The present invention relates to a surface treatment method of a silicon based substrate, and more particularly, it relates to an etching process using plasma.
In a manufacturing process of silicon substrates which are used in semiconductor devices, a thinning-substrate process is prepared for thinning a substrate in order to produce slim semiconductor devices. After circuit patterns are formed on a surface of a silicon substrate, this thinning-substrate process is done by mechanical polishing on a face opposite to the circuit-formed surface. However, the mechanical polishing invites a stress-alteration layer including micro cracks on the surface or around the surface layer of the silicon substrate. The stress-alteration layer would lower the strength of the silicon substrate, therefore, an etching process is provided after the mechanical polishing in order to remove the stress-alteration layer formed on the silicon surface. A plasma etching process has been discussed to be used as a new etching method alternative to a conventional wet-etching method. Because the plasma etching process does not produce chemicals or industrial waste, both of which have been produced by the conventional wet-etching method.
This plasma etching for silicon substrate uses mixed gas including fluorine based gas such as carbon tetrafluoride (CF4) or sulfur hexafluoride (SF6) in order to produce plasma. The fluorine based gas is ionized or excited by plasma discharge, and these ions or radicals etch the silicon surface.
This plasma etching process leaves reaction product on the surface, and the reaction product sometimes attaches again to the surface of the silicon substrate. Etching speed differs depending on a place, e.g., the speed at a place where the reaction product attaches is different from the speed at another place where no reaction product attaches. As a result, an ununiform milky dull color appears on the surface after the etching. This appearance degrades the product per se. To overcome this appearance problem, when a silicon substrate is etched by plasma, the entire surface of the silicon substrate is etched as uniform as possible so that the etched surface can be finished as a mirror surface. A condition of plasma processing is prepared for obtaining this mirror surface.
When the etched surface is finished as a mirror surface, an appearance of the product becomes good; however, the product encounters the following problems. First, marks printed on the surface after the etching are difficult to read. To be more specific, marks indicating a product name and a lot number are printed by a laser marker on a silicon substrate after the etching; however, a mirror surface with the marks printed gives a poor contrast, which makes the marks difficult to be recognized.
Second, in die-bonding, i.e., when a semiconductor chip is mounted to a substrate such as a lead frame after the semiconductor chip is cut from the silicon substrate, the semiconductor chip is hard to remove from adhesive tape. In the die-bonding, the silicon substrate is bonded to adhesive tape on its etched surface (the surface opposite to the surface where circuits are formed), and a semiconductor chip is detached from the adhesive tape by a sucking nozzle one by one, and picked up before die-bonded to a given board. When the etched surface is finished as a mirror surface, adhesive strength between a semiconductor chip and adhesive tape becomes excessively strong, so that the semiconductor chip sometimes cannot be removed in a regular way. Even if the semiconductor chip is stuck up upward by an ejector pin when the chip is picked up, the chip is hard to remove from the tape. This situation invites an error in picking up operation.
In the conventional surface treatment of a silicon based substrate, when a process condition is prepared for obtaining a mirror surface as an etched surface, the problems discussed above occur. This is because an appearance quality is given priority. If another condition is prepared for avoiding the problems and admitting a milky-dull color appearance, the milky-dull color appears ununiformly.
The present invention provides a surface treatment method of a silicon based substrate. Through this method, the surface is etched using plasma, and a uniform milky-dull color appears on an etched surface, so that a good appearance is obtained. Further, die-bonding operation free from pick-up errors can be expected.
The present invention addresses the problem discussed above, and a surface-treatment method of the invention comprises the following steps:
a step of first plasma etching for forming fine dimples on a polished surface of a silicon based substrate which has undergone a thinning process by mechanical polishing; and
a step of second plasma etching by plasma process using gas for producing plasma, the gas including fluorine based gas.
Through these two steps, a uniform milky-dull color appears on an entire etched surface of the substrate. This is an object of the present invention.
A method of surface-treatment of a silicon based substrate comprises at least the following steps.
a first plasma etching for forming fine dimples on a surface to be etched; and
a second plasma etching, carried out after the first plasma etching, for further etching the surface through plasma process using gas for producing plasma, the gas including fluorine based gas.
This surface treatment allows printed marks on the surface to be read with ease even if the substrate is mechanically polished for being thinned. Also the surface treatment prevents an error in picking up operation when a semiconductor chip is die-bonded on a given board. As a result, thin semiconductor devices can be manufactured efficiently.