Evergreen Solar (Evergreen), a Massachusetts silicon PV maker, recently announced that it filed for Chapter 11 bankruptcy and would try to sell its assets under the reorganization.
According to the Greentech Media report, those assets include “the String Ribbon technology that is at the core of Evergreen’s existence.” Without String Ribbon, Michael Kanellos wrote, “Evergreen’s assets are more generic,” and if the technology can be sold off, “there won’t be a lot left to reorganize.”
So what is String Ribbon, and what are the technology assets that are up for sale?
The String Ribbon technology is covered, at least in part, by a family of at least three patents – U.S. Patents Nos. 6,814,802, 7,022,180 and 7,507,291, each entitled “Method and apparatus for growing multiple crystalline ribbons from a single crucible” (collectively “Ribbon Patents”).
The basic ribbon technology, before the invention of the Ribbon Patents, is shown in FIG. 1A. It involved continuous growth of a single ribbon of silicon sheet material (17) drawn through a single crucible (11).
Inside the crucible (11) is a melt (12) of silicon and a pair of strings (15) extending through the crucible (11).
The cooler liquid silicon crystallizes at the top of the meniscus (19), and the strings (15) become incorporated in and define the edge boundaries (18a, 18b) of the crystalline ribbon (17).
According to the Background section of the Ribbon Patents, the continuous growth of silicon ribbon obviates the need for slicing of bulk produced silicon to form wafers.
Instead of just one ribbon at a time, the invention of the Ribbon Patents enables continuous and concurrent growth of multiple semiconductor ribbons from a single crucible.
Figure 2A, for example, shows a continuous two-ribbon dual growth system (20) including a crucible (21) having a melt (22) of silicon and two pairs of strings (25a, 25b) extending through the crucible.
Each of the pairs of string (25a, 25b) has a fixed distance between the strings. Two crystalline ribbons (27a, 27b) of silicon are drawn from the melt (22) as the cooler liquid silicon crystallizes at the tops of the menisci (29a, 29b).
The pairs of strings (25a, 25b) pass through holes in the bottom of the crucible and become incorporated in and define the edge boundaries of the crystalline ribbons (27a, 27b).
Additional embodiments, as shown in FIGS. 3A and 3B, include meniscus shapers (3a, 3b) placed around the two pairs of strings (35a, 35b) to partition the melt (32) to form subregions (3c, 3d) from which two ribbons (37a, 37b) grow.
The Ribbon Patents also teach a nine-ribbon growth system, shown in FIG. 6, which includes nine pairs of strings, nine meniscus shapers (6a-6i), and an afterheater (64).
According to the Ribbon Patents, the disclosed methods and apparatus allow for a substantially better rate of production and efficiency and reduce capital, material, and labor costs.
But the Greentech Media piece points out that adopting the ribbon technology requires some changes to standard manufacturing processes, and the wafers and cells produced by the ribbon process are sized differently than those made by conventional manufacturing methods.
It will be interesting to see how much green will be offered for these green patents.
Eric Lane is a patent attorney at Luce, Forward, Hamilton & Scripps in San Diego and the author of Green Patent Blog. Mr. Lane can be reached at firstname.lastname@example.org.