Method for manufacturing lateral germanium detectors

An improved method for manufacturing a lateral germanium detector is disclosed. A detector window is opened through an oxide layer to expose a doped single crystalline silicon layer situated on a substrate. Next, a single crystal germanium layer is grown within the detector window, and an amorphous germanium layer is grown on the oxide layer. The amorphous germanium layer is then polished to leave only a small portion around the single crystal germanium layer. A dielectric layer is deposited on the amorphous germanium layer and the single crystal germanium layer. Using resist masks and ion implants, multiple doped regions are formed on the single crystal germanium layer. After opening several oxide windows on the dielectric layer, a refractory metal layer is deposited on the doped regions to form multiple germanide layers.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to optical detectors in general, and in particular to a method for manufacturing lateral germanium detectors.

2. Description of Related Art

Photodetection in the near-infrared (IR) regime has many applications, such as telecommunications, thermal imaging, etc. InGaAs-based PIN photodetectors are commonly used for telecommunication applications due to their high responsivity and speed. However, the majority of the InGaAs-based detectors are normal incidence detectors, and the integration of such devices on silicon surfaces can be very expensive. Also, integration of high-speed InGaAs detectors requires special optics to focus light into a small active area, which has been found to reduce device performance.

Germanium-based detectors are known to be a suitable alternative. However, germanium-based detectors exhibit a higher dark current than InGaAs-based detectors, which limit their application in the telecommunications industry. In recent years, attempts have been made to improve the performance of polycrystalline germanium-based detectors for these applications. One exemplary prior art poly-germanium detector is described by Colace et al. in an article entitledEfficient high-speed near-infrared Ge photodetectors integrated on Si substrates(Applied Physics Letters, vol. 76, p. 1231 et seq., 2000).

The present disclosure provides an improved method for manufacturing lateral germanium-based detectors.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a detector window is opened through an oxide layer to expose a doped single crystalline silicon layer located on a substrate. Next, a single crystal germanium layer is grown within the detector window, and an amorphous germanium layer is grown on the oxide layer. The amorphous germanium layer is then polished to leave only a small portion around the single crystal germanium layer. A dielectric layer is deposited on the amorphous germanium layer and the single crystal germanium layer. Using resist masks and implants, doped regions are formed on the single crystal germanium layer. After opening several oxide windows on the dielectric layer, a refractory metal layer is deposited on the doped regions to form multiple germanide layers.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings and in particular toFIGS. 1-9, there are illustrated successive steps of a method for fabricating a lateral germanium-based detector, in accordance with a preferred embodiment of the present invention. Initially, a dry etch process is utilized to open a detector window11through a nitride layer12(˜250 Å) and an oxide layer14(˜6,000 Å) to expose a single crystalline silicon layer10situated on an insulator substrate, as shown inFIG. 1.

A single crystal germanium layer15is then grown within detector window11, as depicted inFIG. 2. Four different gases are used for the growth of single crystal germanium layer15, namely, hydrogen, 100% silane (SiH4), 100% germane (GeH4), and 100% diborane (B2H6). The germanium growth process uses silicon and silicon-germanium seed layers to create an abrupt transition from the underlying single crystal silicon surface and the single crystal germanium growth. The usage of the seed layers allows for a subsequent single crystal germanium growth, even across very large exposed single crystal silicon regions. For the present embodiment, the bottom portion of the germanium growth is in-situ doped with boron at a concentration of approximately 1×E21atoms/cm3. The boron concentration is graded with the highest concentration at the bottom of single crystal germanium layer15as a doped germanium layer13.

Because of the usage of the silicon seed layer, the germanium growth is not completely selective, and some germanium can form over nitride layer12as an amorphous germanium layer16, as depicted inFIG. 2. Amorphous germanium layer16may serve as a sacrificial polish layer for subsequent processing.

Doped germanium layer13minimizes the electric fields at the bottom of single crystal germanium layer15, which can decrease detector noise and dark currents caused by defects commonly located at the bottom of single crystal germanium layer15. The boron in doped germanium layer13can be replaced by other dopants.

Single crystal germanium layer15and amorphous germanium layer16are then polished via a chemical mechanical polish (CMP), as shown inFIG. 3. It is preferable to stop the CMP process before amorphous germanium layer16is completely removed. This is because polishing too far can expose voids and crystalline defects that tend to form at the edge of detector window11(fromFIG. 1) and single crystal germanium layer15, and polishing too close to the crystalline defects can create voids by tearing out entire crystalline defects.

Using a mask, the remaining portion of amorphous germanium layer16is removed via a dry etch, leaving a portion of amorphous germanium layer16located around single crystal germanium layer15, as depicted inFIG. 4.

After a tetraethyl orthosilicate (TEOS) layer17has been deposited on single crystal germanium layer15and amorphous germanium layer16, multiple n+ implant regions18are formed on single crystal germanium layer15, as shown inFIG. 5, via an appropriate mask and ion implants.

TEOS layer17may be replaced by other types of oxides or dielectrics including nitride. For example, germanium oxy-nitride can be used instead of TEOS in layer17in order to lower the stress over amorphous germanium layer16, which should reduce noise and dark current. TEOS layer17is utilized to seal the edge of a germanium detector at which defects and voids are most prone to be formed.

Next, multiple p+ doped germanium regions19are formed on single crystal germanium layer15, as depicted inFIG. 6, via a mask. All n+ implant regions18and p+ doped germanium regions19can be activated via annealing. Preferably, implant masks are utilized to keep the n+ and p+ dopants away from the defects and voids at the edge of single crystal germanium layer15. Keeping the n+ and p+ dopants away from the defect-prone edge regions can decrease detector noise and dark current.

TEOS (or germanium oxy-nitride) layer17is then patterned using a resist mask, and a dry etch is utilized to open multiple oxide windows20, as shown inFIG. 7. Oxide windows20expose n+ implant regions18and p+ doped germanium regions19for respective germanide formations later.

A titanium deposition is performed on n+ implant regions18and p+ doped germanium regions19that are exposed through oxide windows20. One or more heat treatments are then utilized to form TiGe material21within oxide windows20, as depicted inFIG. 8. Since no TiGe can be formed over TEOS layer17, non-reacted Ti layer22remains to be situated on top of TEOS layer17, as depicted inFIG. 8.

The remaining non-reacted Ti layer22located on top of TEOS layer17may be removed using a resist mask and dry etch, as depicted inFIG. 9. Alternatively, the remaining non-reacted Ti layer22located on top of TEOS layer17may be removed with a wet strip. Generally, a wet strip is not the preferred choice because there is some risk that the wet strip may remove TiGe material21or create pits in single crystal germanium layer15. The processing options are not limited to titanium germanide as any refractory metal may be used to form the germanide.

At this point, a P-i-N germanium detector having a lateral configuration is formed, and conventional semiconductor processing techniques can be utilized to fabricate dielectrics and contacts at the top of the P-i-N germanium detector. The detector shown inFIG. 9may be used to detect light from above single crystal germanium layer15, and may be configured as a focal plane array. Alternatively, the detector shown inFIG. 9may be configured to detect light from an underlying waveguide by using single crystalline silicon layer10as the end of a waveguide. The detector shown inFIG. 9may also be configured as a butt coupled device where the light comes from a waveguide that is coupled from the side of single crystal germanium layer15.

As has been described, the present invention provides an improved method for manufacturing a lateral germanium detector.