Fabrication of high hardness Ni mold with electroless nickel–boron thin layer
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- Sawa, Y., Yamashita, K., Kitadani, T. et al. Microsyst Technol (2010) 16: 1369. doi:10.1007/s00542-009-0932-0
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The nickel electroforming method using a high-concentration nickel sulfamate bath is commonly used to fabricate micro metal molds in the LIGA process; however, this method does not produce micro metal molds of sufficient hardness. One means of improving the hardness of micro metal molds made using the nickel electroforming method is to include additives in the nickel plating solution. Another method is nickel alloy plating or a similar technique. In this research, we used a nickel–boron (Ni–B) electroless alloy plating method to obtain a hard nickel plated film having hardness of 832 Hv. It was also ascertained that Ni–B electroless alloy plated film retains its high hardness even during heat treatment in conditions of 250°C for 1 h. To deal with the high stresses developed in high-hardness plated films, we proposed double-layer nickel electroforming. This method is covered and used on conventional nickel electroforming layer by high hardness micro mold. High hardness micro metal mold using double-layer was fabricated by nickel electroforming and Ni–B electroless alloy plating method.
Recently, strong demand for the sophistication, downsizing and intensified integration of systems used in such cutting-edge areas industries as advanced information communications, medical care, bioscience, environment and energy has been sharply accelerating microsystem applications. Microsystems used in these cutting-edge industries are often three-dimensionally structured to enable simultaneous implementation not only of signal processing based on electronics, but also mechanical, optical, and chemical functions. In manufacturing these three-dimensional fine structures, semiconductor fine processing technology has been extended to the development of machining technologies such as grinding, electric discharge machining and laser machining technologies are applied to three-dimensional processing technology is awkward. However, in its demand for microdevices, cutting-edge industry seeks not only three-dimensional fine structures with increased fineness and high aspect ratios, but also three-dimensional fine processing technology that costs less. In connection with this market demand, a manufacturing method based on transfer technology using micro metal molds has recently been in the spotlight. In particular, the LIGA (acronym for the German words Lithographie, Galvanoformung, and Abformung) process (Becker et al. 1986; Stephenson 1966 and Hattori 2006), based on exposure technologies using ultraviolet (UV) rays or synchrotron radiation (SR), is receiving attention. In the LIGA process, a master of the mold that will be formed into a three-dimensional structure is fabricated by lithography technology, and the master mold is then used to fabricate the micro metal mold by electroforming. This micro metal mold is then used for the molding operation.
The mainstream method for manufacturing micro metal molds using the LIGA process is based on the nickel (Ni) electroforming method. Because Ni electroforming uses electrolytic deposition depending on the Ni electroforming conditions during micro mold creation, a camber can occur on the plated film due to electrode position stress, rendering the micro mold unsuitable for use in making molds. By optimizing the Ni electroforming solution and other conditions, we have developed a Ni electroforming process capable of producing the 4 mm thickness necessary in micro metal molds used for injection molding (Sawa et al. 2008a). The Ni electroforming process we have developed has the salient features of high-speed Ni electroforming layer production capability (approximately 50 μm/h) and micro metal molds production free of electrode position stress caused camber in the plating layer. However, the electroforming bath used in the present development is a high-concentration Ni sulfamate electroforming bath without additives (referred to as ‘additive-free bath’ in the following). The Ni electroforming layer prepared using this additive-free bath exhibits a low surface hardness value of approximately 200 Hv, which is not sufficiently high, since the injection mold hardness requirement is approximately 400 Hv or more, in common practice.
Suggested ways of improving the Ni electroforming layer hardness are to introduce additives into the plating bath to downsize the crystal grains in the electroforming film, and Ni-based alloy plating based on a eutectic process involving phosphorus, boron or other elements (Kimura et al. 2006a, b). Numerous reports are available on improving the mechanical properties of Ni electroforming film. Sulfur compounds introduced to the Ni plating solution as additives, also act as stress reducing agents for the Ni electroforming film. Depending on the conditions in which the additives are introduced, the compressive stress on the Ni electroforming layer may increase and produce a large camber on the surface of the electroforming mold. In addition, it is known that Ni electroforming layer hardness can decrease in a high temperature atmosphere, depending on the kind of additive introduced into the Ni plating solution (Kimura et al. 2006b). For this reason, it is thought that a plating layer whose hardness is increased by the introduction of additives is not suitable for use with an injection mold that will contact resin heated to a temperature of over 200°C during the molding operation. On the other hand, alloy plating is often used with electroless alloy plating, which requires unique conditions depending on the kind of reducing agent used in the electroless alloy plating solution (Sawa et al. 2008b).
In this study, not all the 4-mm-thick micro metal molds were created using hard Ni plated film; we used double-layer Ni plating to form thin membranes of hard Ni plating as a film to cover the camber-free surface of an existing Ni micro mold, in an attempt to make a micro mold that exhibits minimum camber, yet has high mold surface hardness.
2 Investigation of the plating bath
Surface hardness and plating bath conditions
Kinds of plating
Bath temperature: 90°C
Bath temperature: 60°C
3 Results and discussions
3.1 Method of assessing Ni–B electroless alloy plated film
The Ni–B electroless alloy plated film was assessed by internal stress, surface hardness, and abrasion examination test. A 3 L beaker was used as the plating cell for plated hard nickel films. The plating solution was mechanically agitated using a stirrer at 300 rpm. We used a Ni–B electroless alloy plating solution (Top Chemi Alloy B-1, made by Okuno Chemical Industries). The cathode was formed by first producing an approximately 300 nm thick Ti film on a 4 in. silicon substrate using a sputtering device, then laying down an approximately 300 nm thick Cu film atop the Ti film by sputtering. Test samples were prepared by depositing a Ni–B electroless alloy plated film on the Cu sputtered film. Two plating bath temperatures were investigated at 60°C as recommended for the bath temperature and 50°C, that is lower than the recommendation. It was thought that the photoresist structure would not be transformed by low liquid temperature.
where WR is the abrasion resistance (DS/mg), N the DS number (number of abrasion ring reciprocating movements (double stroke)), w1 the mass of sample after preliminary prior abrasion test (mg), w2 the mass of sample after abrasion test (mg).
In this test, the number of abrasion ring reciprocating movements (N) was set at 100, which was taken as one unit. Tests corresponding to seven units were conducted to calculate the WR value of each unit. The average over five units after excluding the maximum and minimum calculated WR values was taken as the abrasion resistance (WR) of the sample.
3.2 Results of Ni–B electroless alloy plated film assessment
Surface hardness and abrasion resistance (WR)
Abrasion resistance (WR)
250°C, 1 h
250°C, 1 h
4 Fabrication of high hardness micro mold using double-layers
We have studied electroforming molds based on the LIGA process as a technology for manufacturing 3D fine micro-structures. Since nickel electroforming molds fabricated using an additive-free bath do not meet the hardness requirement associated with injection molding, we studied methods of increasing hardness by means of alloy plating. Noting that high-hardness Ni–B films have large internal stresses, we studied the possibility of making micro metal molds by using double-layer plating consisting of hard Ni plated film and Ni electroformed film using an additive-free bath. Thanks to the plated film deposited from the Ni–B electroless alloy plating solution, we successfully obtained a hard plated film with a hardness of 800 Hv or more. These results show that we succeed in producing double-layer plating consisting of a hard plated film and electroformed film in an additive-free bath, verifying that the hard Ni plated film that served as a mold covering layer covered the entire mold pattern, and in successfully fabricating molds that accurately transfer mold master patterns prepared by UV lithography.
We intend to experiment with molding using the electroformed molds fabricated in the study, and to assess the mold release characteristics and service life of these molds. Furthermore, we will apply the current technology to large electroformed molds, with the aim of establishing a process for making large-area micro metal molds.