Patent Application
20110223706
1. Field
This disclosure relates generally to forming semiconductor devices, and more specifically, to forming a photodetector.
2. Related Art
Some devices, such as photodetectors, are formed by epitaxially growing germanium over a silicon layer. The prior art has focused on making the interface between the germanium and silicon clean. If the interface is not clean then epitaxial growth is impeded and device performance is degraded.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
While making the interface between the germanium and silicon clean improves the photodetector performance, the inventors have discovered that a clean top surface of the germanium also affects device performance. The inventors found that germanium oxide is formed if the germanium is exposed to an oxygen environment, which occurs when the photodetector is removed from the tool used to form the germanium detector. This germanium oxide, if present, creates leakage degrading the performance of the device, such as by decreasing the signal to noise ratio. As a result, the inventors have created a treatment process to remove the germanium oxide to improve performance. This treatment process uses an oxygen-free plasma so that radicals (e.g., hydrogen radicals) bond with the oxygen from the germanium oxide and remove the germanium oxide. The result is a pure germanium layer.
A dielectric 22 is formed over the waveguide region 16, the optical trenches 20, and isolation regions 18. The dielectric 22 may or may not be planarized, patterned, or both. In one embodiment, the dielectric 22 protects the waveguide region 16. The dielectric 22 may have additional functions depending on what layers it includes because the dielectric 22 may include many different layers. For example, the dielectric 22 may include the dielectric layers that were used to form features of other devices (not shown) formed over the semiconductor substrate 12 (e.g., spacers and liner dielectrics). The presence or absence of dielectric layers depends on the particular processing integration being used. The dielectric 22 may be formed by any suitable process, such as a chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD), the like, and combinations of the above. In one embodiment, the dielectric 22 is a silicon protective layer and includes a top layer that is silicon dioxide and a bottom layer of a different material. In one embodiment, the bottom layer includes silicon and nitrogen; for example, the bottom layer may be silicon nitride. In another embodiment, the dielectric 22 only includes one material, such as silicon dioxide. It is desirable that the top layer or the entire dielectric 22 is silicon dioxide, because silicon dioxide has good selectivity for the subsequent germanium formation process. In one embodiment, the dielectric 22 is greater than approximately 50 nanometers (nm) thick. The first dielectric 22 is patterned, using conventional patterning technique to form a window and expose the waveguide region 16. As shown in the illustrated embodiment, the window may also extend into the waveguide region 16.
After forming the window, a clean process may be performed to improve the interface (a first surface) between the germanium detector 24 and the waveguide region 16 to improve performance. For example, the clean process may include an HF (hydrofluoric acid) etch process following by a low temperature oxide growth with O3, followed by a dry process using N2, a subsequent HF clean, and another dry process using N2. Afterwards, the germanium may be grown in the window. In one embodiment, the epitaxial growth is formed by epitaxial growth at a temperature less than 400 degrees Celsius, which in one embodiment is approximately 390 degrees Celsius. Because the growth process is a selective epitaxial process, the top of the germanium detector 24 may be slanted inward at the corners. In prior art methods, epitaxial growth includes two steps: i) a high temperature bake at temperatures greater than 750 degrees Celsius; and ii) growth. However, the high temperature bake may be removed. In one embodiment, instead of a high temperature bake, germane is flowed at a temperature less than 600 degrees Celsius and then HCl (hydrochloric acid) is used to etch the germanium away and further clean the surface. The HCl etch is also performed at less than 600 degrees Celsius. Afterwards, the germanium photodetector 24 is grown. A skilled artisan appreciates that while 600 degrees Celsius may currently be the maximum for the germane flow process if transistors are formed over other portions of the semiconductor substrate 12 that as processes change it is likely that this maximum temperature will decrease, such as to 500 degrees Celsius or even 400 degrees Celsius.
In one embodiment, the radio frequency (RF) power used is approximately 200 Watts. In another embodiment, the RF power is between approximately 100 Watts and less than or equal to approximately 350 Watts. If a lower RF power is used (e.g., 100 Watts) the processing time may need to be extended to significantly remove the germanium oxide 26 as compared to using a higher RF power (e.g., 200 Watts). In one embodiment, if 200 Watts is used, the time may be approximately 40 seconds. The RF power may be chosen so that it is great enough to promote the creation of free hydrogen to bond with the oxygen in the germanium oxide 26, but low enough to minimize or prevent other elements, if present, such as nitrogen, to undesirably roughen the surface of the germanium detector 24.
In one embodiment, a pressure of approximately 3 Torr to approximately 30 Torr may be used. For example, approximately 8 Torr or approximately 4.2 Torr may be used. In one embodiment, the oxygen-free plasma process may be approximately 20 seconds to approximately 100 seconds, such as approximately 40 seconds. In one embodiment, a temperature of approximately 350 degrees Celsius to approximately 425 degrees Celsius, such as approximately 400 degrees Celsius, may be used. In one embodiment, the flow range may be approximately 200 sccms to approximately 8000 sccms. In one embodiment, the flow rage may be approximately 400 sccms of NH3 or approximately 6000 sccms of 91% N2 and 500 sccms of 9% of NH3.
In one embodiment, the process of removing the germanium oxide 26 is performed in the same chamber (i.e., in-situ) as the process for forming the cap layer 28. In another embodiment, these two processes are formed in separate chambers, but in the same platform. In any of these embodiments, the photodetector 10 is not exposed to oxygen after the germanium oxide 26 is removed and prior to forming the cap layer 28 to avoid germanium oxide from being formed over the germanium detector 24. More specifically, the exemplary platform 40 illustrated in
After forming the regions 30 and 32, a dielectric 34 is formed over the cap layer 28 and the dielectric 22. Any suitable process or materials, such as those described for the dielectric 22 may be used to form the dielectric 34. In one embodiment, the dielectric 34 is approximately 500 nm of silicon dioxide. Next, contacts 36 are formed within the dielectric 34 by forming vias in the dielectric 34 and filling the vias with a conductive material. Additionally, planarization may be performed. In one embodiment, the contacts 36 include tungsten. The contacts 36 electrically connect the germanium detector 24 to outside the photodetector 10, for example, through interconnect structures (not shown), which are formed using subsequent processing known to a skilled artisan.
By now it should be appreciated that there has been provided a method for reducing surface contamination on a germanium film to reduce leakage current of the photodetector. The chemistry chosen to remove the germanium oxide is chosen so that a chemical bonds with the oxygen and removes the resulting species so that only germanium remains. If the germanium oxide 26 was not removed then it would exist at the top surface of the germanium detector 24. The portion of the germanium oxide 26 between the regions 30 and 32 would provide a leakage path between these regions together because the germanium oxide 26 is a better conductor than germanium. The process to remove the germanium oxide can be used in a manufacturing environment. The inventors have found that by using the above process the time between exposure to atmosphere after forming the germanium detector 24 to the start of the plasma treatment can be approximately 72 hours or less, such as 48 hours, without degrading the performance of the photodetector 10.
Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The term “coupled,” as used herein, is not intended to be limited to a direct coupling or a mechanical coupling. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.
Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.
