US 7446051 B2
Silicon (12) is etched through a mask (11) comprising a layer of organic resin material (such as novolac) through which openings (32) are formed in the areas to be etched. The layer of organic resin is first deposited over a free surface of the device to be etched. The openings (32) are then formed by depositing droplets of a caustic etchant such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) with an inkjet printer. The etchant reacts with the resin to expose the silicon surface in areas to be etched. The etching of the silicon surface is performed by applying a dilute solution of hydrofluoric acid (HF) and potassium permanganate (KMnO4) to the exposed surface through the openings in the mask to etch the silicon to a desired depth (83).
1. A method of etching silicon through a mask comprising:
(a) forming a layer of organic resin as a mask over a free surface of a device to be etched;
(b) placing the device on a stage;
(c) locating an ink-jet print device over the device to be etched and in close proximity thereto, the ink-jet device and stage being moveable relative to one another;
(d) using an additive to adjust the viscosity of a reactive material to that required by the ink-jet device;
(e) supplying the ink-jet device with the reactive material with the additive added;
(f) moving the device and the ink-jet device relative to one another under control of control means;
(g) controlling the ink-jet device to deposit predetermined amounts of the reactive material onto a surface of the mask in a predetermined pattern as the device and the ink-jet device move relative to one another, to form openings in the mask, thereby exposing the silicon surface in areas to be etched; and
(h) applying a dilute solution of hydrofluoric acid (HF) and potassium permnanganate (KMnO4) to the silicon surface exposed through the mask to thereby etch the silicon to a desired depth.
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The present invention relates generally to the field of semiconductor device fabrication and in particular the in invention provides an improved processing step for use in a method of forming metal contacts and other structures in thin film semiconductor devices. A new device structure for thin film photovoltaic devices is also provided.
A major advantage of thin-film photovoltaic (PV) modules over conventional wafer-based modules is the potential for low cost of production. However in practice cost savings have been difficult to achieve as a major component of cost is the number and complexity of process steps involved in the manufacturing sequence and can quickly outweigh savings in material costs. In particular the number of steps that require precise alignment, or the speed of the equipment used to perform a step can have a strong bearing on cost, as can the robustness of a process, which might in some cases lead to additional remedial steps being required or result in lower performance of the end product because of materiel degradation. Therefore, process improvements which reduce alignment requirement, reduce the number of steps, reduce damage to the device or, allow a step to be performed more quickly provide significant advantages.
The present invention provides a method of etching silicon through a mask comprising the steps of:
Preferably the area of silicon to be etched has a width and length which are significantly greater (say by at least an order of magnitude) than the depth to be etched. In preferred embodiments the silicon to be etched is a thin film of silicon (such as polycrystalline silicon) on a foreign substrate and the etch is limited by the silicon being etched substantially down to) the substrate. However the process is equally applicable to single crystal material (ie wafer material) and can be made to progress at a rate which allows depth of etch to be controlled by timing of the etch. The depth can be controlled to remove only a thin surface layer (eg to provide a clean surface for further processing) or can remove a significant thickness of the silicon material)
Preferably the dilute solution of HF and KMnO4 comprises a solution of 1% HF and 0.1% KMnO4. With this solution 1.5 μm of silicon will substantially etch away in 12 minutes at room temperature (21° C.).
The organic resin is preferably novolac, but other similar resins are also suitable such as commonly available photoresists. The openings in the resin layer can be formed by chemical removal using solutions of caustic substances such as potassium hydroxide (KOH) or sodium hydroxide (NaOH). In a preferred method according to the invention, droplets of dilute (15%) potassium hydroxide are dispensed at locations intended for etching. The KOH solution is preferably deposited using ink-jet print technology. Other methods of making openings in the mask layer include laser ablation and photographic techniques (using photoresist).
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings (not drawn to scale) in which:
Referring to the drawings,
Division into Cells
As seen in
As seen in
Before the cap layer 72 is applied, the structure 11 is transferred to a tank containing a 5% solution of hydrofluoric acid for one minute. This removes any remaining debris and any surface oxides that may have formed. The structure is rinsed in de-ionised water and dried.
The second insulation layer 17 is a thin layer of organic resin. The insulating resin is resistant to dilute solutions of hydrofluoric acid (HF) and potassium permanganate (KMnO4), and is preferably vacuum compatible to 10−6 mbar. The insulation material most often used is novolac resin (AZ P150) similar to that used in photoresist (but without any photoactive compounds). The novolac resin is preferably loaded with 20-30% white titania pigment (titanium dioxide) which improves coverage and gives it a white colour that improves its optical reflectivity to help trap light within the silicon. The resin layer 17 serves as an etch mask for etching steps described below and also covers over the rough jagged surface that is formed along the edges of the cell definition grooves 16, an area that is prone to pinholes in the cap nitride layer 72. The organic resin layer 17 also thermally and optically isolates the metal layer from the silicon to facilitate laser patterning of a metal layer in contact forming process steps described below.
The novolac resin is applied to each module to a thickness of 4 to 5 μm using a spray coater. After the structure 11 is coated, it is passed under heat lamps to heat it to 90° C. to cure. As seen in
Opening Mask and Etching n Type Contact Openings
In order to make electrical contact to the buried n+ type layer and the upper p+ type layer with a metal layer which will be subsequently formed, holes must be made through the novolac resin layer 17 and the cap nitride layer 72 in the locations where the n type “crater” contacts and the p type “dimple” contacts are required. Firstly with regard to the “crater” contacts to the buried n+ type silicon layer, as well opening the novolac resin layer 17 and the cap nitride layer 72, most of the silicon film 12 must be removed from areas beneath what will later become the n type metal pads to form the n type contact openings 32. Referring to
The openings 32 are spaced holes so that lateral continuity is maintained in the semiconductor layer after contact formation. The ink-jet printing process applies a droplet 76 of the caustic solution in a controlled manner to remove the insulation only where the n type contacts are to be formed. The caustic solution preferably contains potassium hydroxide (KOH) but can also use sodium hydroxide (NaOH) and includes glycerol for viscosity control. The print head used for this purpose is a model 128ID, 64ID2 or 64-30 manufactured by Ink Jet Technologies Inc., and will print substances having a viscosity in the range 5 to 20 centipoise. The droplet size deposited by the print head is in the range of 20 to 240 picolitre corresponding to a deposited droplet diameter range of 50-150 μm. In the preferred embodiment the droplets are printed at a diameter of 100 μm. It should be noted that novolac is an organic resin closely related to the resins used in photo-resist material and the etchant printing process described above will apply equally to the patterning of other such materials.
To extend the opening 32 into the silicon layer 12 as seen in
The resulting opening 32 in the silicon 12 has a rough bottom surface 82, in which some points may be etched through to the anti-reflection layer 71 and some ridges 83 extend into the lightly doped p type region 14 as seen in
Reflow of Mask
Because the side walls of the hole 32 are passing trough the p+ type region 13 and the lightly doped region 14, the walls need to be insulated to prevent shorting of the p-n junction. This is achieved by causing the insulation layer 17 to re-flow whereby a portion of the insulation layer 78 in the vicinity of the edge of the opening 32 flows into the hole to form a covering 79 over the walls as seen in
The rate of re-flow will vary with the aggressiveness of the solvent used, the concentration and, temperature. There are many suitable, volatile solvents that will dissolve organic resins such as novolac, including substances such as acetone. Acetone is a suitable solvent for the process, but acts quite aggressively, requiring only a few seconds to cover the walls of the hole 32 with resin, and making it difficult to control the pus accurately. The preferred solvent is propylene glycol monomethyl ether acetate (PGMEA) and the device is introduced into an atmosphere containing a saturated vapour of PGMEA at room temperature (eg, 21° C.) for 4 minutes until a slight shrinkage of the holes in the insulation is observed.
Opening Mask and Cleaning p Type Contact Openings
A further set of holes 19 (see
Formation of Metal Contacts
The final stage of device fabrication involves depositing a metal layer and slicing it up so that it forms a plurality of independent electrical connections, each one collecting current from one line of p type dimple contacts and delivering it to a line of n type crater contacts in the adjacent cell. In this manner, monolithic series interconnection of the cells is achieved.
Before the metal layer is applied, the structure 11 is immersed into a tank containing a 0.2% solution of hydrofluoric acid for 20 seconds. This acid removes the surface oxide from both the crater and dimple contacts. There is wide latitude for the strength and duration of this etch. The structure is then rinsed in deionised water and dried.
Isolation of n an p Type Contacts
The isolation of the n type end p type contacts is achieved by using a laser 86 (see
The structure 11 is processed using a laser operating at 1064 nm to scribe the isolation grooves in the metal layer 28. The laser is adjusted so that it scribes through the metal layer 28 without damaging the silicon 12. These scribes 31 separate the n type contacts 32 from the p type contacts 19 within each cell, while retaining the series connection of each cell to its neighbours. Preferred laser conditions are a pulse energy of 0.12 mJ with the beam defocused to a diameter of about 100 μm. The pulse overlap is 50% and the scribes are spaced 0.5 mm apart. In addition, there are discontinuous scribes 34 along each cell definition groove 16 (see
Following the formation of the first set of scribes 31, a fewer set of metal isolation scribes 34 are formed over the cell separation scribes 16 between adjacent cells 11, to isolate every second pair of cells. The metal isolation scribes 34 extending to either side of any one of the elongate transverse scribes 31 are offset by one cell with respect to those on the other side of the same transverse scribe 31 such that the cells become series connected by a matrix of connection links 36 with alternating offsets, connecting one set of p type contacts 19 of one cell 35 to a set of n type contacts 32 of an adjacent cell 35, as shown in
The metal isolation scribes 31 comprises a first set of long scribes transverse to the cells 35 from 50-200 μm wide, preferably about 100 μm wide. The scribes are typically spaced on centres of 0.2-2.0 mm and preferably about 0.5 mm to form conducting strips about 0.2-1.9 mm and preferably about 0.4 mm wide. The isolation scribes 34 comprises a second set of interrupted scribes parallel to the long direction of the cells 35 and substantially coincident with the cell isolation grooves 16 in the silicon, The isolation scribes 34 are also from 50-200 μm wide, preferably about 100 μm wide. It is equally possible to form the isolation scribes 34 before forming the transverse isolation scribes 31. The scribed areas are illustrated in
A portion of the completed-structure is illustrated in
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.