JPH0322446A - Formation of ball for wire bonding - Google Patents

Abstract

PURPOSE:To keep the stability of discharge, and to form the diameter of a ball shaped at the tip section of a wire in specified size by irradiating a torch electrode with ultraviolet rays when high tension is applied between the tip section of the wire and the torch electrode. CONSTITUTION:An ultraviolet irradiation lamp 4 is mounted so as to be able to irradiate the surface of a torch electrode 2 oppositely faced to the tip section 1a of an Au wire 1. The surface of the torch electrode 2 is irradiated with ultraviolet rays 4a by the UV lamp 4 during application after a time immediately before high tension is applied between gaps by a power supply 3. A work function is kept constant by applying ultraviolet rays, and stable discharge can be conducted, thus preventing the dispersion of the diameter of a ball. Discharge maintaining voltage is fluctuated when gap length is changed when discharge currents are kept constant in the relationship of gap length and the diameter of the ball, and the diameter of the ball is varied.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a wire bonding method used in the manufacturing process of ICs, transistors, etc., and relates to a method for forming a ball formed at the end of a metal wire. It is something.

[Conventional technology]

In the wire bonding method, in order to form a ball of a certain size on a metal wire, a high voltage is applied between the tip of the metal wire and a discharge electrode. An electric discharge occurs between the metal wires, and an arc is generated, which melts the tip of the metal wire and increases its volume while maintaining its spherical shape.

In this case, it is difficult to maintain a stable state of the generated arc, so the size and shape of the ball formed at the tip of the metal wire cannot be made constant.

Therefore, a method of using an electrode that easily emits thermionic electrons as a discharge electrode is disclosed in JP-A-62-136832, but the discharge during ball formation is performed in the atmosphere, and the electron emission characteristics change. , hence the size of the ball. It is difficult to keep the shape constant.

[Problem to be solved by the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method that enables stable electron emission from a discharge electrode and obtains balls of a constant size and shape even if the surface condition of the discharge electrode changes. be.

[Means to solve the problem]

In order to achieve the above object, the present invention applies a high voltage between the end of the wire and the torch electrode to form a pole at the end of the wire. It is characterized in that the torch electrode is sometimes irradiated with ultraviolet light.

[For production]

In the present invention, when a high voltage is applied between the end of the wire and the torch electrode, the torch electrode is irradiated with ultraviolet rays to maintain the stability of the discharge and prevent the formation of The diameter of the pole can be set to a constant size.

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

In FIG. 1, in order to form a ball on the end 1a of the Au wire 1 after bonding, the end 1a of the Au wire 1 is
The torch electrode 2 moves to the lower part of a, and there is a gap of 0.3 to 0.5fi between the end 1a of the Au wire 1 and the torch electrode 2.
A high voltage is applied between the gaps by the power source 3, and the arc generated here melts the end portion 1a of the Au wire to form a spherical pole.

In this case, the inventors have found through experiments that the diameter of the ball formed at the end of the Au wire changes depending on the current density. In other words, the ball diameter is determined by the discharge current, but as shown in Figures 2(a) and (b), (
a) when the discharge area A is large, and (b) when the resist 5
In the case where the discharge area A2 is narrowed by Ru.

For this reason, in a state of constant discharge current,
In order to obtain a ball with a constant small diameter formed at the end of the Au wire, it is important to maintain a wide discharge area stably at all times.

The present invention is characterized in that a wide discharge area is always stably formed in the ultraviolet irradiation area in order to obtain a ball of a certain small diameter at the end of the Au wire.

To explain the present invention by returning to FIG. 1, an ultraviolet irradiation lamp (hereinafter referred to as UV lamp) 4 is provided so as to irradiate the surface of the torch electrode 2 facing the end 1a of the Au wire 1. Immediately before and during the application of a high voltage by the power source 3 between the gaps, the UV lamp 4 irradiates the surface of the torch electrode 2 with ultraviolet rays 4a.

The diameter of the Au wire is 20 μmΦ, and in order to obtain a ball diameter one and a half times that of the Au wire, 0.0 μm is used. A high voltage was applied across a gap of 3 mm, and results of variations in ball diameter were obtained in the conventional case in which no ultraviolet rays were irradiated and in the present invention in which ultraviolet 4a was irradiated.

According to this, in the conventional case, it is 35 ± 4 μm, and in the case of the present invention, it is 35 ± 1.4 μm, and in the case of the present invention, the work function is maintained constant by irradiating ultraviolet rays,
Stable discharge can be achieved, and there is no variation in ball diameter.

The area irradiated with ultraviolet rays 4a by the UV lamp has a gap length of 0. When a normal high voltage is applied to an Au wire with a diameter of 20 μmΦ at 3 M, Q. A sufficient effect can be achieved with 7n''. Furthermore, in order to widen the discharge area, an ultraviolet irradiation area larger than 7 x 10-tmm is usually required.

The applied voltage varies depending on the length of the gas gap, and the longer the gap length, the higher the voltage is required, and the higher the voltage is required at the start of discharge, but it may be lower in the discharge maintenance state than at the time of start of discharge.

The relationship between gap length and ball diameter is that when the discharge current is constant, changing the gap length changes the discharge sustaining voltage.
Ball diameter changes. Gasop length that causes little change in ball diameter with normal applied voltage is 0.3 to 0. The gasop length was 5 m, and in this case, the optimal range of ultraviolet irradiation area was 0.12 to 0.50 ml2. A certain irradiation area is required for a given gap length; if the irradiation area is too small, the effectiveness will be reduced.

Note that the ball diameter is controlled by the discharge time and discharge current as in the past.

〔effect〕

With the structure of the present invention, stable electron emission can be performed from the torch electrode, a ball with a constant small diameter can be formed, and the connection to the electrode of a minute area can be made efficiently. It has the effect of expanding its uses.

[Brief explanation of drawings]

FIG. 1 is a schematic side view showing the present invention, and FIG. 2 (tag, fb) is a schematic side view showing the relationship between current density and ball diameter. DESCRIPTION OF SYMBOLS 1... Metal wire, 2... Torch electrode, 3... Power supply, 4... Ultraviolet irradiation lamp, 4a... Ultraviolet rays. 7

Claims (1)

[Claims] By applying a high voltage between the end of the wire and the torch electrode,
A method for forming a ball for wire bonding, which comprises irradiating a torch electrode with ultraviolet rays immediately before and during application of a high voltage when forming a ball at the end of a wire.