LPCVD for TOPCon Cells

As mentioned in an earlier TOPCon article (see Several Path To TOPCon Cells), nearly every known deposition method is promoted for the application of the rear passivation stack of TOPCon solar cells. However, LPCVD – short for Low Pressure Chemical Vapor Deposition – is the first and most widely tool used, at least as of now. Why? LPCVD is a widely used process in the semiconductor industry for depositing polysilicon and even doped silicon, exactly what is also needed for TOPCon.

With a strong semiconductor industry background and the technology being readily available, several European s0lar quipment vendors turned to LPCVD as the first choice. This thermal process is used to deposit thin films from gas-phase precursors operating at vacuum. The reduced pressure during the process helps reduce unwanted gas-phase reactions and improve film uniformity across the wafer. All LPCVD tools promoted for passivated contacts today enable the growth of tunneling oxide and polysilicon in the same run without breaking the vacuum. And in-situ doping of polysilicon is also offered by all these tool vendors, at least as an option. However, LPCVD faces a inherent issue called wrap around. It is nothing but the parasitic deposition of the polysilicon on the front side caused due to the precursor penetrating the gaps between the wafers in typically employed back-to-back loading – 2 wafers in a single slot of a carrier used to process the thin silicon slices.

Semco, also a company with in-house polysilicon deposition experience for its semiconductor clientele, developed an LPCVD solution for TOPCon. The tool platform of the company has even alleviated major shortcomings of the LPCVD technology. On top of the list is the horizontal wafer processing, which though many may not eliminate the wrap-around completely, it does afford better control or pushes it down to a predictable level at least. The approach also facilitate processing of larger wafers with reducing the risk of breakages and offer better control over the process window and recipe management.  Another  major change that the company brought in was to replace the breakable quartz reactor with a stainless steel chamber. The LPCVD platform of the company, called HORTUS, accomplishes all necessary processes required for establishing the passivated contact structure on the rear side of the cell.

Semco’s HORTUS LPCVD platform comes in a 6-stack furnace configuration. The tool accomplishes all 3 processes — tunneling oxide growth, polysilicon deposition and in-situ doping of polysilicon film that is capable of processing 10,800 silicon substrates per production cycle. This translates into an hourly throughput of 5,700 with G12-size wafers, while the capacity goes up to 6,000 when processing M10 wafers. The above numbers are with reference to a polysilicon layer thickness of 160 nm. Semco has been an equipment partner for early adopters of TOPCon, such as Korea’s LG, and the company claims more than 4 years of mass production experience with its tool. It has also supplied “several” of its toolsets to leading manufacturers that are operating with high production yields of up to 96% in mass production, according to CEO Raymond de Munnik.

Centrotherm is another important solar cell processing equipment maker that also has an LPCVD based solution on offer for TOPCon cell production. Employing LPCVD, especially to deposit undoped polysilicon, has been the main route to realize TOPCon structure in production, according to Josef Haase, senior technical director of the company. While its first-generation tools were aptly designed to accomplish these basic tasks, the company added the in-situ doping feature to its platform to simplify the process. However, opting for in-situ doping reduces the throughput. While poly doping can be accomplished with half pitch loading of 1,600 wafers per tube per batch, the in-situ process requires wider spacing, meaning a full pitch of 4.76 mm. Thus, the batch size is limited to 800 wafers. And it also makes the reactor a bit complex with the added responsibility of handling gaseous mixtures of silane and phosphene or diborane. It is still worth the effort; consecutive doping not only eliminates the POCl diffusion step entirely, PSG etch can also be avoided. With the in-situ step, the dopant is already evenly spread, which just requires a short RTP activation step. The annealing or activation step is very well accepted by the manufacturers, given the low prices of such simple thermal treatment furnaces. And with proper optimization, the step can also be integrated into the firing profile.

Centrotherm has introduced a new variant of c.DEPO X LPCVD system with 10-stack tube configuration. Similar to any other equipment maker, the Germany company has scaled up the processing tools to accommodate larger wafers up to 210 mm side length. Depending on the opted process recipe, doping configuration and process integration, the tool supports a throughput of up to 6,000 wafers per hour. The thickness of the polysilicon is yet another parameter that influences throughput, the prevailing standard for which is between 100 and 150 nm. Centrotherm has already supplied its LPCVD furnaces to “a few” cell makers.

Laplace is a notable LPCVD supplier in China, primarily focused on TOPCon with larger and thinner wafers. Laplace’s reactor design closely resembles that of Semco, especially when it comes to wafer orientation during the processing. Laplace is also a strong advocate of processing wafers horizontally for the same 2 reasons – to keep the wraparound within limits and to address breakage issues associated with larger wafers. However, unlike Semco, which uses a stainless steel reactor chamber, Laplace still uses quartz tubes. Laplace uses paddles to support the boats during the processing so that the boats are not directly placed in the tube, an approach it originally developed for boron diffusion. However, this does not completely fix the main issue of cracking of the quartz tube due to the polysilicon deposition on the inner walls. Sparing the details, Laplace says it has optimized the process to improve the lifespan of the quartz-ware – to about 600 runs – which according to Laplace accounts for 1.5% of the costs, and the company plans to reduce it to 1% by enhancing the life further.

As for the throughput, for wafer sizes below 190 mm, each tube can be loaded with 2,000 wafers. The tool requires 80+ minutes to accomplish the deposition of intrinsic poly, which goes up to 3 hours with in-situ doping. The loading capacity drops to 1,600 in case of G12 wafers. And the net throughput depends on the choice of number of tubes and process recipe. For reference, the 5-tube configuration can process above 3,300 of M10 wafers with in-situ doping. Laplace has an installed capacity of about 3 GW.

Polar PV is a Chinese equipment maker that is offering two different equipment platforms — PVD and LPCVD – for TOPCon. The LPCVD platform of the company accomplishes the growth of tunneling oxide and deposition of polysilicon. From loading to unloading, the process is accomplished in 13 steps in an average cycle time of 110 minutes. However, some companies have even managed it in 100 minutes, it notes. One way to reduce the cycle time further is through the optimization of polysilicon layer thickness, which is currently between 80 to 150 nm. Polar PV’s LPCVD has a throughput of 3,600 wafers per hour, applying 1 to 1.5 nm of silicon oxide and 100 nm of polysilicon. Polar PV claims that it has supplied LPCVD tools for Jolywood’s entire 2.4 GW production capacity.

S.C New Energy, the leading Chinese equipment supplier, is also offering 2 solutions for TOPCon – and LPCVD is naturally the first. Called LD-420, the company’s LPCVD furnace is built with a 6-furnace stack in its standard configuration, while a tool with 5 or 4 tubes can also be ordered. The tube has an inner diameter of 420 mm, enabling the processing of wafers up to G12, while the company would also consider a special request to configure the furnace to process further larger wafers of up to 230 mm side length. The company is using a double-layer quartz tube structure with water-cooled tube sealing technology. Each tube has a 2,200 mm flat zone that accommodates 1,600 wafers in a back-to-back loading configuration following half pitch arrangement of 2.38 mm. The tool supports quite a wide range of low-pressure processing environments from 15 to 600 Pa. This LPCVD furnace is designed to accomplish three important process steps of the TOPCon process — growing tunneling oxide with thickness between 1.4 and 2.2 nm, polysilicon layer from 80 to 200 nm, and in-situ doping. However, S.C New Energy considers 150 nm as the polysilicon layer thickness for indexing the process with the spec for thickness uniformity deviation of 3% within the wafer and run to run, while 4% is given for wafers of the same batch.

This brief article is taken from our recent TaiyangNews report on TOPCon Solar Technology, which is available for free download here.