Method and apparatus for ultrasonic wet etching of silicon

Details Method and apparatus for ultrasonic wet etching of silicon Method and apparatus for ultrasonic wet etching of silicon

1999-09-14

2001-05-01

Powell, William (Department: 1765)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

C216S090000, C216S091000, C438S747000, C438S753000

Reexamination Certificate

active

06224713

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of wet chemical etching and, in particular, to wet chemical etching of micromachined devices, such as silicon semiconductor structures. The process is particularly adapted for use in producing improved pattern definition in the etching of intricate silicon microstructures.
BACKGROUND OF THE INVENTION
As the demand for ever-smaller silicon devices continues, and as resolution continues below the sub-micron level, the need for uniform and precise micromachining is increasing. Microdevices and microstructures used in semiconductor devices and scanning probe microscopy demand smooth surfaces and precise etching at the sub-micron level. In addition, defect-free surfaces are required to bond micromachined parts together during the formation of microdevices.
Micromachining is especially important in the fabrication of contact structures for testing jigs. Because of a trend towards multi-chip modules, semiconductor manufacturers are required to supply bare, unpackaged dice that have been tested and certified as Known Good Die (KGD). Known good die is a collective term that denotes bare, unpackaged die having the same reliability as the equivalent packaged die. The need for known good die has led to the development of test apparatus in the form of temporary jigs or carriers suitable for testing discrete, unpackaged semiconductor dice. The test apparatus must make a non-permanent electrical connection between contact locations on the die, such as bond pads, and external test circuitry associated with the test apparatus. The bond pads provide a connection point for testing the integrated circuitry formed on the die.
Typically the contact structures on the test apparatus take one of two forms: (1) a recessed contact structure or “pit” such as the structures disclosed in U.S. Pat. No. 5,592,736 to Akram et al., which is assigned to Micron Technology, Inc.; or (2) a raised contact structure or “pillar” such as the structures disclosed in U.S. Pat. No. 5,686,317 to Akram et al., which is assigned to Micron Technology, Inc. The contact pit structure is especially usefull for making contact with die such as, e.g.,. bumped die, flip-chips, chip scale packages or ball grid arrays, which have bumps or balls of solderable material such as a lead-tin alloy located on the bond pad of the semiconductor die. Both the pillar and pit structures require precise micromachining so that they make a good electrical connection with the semiconductor die being tested.
Uneven etching produces irregularly shaped contact pits, which require that the die be forced down onto the test apparatus so that the balls or bumps of the device make contact with the test apparatus. The force required to make such contacts often is so great that it damages the die and renders it inoperable.
In addition, uneven surfaces on a device may cause layers deposited thereon to be rough or irregular, thereby impairing electrical functioning and resulting in low processing yields. For example, a layer such as an insulating or conductive layer that is formed on a rough surface may have pinholes or breaks that result in electrical shorts or that otherwise lead to improper electrical functioning of the device.
Micromachining processes using wet chemical etching have the advantages of keeping production costs low, permitting high control of material purity, and being relatively reproducible. However, there are two major disadvantages of wet etching processes: (1) non-uniform concentration of etchant; and (2) hydrogen bubble adhesion.
Uniform wet chemical etching is difficult to achieve because the etchant solution is often non-uniformly concentrated at the microscopic levels of interest. Localized regions of low etchant concentration may occur due to low mobility of the active elements of the solution, causing “dead spots.” These dead spots may also be the result of the etchant solution becoming saturated within localized regions. When this happens, the etching action is diminished in spots and results in non-uniform etching.
A known method of increasing local concentrations is to increase the overall concentration of the etchant solution, but this does not resolve the problem of non-uniform concentration. In addition, the resultant solution may become so highly concentrated as to increase the etching rate to an undesirable level in some areas, causing undercutting of the photoresists and loss of control over line resolution and spacing. Other methods to solve non-uniformity, such as the addition of magnetic stir-bars to mix the solution, may improve macroscopic concentration uniformity of the solution, but do not significantly affect non-uniformity on the microscopic level.
A second problem causing non-uniform etching and poor pattern definition is the adhesion of hydrogen bubbles to silicon surfaces during the etching process, which causes rough surfaces on the final product. Bubbles cling to the silicon surface due to the poor wettability of the hydrophobic silicon surface. Because the area of contact between a bubble and the surface is shielded from the liquid etchant, it remains unetched, or etches at a slower rate during the etching process. These inadequately etched areas may appear as pyramid-like islands of silicon on a planar surface or irregular pattern edges on the final product.
There is needed, therefore, a process for improving pattern definition and etch uniformity in the etching of intricate silicon microstructures by increasing etchant concentration uniformity and by decreasing the adhesion of hydrogen bubbles.
SUMMARY OF THE INVENTION
The present invention provides a method for improving etch uniformity and pattern definition when etching silicon microstructures by subjecting a wet etching solution containing wetting agents to ultrasonic waves. The wetting agents minimize bubble adhesion, while the ultrasonic waves serve to mix the solution on a microscopic level, to enhance uniformity of concentration, and also to dislodge bubbles from the surface to be etched. Also provided is an apparatus for carrying out ultrasonic-assisted wet etching.
Advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention.


REFERENCES:
patent: 4501636 (1985-02-01), Valley
patent: 4554046 (1985-11-01), Taguchi et al.
patent: 4602184 (1986-07-01), Meitzier
patent: 4686406 (1987-08-01), Meitzler
patent: 4957583 (1990-09-01), Buck et al.
patent: 5021120 (1991-06-01), Buck et al.
Theo Baum et al. AFM Study of Surface Finish Improvement by Ultrasound in the Anisotropic Etching of Si (100) in Koh for Micromachining Applications, Micromech, Microeng. 1997, pp. 338-342.

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