The entire disclosures of Japanese Patent Application No. 2006-124095, filed Apr. 27, 2006, and Japanese Patent Application No. 2007-032513, filed Feb. 13, 2007, are expressly incorporated herein by reference.
1. Technical Field
The present invention relates to pattern forming. More specifically, the present invention relates to a method a forming a pattern, a droplet discharging device and a circuit module.
2. Related Art
Recently there have been circuit modules developed which include mounted electronic components, including semiconductor elements, such as a Low Temperature Co-fired Ceramics (LTCC) multilayer board made of a glass ceramics. Within the circuit modules, the LTCC multilayer board can be fabricated by firing stacked-up green sheets at temperatures of 900° C. or less. Within the boards, low melting metals such as silver and gold can be used for internal wiring, thereby simplifying the internal wiring process.
During the manufacturing process of such LTCC multilayer boards, a metal paste or a metal ink is used to write wiring patterns on each green sheet before it is stacked. One proposed method of writing is found in Japanese Patent Application JP-A-2005-57139 where an inkjet method is used wherein metal ink is discharged in the form of micro droplets. The inkjet method writes wiring patterns by joining such micro droplets. One advantage of this method is the quick response to changes in the design of the internal wiring, such as changes to the density of the internal wiring and changes to the width and pitch of the internal wiring.
One difficulty of the inkjet method is that the size and form of each droplet that lands on the green sheet changes with time according to the surface state of the green sheet and the surface tension of the droplet. The variations in the droplet size and form also vary in accordance with the timing of its drying. For example, a metal ink droplet with an external diameter of 30 μm expands to a diameter of 70 μm in 100 msec after landing on the lyophilic green sheet. The external diameter further expands to 100 μm 200 msec after landing. Thus, in this example, when the timing for drying of the droplet varies within the range of 100 msec to 200 msec after its landing, the width of the corresponding wiring pattern will vary within the range of 70 μm to 100 μm.
It has been proposed to use irradiated laser beams to dry such droplets, in order to minimize the variations in the size of the pattern. In the laser drying methods, only the droplets within the region irradiated by laser beams are dried. Therefore, the laser drying methods allow a highly precise control in the timing of drying of the droplets, resulting in a reduction of variation in the pattern size.
One disadvantage of the laser drying methods, however, is that in order to secure the placement of the droplets, the space between a droplet discharging head and an discharging target is often reduced to the level of several hundred micrometers. Thus, in the case where the droplets need to be dried right beneath the droplet discharging head, the laser beam must be irradiated into the narrow space between the droplet discharging head and the target of discharging from an angle that often makes it difficult to tightly focus the beam. As a result, the laser beam spot formed on the target is often large, preventing the sufficient amount of laser beam strength which is required for drying the droplets. This tends to result in insufficiently dried droplets, thereby incurring cases of poorly formed patterns.