Microsystem Technologies

, Volume 16, Issue 8, pp 1369–1375

Fabrication of high hardness Ni mold with electroless nickel–boron thin layer

Authors

  • Yoshitaka Sawa
    • Sawa Plating Co., Ltd.
  • Kenji Yamashita
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
  • Takeshi Kitadani
    • Sawa Plating Co., Ltd.
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
  • Tadashi Hattori
    • Laboratory of Advanced Science and Technology for IndustryUniversity of Hyogo
Technical Paper

DOI: 10.1007/s00542-009-0932-0

Cite this article as:
Sawa, Y., Yamashita, K., Kitadani, T. et al. Microsyst Technol (2010) 16: 1369. doi:10.1007/s00542-009-0932-0

Abstract

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.

1 Introduction

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.

Figure 1 shows a SEM image of the surface of a mold after it has produced approximately 3,000 acrylic sheets. This mold was fabricated in an additive-free bath. Molding conditions are injection temperature of 250°C, mold temperature of 105°C, molding retention pressure of 1.2 × 102 MPa, resin loading time of 0.49 s, molding retention time of 5 s, and cooling time of 45 s. Figure 2 shows a SEM image of a resin component molded using the mold shown in Fig. 1. It has a resin defect caused by resin film adhesion to the mold surface. These figures show the necessity for high-hardness electroforming layer from the aspects of mold service life and mold release characteristics.
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Fig. 1

SEM image of Ni metal mold surface after molding

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Fig. 2

SEM image of resin molded surface

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

We investigated Ni plating baths that can be used to produce the high-hardness Ni plating layer. In this study, we examined Ni–alloy-based plating baths of three types: commonly known nickel-phosphorus (Ni–P) electroless alloy plating, Ni–P alloy electroforming, and nickel–boron (Ni–B) electroless alloy plating. Table 1 summarizes the types of plating bath, main plating conditions and hardness values of the plating layer examined in this study. First, we excluded Ni–P alloy electroforming method. Because the pH value of Ni–P electroplating bath can be used as high as 1–2, indicative of strong acidity.
Table 1

Surface hardness and plating bath conditions

Kinds of plating

Plating condition

Hardness (Hv)

Alloy plating

 Electroless Ni–P

Bath temperature: 90°C

500

 Ni–P

pH: 1–2

500

 Electroless Ni–B

Bath temperature: 60°C

750

The photoresist that forms the mold master structure will dissolve in such strong acid. As the molds master using AZ-P4903 photoresist, we first made a line and space structure with a width of 20 μm and a height of 20 μm on a 4 in. silicon substrate. A copper thin film with 300 nm thickness was formed on the surface of the master by sputtering and a 5 μm Ni–P alloy plating layer was formed using an Ni–P electroless alloy plating bath (bath temperature: 90°C). Figure 3 shows a SEM image of the mold master after the plating operation. Numerous wrinkles creases developed on the photoresist structure side of the Ni–P electroless alloy plating layer. Since these wrinkles creases are transferred in the molding operation, this mold master cannot be used as an LIGA process micro metal mold. Figure 4 shows a SEM image of the mold master after Ni–B electroless alloy plating (bath temperature: 60°C). The mold master was made under the same conditions as Ni–P electroless alloy plating. Wrinkles creases did not develop on the photoresist structure side of the Ni–B electroless alloy plating layer. The plating temperature of the Ni–P electroless alloy plating bath, which can be as high as 90°C, was considered responsible for the development of wrinkle creases on the plating layer. In response to this problem, this study examined use of a Ni–B electroless alloy plating layer to produce a hard Ni plating layer, since that plating process enabled a lower bath temperature. We assessed this layer mechanical property as a metal mold covering layer.
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Fig. 3

SEM image of mold master surface after plating

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Fig. 4

SEM image of Ni–B electroless plating

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.

Test samples of approximately 40 μm thick plated film, deposited from the 50 and 60°C baths, were used. The internal stress of plated film was measured by the strip test method. The surface hardness was measured with a micro Vickers hardness tester (model HM-114, made by Akashi) at a load of 4.9 N. Each sample was measured three times; the average was taken as the sample hardness. We also assessed the change in hardness of the hard Ni plated film at high temperatures, since a micro metal mold is exposed to high temperatures during injection molding operations. A heating furnace (model KDF-S70G100, made by Denken) was used to heat the hard Ni plated film. The change was assessed after holding the samples at temperatures of 100, 150, 200, 250, and 300°C in a nitrogen atmosphere for one hour, then cooling them in air to room temperature. Abrasion resistance testing was conducted in accordance to JIS H 8503 (testing of plating abrasion resistance). A testing device (model NSU-ISO-3, made by Suga Test Instruments) was used as shown in Fig. 5. Abrasive paper with the #320 grain size specified in JIS R 6252 was used. For the abrasion resistance testing, the 12 by 158 mm abrasive paper was bonded to the friction ring of the test device. The test was conducted with an applied load of 1.5 kg. Test samples were prepared with the Ni–B electroless alloy plating solution at 50 and 60°C. One set of plated samples was left in ambient air; other sets were heated for 1 h in a high-temperature oven at 250°C in a nitrogen atmosphere, and cooled naturally to room temperature. For abrasion loss measurement, 100 reciprocal movements of the abrasion tester were taken as a unit, and the abrasion loss was defined by measuring the mass variation per unit. An electronic balance (model AY62, made by SHIMAZU, minimum measurement: 0.1 mg), was used to measure abrasion loss. Measured abrasion loss values were calculated using Eq. 1, which gives the abrasion resistance (WR).
$$ {\text{WR}}\left( {{\text{DS}}/{\text{mg}}} \right) = {\frac{N}{{w_{1} - w_{2} }}} $$
(1)
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Fig. 5

Picture of abrasion testing machine

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

Figure 6 shows the result of internal stress measurement of the Ni–B electroless alloy plated film. The internal stress of the Ni–B electroless alloy plated film ranged from 106 to 147 MPa. It is showing the smallest tensile stress value of 106 MPa at plated film thickness of 4.5 μm as a result of using a plating solution at a temperature of 60°C.
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Fig. 6

Measurement results of internal stress

Figure 7 shows the results of the hardness measurement of Ni–B electroless alloy plated film. The hardness of the plated film deposited from the plating bath at a temperature of 50°C was 808 Hv and that from the plating bath at a temperature of 60°C was 832 Hv. The Ni–B electroless alloy plated film did not exhibit any reduction in hardness as a result of heat treatment below 300°C. The heat treatment at 300°C yielded hardness values of 904 Hv (for bath temperature of 50°C) and 960 Hv (for bath temperature of 60°C), both values being higher than that obtained immediately after deposition without heat treatment.
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Fig. 7

Measurement results of Vickers hardness

Table 2 summarizes the abrasion resistance (WR) and hardness of individual samples. The abrasion resistance (WR) of samples from the bath at a temperature of 60°C was 77 at room temperature, while the abrasion resistance (WR) of samples from the bath at a temperature of 50°C was 59. Regarding the heat-treated samples, those from the bath at a temperature of 50°C exhibited the highest abrasion resistance (WR) value of 143, while the abrasion resistance (WR) value of samples from the bath at a temperature of 60°C was 111. The improvement in surface hardness is considered attributable to an increase in abrasion resistance. Figure 8 shows SEM images of the sample surface used in the abrasion resistance test. SEM images of the abrasion resistance test were an additive-free bath (Fig. 8a) and a Ni–B electroless alloy plating bath (bath temperature of 60°C) (Fig. 8b). Both samples were heat-treated at 250°C for one hour. Sample of plated surface used additive-free bath had been scraped off significantly. Grinded side is ruggedness with sizes of 2 to 3 μm. Particulate residues resulting from abrasion testing were also observed. Sample of surfaces plated using Ni–B electroless alloy bath had been scraped off by polishing grains, but was flat, with no extreme irregularities observed on the polished surface; no particles scattered by the polishing operation were found. Compared with the abrasion tracks on the plated film deposited from the additive-free bath, the surface of the Ni–B electroless alloy plated film after abrasion testing is very flat. On the basis of this test result, we conclude that making the micro metal mold surface harder with Ni–B electroless alloy plated film suppresses the occurrence of fine flaws on the mold surface, thereby reducing the occurrence of mold resin adhesion to the mold surface.
Table 2

Surface hardness and abrasion resistance (WR)

Plating bath

Heat-treatment

Hardness (Hv)

Abrasion resistance (WR)

Ni–B (50°C)

 

808

59

250°C, 1 h

949

143

Ni–B (60°C)

 

832

77

250°C, 1 h

963

111

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Fig. 8

SEM images of the abrasion resistive test results after heat-treated at 250°C for one hour. a Additive-free bath. b Ni–B electroless plating bath

4 Fabrication of high hardness micro mold using double-layers

We prepared a Ni micro mold using the double-layer plating method, which combines a hard Ni plated film, with mold covering layer composed of a Ni–B electroless alloy plated film, and a Ni electroforming layer produced in an additive-free bath. Figure 9 shows the process of fabricating a high hardness nickel micro mold. In the present study we used a light guide plate pattern with fine reflecting protrusions (Tanaka et al. 2006). Using a 4 in. silicon wafer as the substrate, we formed an approximately 300-nm-thick copper film, which provided the reaction layer for the Ni–B electroless alloy plating by sputtering on a photoresist structure prepared by UV lithography. We used a Ni–B electroless alloy plating bath at a temperature of 60°C to deposit a hard Ni plated film approximately 1 μm thick on this mold master. The surface of the hard Ni plated film, deposited before electroforming in an additive-free bath, was soaked in an organic degreasing agent for one minute as pretreatment, and then soaked in a 10% sulfuric acid solution for one minute as surface activation treatment. This was followed by electroforming in an additive-free bath to enable deposition of a Ni electroforming layer to be 4 mm thick. The additive-free bath composition and plating conditions used have already been reported (Sawa et al. 2008a). After removing the silicon substrate, copper conductive layer and photoresist at the end of the process, we were able to successfully fabricate a double layer nickel mold.
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Fig. 9

Process flows of high hardness micro metal mold

Figure 10 shows SEM images of the Ni electroforming mold we prepared and of the truncated-cone-shaped reflecting protrusions of the light guide plate. Also shown are SEM images of a cross section of the truncated-cone-shaped reflecting protrusions. Samples used to observe the cross sections were obtained by cutting through the pattern portion and polishing the cross section with a cross-section polisher (model SM-09010, made by JEOL Ltd.) The pattern shape of the master was formed by a truncated cone with an upper base of 10 μm, a lower base of 52 μm, and a height of 30 μm; it was ascertained that the pattern shape was accurately transferred to the mold. The cross section images show that a hard Ni plated film has been formed uniformly along the structure pattern on the Ni electroforming layer built up in the additive-free bath. The above results show that in manufacturing a micro mold for a light guide plate, it is possible to enhance mold surface hardness and ensure minimum camber.
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Fig. 10

SEM images of fabricated Ni micro mold and cross section of double-layer pattern

5 Conclusions

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.

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© Springer-Verlag 2009