Method for the calibration of a wire bonder

Details Method for the calibration of a wire bonder Method for the calibration of a wire bonder

2003-02-03

2004-05-25

Stoner, Kiley (Department: 1725)

Metal fusion bonding

Process

Plural joints

C228S102000, C228S110100

Reexamination Certificate

active

06739496

ABSTRACT:

PRIORITY CLAIM
The present application claims priority under 35 U.S.C §119 based upon Swiss Patent Application No. 2002 0188/02 filed on Feb. 1, 2002 which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention concerns a method for the calibration of a Wire Bonder.
BACKGROUND OF THE INVENTION
A Wire Bonder is a machine with which wire connections are made to semiconductor chips after they have been mounted on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On making the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding from the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics are applied to the horn from an ultrasonic transducer. This process is known as ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last process is known as wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bond cycle can begin.
The ball bonding is influenced by various factors. In order to achieve bond connections of a predetermined quality, the adequate values of several physical and/or technical parameters must be determined for a particular process. Examples of such parameters are the bond force, that is the force which the capillary exerts on the ball or the connection point of the semiconductor chip during the bonding process, or the amplitude of the alternating current which is applied to the ultrasonic transducer of the horn.
The distances between the connection points on the semiconductor chips, known in the art as “pitch”, are becoming increasingly smaller. Today, in the Fine Pitch field, one already tends towards a pitch of only 50 &mgr;m. This means that the dimensions of the capillary in the area of its tip are also becoming increasingly smaller in order that the capillary does not come into contact with the already bonded wires. With the increasingly smaller dimensions of the tip of the capillary, the influence of unavoidable manufacturing tolerances on the mechanical characteristics of the capillary become greater. With bonding, the capillary wears out so that from time to time it has to be replaced by a new capillary. Today, in order to achieve reliable bonding results even in the Fine Pitch field without having to recalibrate the Wire Bonder with time consuming work after every capillary change, the capillaries are selected according to strict geometrical criteria.
SUMMARY OF THE INVENTION
The object of the invention is to develop a method for the calibration of a Wire Bonder which guarantees that, in mass production, semiconductor chips are wired under the same process conditions before and after a capillary change.
A further task which is set in mass production is the transfer of the optimum parameters found on one Wire Bonder to another Wire Bonder. The invention should also offer a solution for this task and support the recipe transfer from Wire Bonder to Wire Bonder in a simple and robust manner.
Each Wire Bonder has a capillary clamped to a horn. Ultrasonics is applied to the horn by an ultrasonic transducer, whereby the ultrasonic transducer is controlled by means of a parameter P. The parameter P is preferably the current which flows through the ultrasonic transducer. The parameter P can however also be the amplitude of the alternating voltage applied to the ultrasonic transducer or the power or another quantity which controls the ultrasonic transducer.
As a rule, on capillary change, the oscillating behaviour of the capillary tip changes because every capillary has somewhat different characteristics and is also slightly differently clamped onto the horn. The endeavoured aim of the named tasks consists in measuring the basic influential quantities of the capillary or the oscillation system formed by the horn and the capillary which have a fundamental influence on the bonding process and using the knowledge gained to reset the relevant bond parameters of the Wire Bonder after a capillary change according to the mechanical characteristics of the new capillary and only starting production with the new capillary after this.
The invention is based on the knowledge that the mechanical characteristics of the tip of the capillary have a strong influence on the ultrasonic force which the capillary exerts on the ball bond. Because the dimensions of the capillary in the area of its tip are becoming smaller and smaller, unavoidable manufacturing tolerances also cause increasing variations in the rigidity from capillary to capillary. The invention provides a solution as to how these variations in rigidity can be compensated.
During bonding, a predefined bond force is applied to the capillary. The tip of the capillary therefore presses in vertical direction against the ball bond which is clamped between the capillary and the connection point of the substrate. When ultrasonics is applied to the horn, then stationary ultrasonic waves are formed in the horn and in the capillary. Because the capillary is pressed against the ball bond, its tip can not oscillate freely. The tip of the capillary therefore exerts a force directed in horizontal direction, a so-called tangential force F
T
, on the ball bond. This tangential force F
T
is a function of the deflection A
H
(t) of the tip of the horn in relation to the tip of the capillary, whereby the parameter t designates the time. The tangential force F
T
(t) is an alternating force F
T
(t)=F
T0
*cos((&ohgr;t), which oscillates with the frequency &ohgr; of the ultrasonic waves.
The amplitude A
H
of the oscillation of the horn at the clamping point of the capillary relative to the tip of the capillary lies typically in the range of 0.1-4 &mgr;m and is therefore small in relation to the length of the capillary of typically 11 millimetres. The amplitude A
H
is also small in relation to the length of the thinnest part of the capillary, namely the tapering at the tip of the capillary. The capillary therefore behaves almost like a spring, ie, the amplitude F
T0
of the tangential force F
T
(t) is in good approximation proportional to the amplitude A
H
of the oscillations of the horn at the clamping point of the capillary relative to the tip of the capillary:
F
T0
=k*A
H
,  (1)
whereby the quantity k designates a constant which is dependent on the mechanical characteristics of the capillary: The constant k is a measure of the flexural strength of the capillary. The amplitude F
T0
of the tangential force is therefore fundamentally dependent on two quantities, namely the amplitude A
H
, which is controlled by the ultrasonic transducer, and on the flexural strength of the capillary.
The solution of the named tasks now exists in determining the flexural strength for each capillary and, after each capillary change, adapting the parameter P, which controls the ultrasonic transducer, to the determined flexural strength of the capillary so that the tangential force exerted on the ball bond by the respective capillary is equally great before and after a capillary change.
When setting up for bonding a new product, the optimum values for various parameters such as bond force, parameter P for control of the ultrasonic transducer, etc, must first be determined. In the following, it is explained how the parameter P for control of the ultrasonic transducer is reset after a capillary change. The parameter P is, for example, the amplitude I
0
of the alternating current I which is applied to the ultrasonic transducer. A linear relationship exists between the amplitude A
H
and the amplitude I
0
of the alternat

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