Several attempts are made to downscale nuclear magnetic resonance (NMR) spectroscopy systems and to enable high resolution chemical analysis of small sample quantities. However, miniaturization is nontrivial due to stringent demands on precise analyte sampling within the detector while performing local excitation of the sample and signal detection with a microsized coil. Imperfect coil geometry and inhomogeneities in the coil's surrounding environment have a detrimental impact on the signal quality, hampering further development of miniature NMR systems. To solve this challenge, a new type of monolithic wafer-scale self-assembled microcoils with a detection volume of 1.5 nL are integrated into a microfluidics circuit. The microcoils are fully encapsulated in polydimethylsiloxane (PDMS) allowing for simplified and precise supply of an analyte through the interior of the detector. Due to their construction, with the inner winding touching the analyte, the microcoils have an almost 100% filling factor. Magnetic field inhomogeneities are reduced through well-defined microtubular architectures and susceptibility matched conductors. This approach results in a spectral linewidth of only 8 ppb with shimming and 22 ppb without shimming. The demonstrated methodology promotes the realization of next generation miniaturized analytical NMR systems for product monitoring, safety verification, medical testing, and material evaluation.