidics, where precisely machined channels facilitate controlled fluid flow for lab-on-a-chip systems Câmara et al. [39]. Micro Electrical Discharge Machining (EDM) employs controlled sparks between an electrode and the workpiece to erode material and create intricate shapes. This non-contact process is pivotal in generating micro-scale features that are challenging to achieve with conventional machining methods. Micro EDM is used to fabricate micro-scale molds, tooling, and dies. These components play a crucial role in micro-injection molding for the mass production of microfluidic devices, micro-optics, and precision electronic connectors. The technique’s ability to work with electrically conductive materials allows the production of miniaturized parts with high accuracy Kumar et al. [40]. Laser micromachining employs high-energy laser pulses to ablate or melt material, allowing the creation of intricate patterns with minimal heat-affected zones. This non-contact technique is versatile and suitable for various materials, including ceramics, metals, and polymers. Laser micromachining role in miniaturized electronics spans micro-via drilling in printed circuit boards (PCBs), creation of micro-optical components, and patterning for flexible electronics. Its precision and non-contact nature ensure minimal mechanical stress on delicate structures, making it an ideal choice for manufacturing micro-scale electronic components Gattass Rafael et al. [41].

2.2 Micro milling, principle and process

2.2.1 Micro milling

Micro milling is a versatile micro-cutting process for producing micro-level components in small and medium quantities. CNC controllers allow for precise control of the cutting tool, while CAD/CAM software is used to generate the tool paths necessary to machine complex microstructures. The size, geometry, and composition of micro tools are all important factors in micro milling. The size of the tool determines the minimum feature size that can be machined. The geometry of the tool affects the cutting forces, surface finish, and tool wear. The composition of the tool affects the tool wear resistance and toughness. Commercially micro milling tools ranging from 25 to 1000 μm are available. A typical micro milling machine has got ultra-high-speed low run-out spindles for achieving acceptable machining rates. These spindles being temperature-controlled result in lower thermal errors and better surface finish. These machines use polymer concrete in place of cast iron for achieving high dynamic stiffness as well as submicron resolutions glass scales for better control and minimum motion errors (Figure 1). The machining of THz waveguide geometries requires a very high level of geometrical tolerances and surface quality which can only be achieved through micro-milling [4, 5]. The generated topography of the micro-milled surface inside the waveguide is mainly influenced by the tool, machining parameters, and strategy of cutting. Hence micro-milling represents a suitable technique to manufacture THz waveguide components. Table 1 shows a typical micro milling machine, and its features. This machine was extensively used to make waveguides at W band of frequency where the waveguide size is 2.54 × 1.27 mm within linear tolerances of 10 μm, surface roughness of the order of 150nmRa and flatness of 0.3 μm. when this frequent raises to 1THz the waveguide size becomes 0.245 × 0.127 mm within linear tolerances of 1–2 μm, surface roughness of 40–50 nmRa and flatness also of the order of half the wavelength at such a higher frequency Bhardwaj et al. [1, 42, 43, 44, 45, 46].

Figure 1.

(a) Kern Micro milling, (b) different systems of micro milling machine, (c) polymeric concrete construction, and (d) centring microscope.

Resource	Description
CAM system	Edge CAM and UG NX.
Machine tool	KERN Micro-2522, CNC Micromilling and drilling Machine, iTNC 530 Heidenhain Controller
Cutting tools	Ultrafine grade (<0.5 μm) WC-Co Solid Carbide Endmill Cutters
Cutting tool adapter	Balanced HSK 25 tool adapter and collet with runout less than 1 μm.(KERN-Schaublin make)
Cutting tool length and diameter calibration	Blum Micro Compact NT laser pre-setting system, repeatability ±1 μm
Workpiece holding	High Precision grinding vice
Configurations	Three linear axes
Base structure	Polymer concrete
Controller	Heidenhain iTNC 530
Speed range	Up to 50,000 rpm
Resolution	<0.1 μm
Work volume	250 × 220 × 200 mm

Table 1.

Features of a typical Micro milling machine.

2.2.2 Cutting mechanisms

The material removal principles and process details of the micro-milling cutter are different from macro milling, and it can be characterized by small uncut chip thickness (UCT), specific cutting energy, tool run out noticeable size effect, and elastic recovery after machining. In micro-milling, the UCT is of the scale of the cutting-edge radius or the tool nose radius, also called the size effect (Figure 2) [11].

Figure 2.

Illustration showing the size effect in macro and micro-cutting [11].

The unusual increase in the specific cutting energy as the depth of cut decreases below a critical value during micro-cutting operations using a geometrically defined tool is referred to as the size effect. It is among the principal issues in micro-milling which dominate cutting physics [12, 13, 14, 15, 16, 17]. Three aspects of size-effect are as follows:

1. If the uncut chip thickness is of the order of grain size, the work material may not exhibit homogeneous property.

2. If the UCT is comparable to or smaller than the cutting-edge radius (tool nose radius). In that case, the rake angle becomes highly negative, resulting in the simultaneous occurrence of the plowing and shearing of the workpiece material.

3. When the UCT in micro-cutting is smaller than a certain value, chip formation may not occur. This defines the minimum undeformed chip thickness (MUCT) hm.

As shown in Figure 3a–c, whether an elastic deformation or recovery of the machined surface occurs in the chip formation stage during micro milling depends on the chip thickness and the material of the workpiece. If the chip thickness is small enough, the workpiece material will only undergo elastic deformation and will fully recover to its original shape after the cutting tool passes. This is typically the case in the elastic stage of micro milling, where the chip thickness is in the order of a nanometer. If the chip thickness is larger than the critical chip thickness, the workpiece material will undergo plastic deformation and will not fully recover to its original shape after the cutting tool passes. However, some recovery may still occur, especially if the workpiece material is soft and ductile. This is typically the case in the elastic-plastic stage of micro milling, where the chip thickness is in the order of a few micrometers to millimeters. Overall, it is important to note that there is no clear-cut boundary between elastic deformation and recovery of the machined surface during micro milling. The two phenomena are often intertwined and depend on a number of factors. However, a general rule of thumb is that the chip thickness and the material of the workpiece are the most important factors affecting the recovery of the machined surface [10, 13].

Figure 3.

Material removal in orthogonal micro-milling processes: (a) elastic deformation, (b) mixed elastic-plastic deformation region, and (c) complete chip formation region in the orthogonal micro-cutting process.

In the case of micro milling of THz waveguides, the chip thickness is typically very small, and the workpiece material is often a soft metal such as aluminum or copper. This suggests that elastic recovery of the machined surface is likely to be significant. However, more research is needed to quantify the exact amount of elastic recovery and to determine its impact on the performance of THz waveguides.

2.2.3 Material removal mechanism in micro-milling

In micro milling, the uncut chip thickness (UCT) initially increases from zero to a maximum value approximately equal to feed per tooth, and then decreases from feed per tooth to zero as shown in Figure 4a. In side milling, the UCT reduces from maximum to zero as shown in Figure 4b. The minimum uncut chip thickness (MUCT) is the smallest chip thickness that can be removed without causing plastic deformation of the workpiece material. The MUCT is based on the geometrical relationship at the stagnant point, which is the point at which the material deforms or flows with minimum deformation or kinematic energy [14]. The MUCT varies from 1/4 to 1/3 of the cutting-edge radius, depending on micro-milling conditions, micro mills (tools), and workpiece material classes [10]. Malekian et al. [15] predicted the MUCT for Aluminum alloy (Al6061) as hm = re(1 − cosβ), where β is the friction angle between the workpiece and rake face. Son et al. [17] predicted the MUCT for Al, Brass, and OFHC as hm = re(1 − cos(π /4 − β/2)).

Figure 4.

(a)Tool pass in full end milling immersion, (b) side micro-milling process [18].

The MUCT is an important parameter in micro milling, as it affects the material removal behavior, fluctuations in the cutting force, and stability of cutting forces. By understanding and controlling the MUCT, it is possible to improve the quality of the machined surface. The MUCT is influenced by a number of factors, including the geometry of the cutting tool, the material of the workpiece, and the micro-milling conditions. It is important to note that the MUCT is not a constant value, but rather varies depending on the specific micro-milling operation [18].

2.3 Cutting tools and their geometry effect on micro-cutting

Cemented carbide tools are an excellent choice for micro milling because of their high hardness, acceptable wear resistance, and strength, as well as their affordability. The performance of solid carbide tools is chiefly dependent upon the composition (i.e., cobalt content) and grain size. A lower grain size enables lower values of cutting-edge radius on the tool, which is important for achieving high surface roughness and accuracy in micro milling. The most popular choice for micro milling tools is fine grain size (<0.6 μm) cobalt bonded tungsten carbide [18, 19, 20]. Fine grain size tungsten carbide is an excellent choice for micro milling tools due to its high hardness, good wear resistance, high strength, and affordability. However, it is important to be aware of its brittleness and sensitivity to heat when using it.

The generic geometry of the micro cutting tool is shown in Figure 5, and raw material properties are given in Table 2.

Figure 5.

SEM image of cutter having diameter of 70 μm [20].

Materials	Cemented carbide
Specification	HW-K10
Composition	WC-6Co
Density (g/cm^3)	14.90
Hardness (HV)	1580
Fracture toughness ($MPa.{\text{m}}^{1/2}$)	9.60
Flexural strength (GPa)	2
Compression strength (GPa)	5.40
Young’s modulus (GPa)	630
Poisson constant	0.22
Heat conductivity (W.m^−1.K^−1)	80
Coefficient of thermal expansion, 293–1073 K, (10^−6.K^−1)	5.50
Specific heat capacity 293 K, (J.g^−1.K^−1)	0.95

Table 2.

Raw material types for micro milling cutters and their properties [20].

The material properties of cemented carbides can be adjusted according to the requirement of micro milling cutter applications by altering cobalt content, average grain size, and concentration of composite carbides, as shown in Figure 6a and b [21].

Figure 6.

(a) Scanning electron microscopy (SEM) images, (b) schematic of chemical components for micro-milling cutter materials [21].

2.4 Tool geometry and design considerations

Micro milling tools are small and delicate, and therefore they are more susceptible to breakage than larger tools. High stiffness is essential to prevent the tool from breaking, especially when machining hard materials. The optimal tool geometry for micro milling depends on the tool diameter. For tools with a diameter smaller than 200 μm, an asymmetrical tool geometry is preferred. This is because asymmetrical tools are more tolerant of fabrication errors. For tools with a diameter larger than 200 μm, symmetrical tool geometry is preferred. This is because symmetrical tools are more efficient. The relief angles, tool peripherals, and tool bottom surfaces should be carefully designed to avoid unnecessary contact with the workpiece. This is important to reduce friction and heat generation, which can lead to tool wear and poor surface finish. The critical small geometry features of the tool, such as the size of the smallest grinding wheels and thinnest electrode wire, must be considered during design. This is because these features can limit the minimum feature size that can be machined. Chip disposal spaces must be considered, especially when machining under severe cutting conditions such as no cutting fluid and high-pressure air. This is to ensure that the chips are removed from the cutting zone efficiently. Small cutting-edge radii are essential for achieving a high-quality machined surface. However, it is important to balance the need for a small cutting-edge radius with the need for wear resistance. The rake angle is an important parameter that affects the cutting forces, chip formation, and surface finish. A small positive rake angle is preferred for machining ductile materials. This is because a small positive rake angle produces lower cutting forces and a better surface finish. A highly negative rake angle is preferred for machining hard and brittle materials. This is because a highly negative rake angle produces higher cutting forces but a better surface finish. In addition to the criteria listed above, there are a number of other factors that need to be considered when designing micro milling tools, such as the material of the tool, the coating of the tool, and the cooling system. By carefully considering all of these factors, it is possible to design micro milling tools that can produce high-quality machined surfaces with high precision and accuracy [18, 22].

2.5 Minimum quantity lubrication (MQL)

Cutting fluid plays an important role because of its capacity to reduce friction and its ability to dissipate the heat generated between the micro tools and the workpiece. As cutting zones are very tiny, one of the most challenging aspects of the micro-milling scale is effective cooling and/or lubricating fluid delivery. More than 90% of the mechanical energy in micro-milling processes is converted into thermal energy, resulting in a temperature increase in the cutting zone and cutting edges. When machining oxygen-free copper and aluminum alloys, the cutting zone temperature is frequently below 100°C, whereas over 200°C is frequently what is observed when machining hard and brittle materials [23]. The conventional flood cooling method is not effective in micro-milling due to the small size of the cutting zone and the significant surface tension force [24, 25]. In order to improve cutting fluid penetration into the small cutting zone, a minimum quantity lubrication (MQL) method is considered as the suitable alternative for precisely delivering the lubricant and/or coolant and meeting the requirements of being environmentally friendly. In the MQL system, lubricant is pulverized, allowing to better reach the cutting edge. The turbulence created by tool rotation is a major challenge in micro-milling. It can lead to poor surface finish, tool wear, and chip formation. MQL systems help to overcome this challenge by delivering a small amount of lubricant to the cutting edge in the form of a mist. The mist is able to penetrate the turbulence and reach the cutting edge, where it can reduce friction, heat generation, and chip formation [26]. According to Li et al. [47], minimum quantity lubrication, also known as near dry machining (NDM), refers to the use of cutting fluids in tiny quantities, which is only about ten-thousandths of the amount of cutting fluid used in flood-cooled machining. MQL has been shown to be effective in improving tool life, surface finish, and burr formation in micro-milling.

2.6 Accuracy and thermal effects

Micromilling is significantly affected by factors that are often overlooked in conventional milling, such as tool wear, tool runout, and chatter [28]. Tool wear is high in micro-milling due to the lower chip loads and large effective rake angles. Tool runout is also higher in micro-milling due to the low rigidity of the tool and higher cutting speeds. Chatter is a common problem in micro-milling due to the low stiffness of the tool and the interrupted cutting process. Another challenge of micro-milling is that the stiffness of the tool decreases rapidly as the diameter of the tool decreases. This is because the tool can be considered as a cantilever, and the stiffness of a cantilever decreases with the fourth power of the diameter [29, 30]. Also, micro-milling is an interrupted cutting process where cutting forces vary with the rotation angle of the cutter. To overcome these challenges cutting paths and machining strategies should be selected and optimized in micro-scale milling to maintain tighter tolerances, reduce the cutting force, maintain constant chip load, and avoid premature tool failure [31, 32]. Gracia et al. [48] have discussed the sources of geometrical and surface errors affecting the machining accuracy in Micro milling (Figure 7). The key characteristics of micro-milling are shown in Figure 8 in three broad categories i.e. Tooling, Work material and machine tool [11].

Figure 7.

Geometrical errors sources [31].

Figure 8.

Key characteristics in micro milling [11].

Micromachining processes for THz waveguide are particularly sensitive to the surface roughness and the precision of achieved machining geometrical accuracies. Apart from the inaccuracies of the setup and positional errors, another problem in CNC precision is that of achieved control versus desired control, where actuator backlash, wearing of gears can all add to the problems of deviations. Tool deflections occur in micro-milling, and vibrations occur based on the large tool length to small diameter ratios; such problems only add to increased geometrical inaccuracies. Finally, another problem to be considered is in terms of temperature gradients that can impact material machining accuracy.

2.6.1 Higher spindle speeds (RPM) in micro-milling

High spindle speeds also have the effect of reducing the chip size by requiring a smaller feed rate per tooth, f_T (since f_T∝ 1/ N). Smaller chip loads result in improved surface finish. Good surface finishes are crucial for cutting down on waveguide losses [33, 34].

2.6.2 Temperature control and thermal effects

The most important issue in precision machining is maintaining a constant temperature of the positional stages and the tool holders. Due to the stringent tolerances required for fabricating high-frequency waveguide components, temperature changes of a few degrees result in significant errors. To overcome the thermal effects, liquid cooled spindles and slideways are part of the precision machine design. As well as before commencing actual machining, the machine is usually run for 10–20 min to reach temperature equilibrium (warmup).

2.7 Chatter in machining

Chatter has been and still is a very important topic in micro-milling. It is a highly complex phenomenon due to the diversity of elements that can compose the dynamic system and its behavior. Negative effects of chatter are poor surface quality, unacceptable accuracies, excess noise, disproportionate tool wear, low material removal rate (MRR), material wastage, cost of rework or repair, wastage of energy, and tool damage. Chatter generates through the self-excitation mechanism. It can be primary chatter or secondary chatter.

Primary chatter is by friction between tool and work, it produces a thermomechanical effect on chip formation, and it is due to mode coupling. At the same time, Secondary chatter regenerates waviness on the workpiece surface. This is the main cause of chatter. In most publications, chatter is referred to as regenerative chatter. However, it is possible to distinguish between frictional chatter, thermomechanical chatter, mode coupling chatter, and regenerative chatter on the self-excitation mechanisms that cause the vibration [36].

2.7.1 Strategies for ensuring stable machining process

The control strategies are based on in-process control and out-of-process control. The first group is composed of all those methods that ensure a stable machining process by selecting cutting parameter combinations in the stable zone of the side lobe diagram (SDL) and making most of it. It is possible to distinguish between out of the process and in-process methods. The Methods aim to predict the location of the stability boundary of the cutting process to select stable cutting parameter combinations. The SLD identification is made out of the process before machining. On the other hand, In-process includes those methods that detect chatter during the metal cutting process, allowing the parameters to be corrected, so the cut migrates to the stable zone. It is possible to distinguish between passive and active mechanisms as follows:

2.7.2 Passive modes of chatter suppression

Strategies based on modifying certain machine tool elements to passively change the behavior of system composed of machine tool, for example, cutting tool and tool holder.

2.7.3 Active modes of chatter suppression

Based on certain elements capable of modulating the quantity of work provided, absorbing or supplying the energy with the aim to actively raise or at least change the stability frontier.

2.8 Burr formation

One of the main micro-milling-induced geometrical defects is the burr occurrence on the machined edges of the material. Burrs can be defined as the undesirable projections on the surface beyond the edge of the workpiece which are formed due to the bending of chips at the end of the cut. It has been reported and accepted that burr formation at the microscale is also affected with size effects analogous to the surface roughness.

When the ratio of cut to the cutting-edge radius is small, plowing dominates the cutting process rather than shearing, resulting in high biaxial compressive stresses that push material toward the free surface and generate large-top burrs. Also, the path the tool follows when entering or exiting the work surface also significantly affects burr formation [38, 39]. The cutting parameters, workpiece material properties, tool geometry, coatings, and coolant lubricants also affect the burr formation significantly micro-milling [41]. Figure 9 shows different types of burr and their location.

Figure 9.

Different burr types and their locations [38].

Higher feed per tooth and smaller cutting width values have a positive effect on the burr formation mechanisms [42]. Figure 10 gives the Ishikawa diagram is showing various possible factors which may affect the surface quality and burr formation of the micro-milled surface.

Figure 10.

Fish bone diagram of factors affecting surface quality.

2.9 Machine tools for micro-scale processing

Micro-scale processing is used to produce small and precise features in a wide range of industries, including automotive, aerospace, astronomy, medical, optics, and metrology. To meet the demands of these industries, it is necessary to develop machine tools that can provide the required accuracy and precision.

Figure 11 shows that a thermally and dynamically stable machine tool structure is important for micromachining because it helps to ensure that the workpiece is machined accurately and precisely. Thermal distortion and vibration can cause errors in the machining process, which can lead to defective products. New structural materials, such as advanced ceramics, advanced composite materials, engineering plastics, fiber-reinforced plastics (FRPs), and fiber-reinforced metals, can be used to improve the thermal and mechanical stability of machine tools for micromachining. Figure 12 shows conceptual design of a thermally and dynamically stable machine tool. Design to minimize thermal distortion and vibration can be done by using materials with a low coefficient of thermal expansion (CTE), designing the structure to be stiff and rigid, and isolating the error sources from the machining area. By using vibration dampers and designing the structure to be symmetric and balanced, isolation of the error sources can be achieved. High-precision components, designing the structure to be easy to assemble and adjust, and using materials with a low CTE will lead to minimization of the error sources and feedback sensors will contribute to controlling of the error sources [49].

Figure 11.

Micro scale processing and machine tool structure.

Figure 12.

Concept of stable machine tool structure for micro-and nanometer-scale processing [49].

2.9.1 Abbe’s principle

The Abbe principle states that the length to be measured and the measuring scale must lie on the same axis. This is important because if the measurement is taken at an angle, it will introduce an error. Conventional machine tools often do not satisfy the Abbe principle because the motion axis and the machine element (its reference point or its center of gravity) do not lie on a single line. This can lead to errors in the machining process. Figure 13 shows two different machine tool designs. Figure 13b shows a C-frame with a single ball screw column. In this design, the tool axis line and measuring line are separated. This means that the Abbe principle is not satisfied, and angular deflection will be translated into a displacement error at the tool. Figure 13a shows a double-column and twin ball screw design. In this design, the distance between the tool axis and each ball screw axis compensates for the angular deviation effect on the linear axis. The Abbe principle is an important consideration for the design of machine tools and other precision instruments. By following the Abbe principle, engineers can design machines that are more accurate and precise.

Figure 13.

Two different machine designs. (a) with Abbe’s error reduction, (b) with Abbe’s error.

2.9.2 Machine bed material

Thermal stability is important for machine tool beds in micro-cutting because it helps to ensure that the machine bed maintains its shape and dimensions even when it is subjected to heat from the machining process. This can be achieved by using materials with a low thermal expansion coefficient, low specific heat capacity, and good damping properties. Granite and mineral casting are two materials that meet these requirements, and they are commonly used for machine tool beds in micro-cutting applications. Granite is used mostly in the field of ultra-precision machine beds because of its low thermal expansion coefficient of 6.5 μm/mK which is nearly 2.5 times less than the one of mineral casting. Additionally, granite with a density of 2.8 kg/dm³ is beneficial in terms of Eigen frequency and dynamic properties (refer Table 3).

Characteristic	Unit	Grey cast iron	Granite	Mineral cast
Compression strength	N/mm^2	600–1000	70–300	140–170
Tensile strength	N/mm^2	150–400	30–35	25–40
Modulus of elasticity	kN/mm^2	80–120	35–90	30–40
Heat conductivity	W/mK	50	1.7–2.4	1.3–2.0
Coefficient of thermal expansion	m/mk	10	6.5–8.5	12–20
Density	g/cm^2	7.15	2.9–3.0	2.1–2.4
Damping capacity	—	0.003	0.015	0.02–0.03

Table 3.

Material properties for structural elements in precision machine tools.

Mineral casting materials are a good alternative to granite for machine tool beds because they have similar material properties, but they are less expensive and easier to manufacture. The material properties of mineral casting are comparable to those of granite, but the heat expansion coefficient is about two to three times higher. However, the thermal distortion of mineral casting machine tool beds can be minimized by using appropriate design techniques and by carefully controlling the temperature of the machine tool environment [45].

2.10 Touch-trigger probing and laser measuring systems

Probing systems are an indispensable part of micromachining. Probing systems can help to improve the accuracy, efficiency, and quality of manufactured products. On-machine probing is used to measure and inspect workpieces while they are still on the CNC machine tool. This can help to reduce setup and inspection times and improve the overall efficiency of the machining process. In-process probing is used to monitor the machining process and to make adjustments as needed. This can help to improve the accuracy and quality of the finished product.

2.10.1 Application of probing measuring

Standard applications in micromachining of touch-trigger, on-machine probing measuring systems, has following benefits:

• Identification of the type and size of parts, and their alignment in the fixture.

• Simplification of traditional fixtures by coupling a pallet fixturing system with machine/tool probes because the probe locates everything and compensates before machining.

• Tool presetting and blunt or broken tool detection.

• Checking tools, verification of setups and adjusting offsets.

• Routine machine calibration.

3D touch probes and laser controlled, in process tool management systems are two types of non-contact tool measurement systems that can be used to improve the accuracy and efficiency of CNC machining processes.3D touch probes (as shown in Figure 14) are mounted in the machine spindle and can be used for a variety of tasks, including workpiece alignment, datum setting, workpiece measurement, and digitizing 3D surfaces. Laser controlled (as shown in Figure 15), in process tool management systems use lasers to measure tool length, diameter, roundness, and run-out with a repeatability of better than 1 μm. These systems can also be used to detect tool wear and to compensate for thermal drift of the machine CNC axis and tool.

Figure 14.

3D touch probe, RENISHAW OMP.

Figure 15.

Laser controlled, in process tool management system.

2.11 Precision vices

Some features of Precision vices are, use of high-grade FCO 60 body, ground top and bottom. The clamping jaws give high performance without wear and accuracy in parallelism and squareness of the order of 0.02/100 mm. the jaw plates should be interchangeable. Tenon and Tenon slots are provided for effortless trueing of vice on machine table, high clamping force of the order of 5000 Kg can be achieved effortlessly so that hammering is eliminated on vice handle.

2.12 Spindle/tool holders/collets

Spindles in micro-milling machines must be able to rotate at high speeds (up to 30,000 rpm) to achieve the adequate cutting speed for most materials. The spindle must also be stiff (> 25 N/μm) and have a small runout (< 1 μm) to ensure high precision in the cutting process. To achieve high speeds, spindles in micro-milling machines typically use ceramic ball bearings that are continuously refrigerated and lubricated. Micro-milling spindles are also typically low-power electro-spindles (200–500 W) or aerostatic spindles. Tools in micro-milling are typically small and delicate. The most used collets for clamping tools in micro-milling machines are precision “ER type” collets and super-precision ER type collets. Precision ER type collets can present big runout errors that depend on the clamped diameter, while super-precision ER type collets present runout errors smaller than 2 μm. Tool wear in micro-milling is quite high, so it is usual to use two or more mills per operation (one for rough machining and one for finishing). Tool change is a critical operation in micro-milling because the tool runout, tool height, and collet runout are modified. Therefore, tool change must be performed carefully, cleaning all the shank, collets, tools, and nuts and applying controlled torques [46].

2.12.1 HSK dual-contact tooling

The HSK tool holder is a popular alternative to the conventional steep-taper spindle connection for high-speed machining (HSM). HSK tool holders offer several advantages, including:

• Simultaneous fitment on both the taper and face at the front of the spindle: This provides increased rigidity of the joint and improves accuracy.

• Reduced dimensions: HSK tool holders are smaller and lighter than conventional steep taper tool holders, which reduces vibration and improves performance at high speeds.

• Inner clamping: HSK tool holders clamp the tool shank from the inside, which provides better grip and reduces the risk of tool slippage (Figure 16).

Figure 16.

HSK, dual-contact tooling.

3. Challenges and advances in micromilling for RF devices

3.1 Challenges in micromilling RF devices

Micromilling has revolutionized the fabrication of high-frequency RF (Radio Frequency) devices, enabling miniaturization and enhanced performance. However, this process comes with challenges that researchers and engineers have been addressing through innovative advancements.

3.1.1 Challenges

1. Dimensional accuracy and surface finish: Micromilling demands high dimensional accuracy and surface finish for precise RF device performance. Achieving sub-micron tolerances while maintaining a smooth surface in the micro-scale features is challenging due to tool wear, vibration, and thermal effects [18].

2. Tool wear and material removal rates: Maintaining tool sharpness and durability during micromilling is critical. The small tool sizes used for micromilling can experience rapid wear, affecting accuracy and material removal rates. As RF devices require intricate structures, tool wear management becomes crucial [50].

3. Vibration: Microscale structures are susceptible to vibration effects, impacting accuracy and surface quality. These challenges become more pronounced as device sizes decrease and operating frequencies increase.

3.2 Advances and solutions

1. Tooling advancements: Advances in tool design, materials, and coatings have led to longer tool life and improved machining performance. Ultra-hard tool materials, such as diamond-coated micro end mills, enhance wear resistance and enable prolonged micromilling operations [18, 51, 52].

2. Process monitoring and control: Real-time monitoring and feedback systems have been developed to detect tool wear, surface quality deviations, and other process anomalies. These systems enable adjustments during micromilling to maintain accuracy and quality [53, 54].

3. Simulation and modeling: Advancements in simulation software have enabled accurate prediction of tool behavior, cutting forces, and surface quality during micromilling. These models aid in optimizing tool paths, reducing vibration, and improving dimensional accuracy [55, 56].

4. Multi-axis micromilling: Multi-axis micromilling machines enable the fabrication of complex 3D structures without requiring multiple setups. These systems enhance efficiency, reduce errors, and enable precise fabrication of intricate high-frequency RF device geometries [57, 58].

5. Hybrid micromachining: Combining micromilling with other micromachining techniques, such as micro EDM and laser micromachining, provides a comprehensive solution to overcome challenges like tool wear and material compatibility. This approach enhances the accuracy and versatility of the micromachining process [59, 60].

4. Conclusion

The challenges and advancements in micromilling of high-frequency RF& Terahertz devices illustrate the ever-changing nature of this field. While challenges like dimensional accuracy, tool wear, and material compatibility still persist, innovative tooling, monitoring systems, simulation software, multi-axis systems, and hybrid approaches are advancing at rapid ratesto and increasing micromilling capabilities. These advances will continually enhance the precision and quality of high-frequency RF & TeraHertz devices, driving progress in modern communication and sensing systems.